Dark matter near the Sun

Fig: The high-resolution simulation of the Milky Way used to test the mass-measuring technique. Image credit: Dr. A. Hobbs

By Royal Astronomical Society, United Kingdom

Published: August 9, 2012

 Astronomers have found large amounts of invisible dark matter near the Sun. Their results are consistent with the theory that the Milky Way Galaxy is surrounded by a massive “halo” of dark matter, but this is the first study of its kind to use a method rigorously tested against mock data from high-quality simulations. The scientists also have found tantalizing hints of a new dark matter component in our galaxy.

Swiss astronomer Fritz Zwicky first proposed dark matter in the 1930s. He found that clusters of galaxies were filled with a mysterious dark matter that kept them from flying apart. At nearly the same time, Jan Oort in the Netherlands discovered that the density of matter near the Sun was nearly twice what could be explained by the presence of stars and gas alone.

In the intervening decades, astronomers developed a theory of dark matter and structure formation that explains the properties of clusters and galaxies in the universe, but the amount of dark matter in the solar neighborhood has remained more mysterious. For decades after Oort’s measurement, studies found three to six times more dark matter than expected. Then last year new data and a new method claimed far less than expected. The community was left puzzled, generally believing that the observations and analyzes simply weren’t sensitive enough to perform a reliable measurement.

In this latest study, the astronomers are more confident in their measurement and its uncertainties. This is because they used a state-of-the-art simulation of our galaxy to test their mass-measuring technique before applying it to real data. This threw up a number of surprises. They found that standard techniques used over the past 20 years were biased, always tending to underestimate the amount of dark matter. They then devised a new unbiased technique that recovered the correct answer from the simulated data. Applying their technique to the positions and velocities of thousands of orange K dwarf stars near the Sun, they obtained a new measure of the local dark matter density.

“We are 99 percent confident that there is dark matter near the Sun,” said Silvia Garbari from the University of Zürich. “In fact, our favored dark matter density is a little high. There is a 10 percent chance that this is merely a statistical fluke. But with 90 percent confidence, we find more dark matter than expected. If future data confirms this high value, the implications are exciting. It could be the first evidence for a disk of dark matter in our galaxy, as recently predicted by theory and numerical simulations of galaxy formation. Or it could be that the dark matter halo of our galaxy is squashed, boosting the local dark matter density.”

Many physicists are placing their bets on dark matter being a new fundamental particle that interacts only weakly with normal matter — but strongly enough to be detected in experiments deep underground where confusing cosmic-ray events are screened by over a mile of solid rock.

An accurate measure of the local dark matter density is vital for such experiments. “If dark matter is a fundamental particle, billions of these particles will have passed through your body by the time you finish reading this article,” said George Lake from ETH Zürich. “Experimental physicists hope to capture just a few of these particles each year in experiments like XENON and CDMS currently in operation. Knowing the local properties of dark matter is the key to revealing just what kind of particle it consists of.”

Coronal mass ejections (CMEs) could "sandblast" the Moon

Fig: Coronal mass ejection as viewed by the Solar Dynamics Observatory
(June 7, 2011. NASA/SDO)

By NASA's Goddard Space Flight Center, Greenbelt, Maryland

Published: December 12, 2011

Solar storms and associated coronal mass ejections (CMEs) can significantly erode the lunar surface, according to a new set of computer simulations by NASA scientists. In addition to removing a surprisingly large amount of material from the lunar surface, this could be a major method of atmospheric loss for planets like Mars that are unprotected by a global magnetic field.

Rosemary Killen is leading the research from NASA’s Goddard Space Flight Center in Greenbelt, Maryland, as part of the Dynamic Response of the Environment At the Moon (DREAM) team within the NASA Lunar Science Institute.

CMEs are basically an intense gust of the normal solar wind, a diffuse stream of electrically conductive gas called plasma that’s blown outward from the surface of the Sun into space. A strong CME may contain around a billion tons of plasma moving at up to 1 million mph (1.6 million km/h) in a cloud many times the size of Earth.

The Moon has just the barest wisp of an atmosphere, technically called an exosphere because it is so tenuous, which leaves it vulnerable to CME effects. The plasma from CMEs impacts the lunar surface, and atoms from the surface are ejected in a process called “sputtering.”

“We found that when this massive cloud of plasma strikes the Moon, it acts like a sandblaster and easily removes volatile material from the surface,” said William Farrell from NASA’s Goddard Space Flight Center. “The model predicts 100 to 200 tons of lunar material — the equivalent of 10 dump-truck loads — could be stripped off the lunar surface during the typical two-day passage of a CME.”

This is the first time researchers have attempted to predict the effects of a CME on the Moon. “Connecting various models together to mimic conditions during solar storms is a major goal of the DREAM project,” said Farrell.

Plasma is created when energetic events, like intense heat or radiation, remove electrons from the atoms in a gas, turning the atoms into electrically charged particles called ions. The Sun is so hot that the gas is emitted in the form of free ions and electrons called the solar wind plasma. Ejection of atoms from a surface or an atmosphere by plasma ions is called sputtering.

“Sputtering is among the top five processes that create the Moon’s exosphere under normal solar conditions, but our model predicts that during a CME, it becomes the dominant method by far, with up to 50 times the yield of the other methods,” said Killen.

CMEs are effective at removing lunar material not only because they are denser and faster than the normal solar wind, but also because they are enriched in highly charged, heavy ions, according to the team. The typical solar wind is dominated by lightweight hydrogen ions (protons). However, a heavier helium ion with more electrons removed, and hence a greater electric charge, can sputter tens of times more atoms from the lunar surface than a hydrogen ion.

The team used data from satellite observations that revealed this enrichment as input to their model. For example, helium ions make up about 4 percent of the normal solar wind, but observations reveal that during a CME they can increase to over 20 percent. When this enrichment is combined with the increased density and velocity of a CME, the highly charged, heavy ions in CMEs can sputter 50 times more material than protons in the normal solar wind.

“The computer models isolate the contributions from sputtering and other processes,” said Dana Hurley from the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland. “Comparing model predictions through a range of solar wind conditions allows us to predict the conditions when sputtering should dominate over the other processes. Those predictions can later be compared to data during a solar storm.”

The researchers believe that NASA’s Lunar Atmosphere And Dust Environment Explorer (LADEE) — a lunar orbiter mission scheduled to launch in 2013 — will be able to test their predictions. The strong sputtering effect should kick lunar surface atoms to LADEE’s orbital altitude, about 12 to 31 miles (19 to 50 kilometers), so the spacecraft will see them increase in abundance.

“This huge CME sputtering effect will make LADEE almost like a surface mineralogy explorer, not because LADEE is on the surface, but because during solar storms surface atoms are blasted up to LADEE,” said Farrell.

The Moon is not the only heavenly body affected by the dense CME driver gas. Space scientists have long been aware that these solar storms dramatically affect Earth’s magnetic field and are responsible for intense aurorae (the northern and southern lights).

While certain areas of the martian surface are magnetized, Mars does not have a magnetic field that surrounds the entire planet. Therefore, CME gases have a direct path to sputter and erode that planet’s upper atmosphere. In late 2013, NASA will launch the Mars Atmosphere and Volatile Evolution (MAVEN) mission that will orbit the Red Planet to investigate exactly how solar activity, including CMEs, removes the atmosphere.

On exposed small bodies like asteroids, the dense, fast-streaming CME gas should create a sputtered-enhanced exosphere about the object, similar to that expected at the Moon.

Tiny flares responsible for outsized heat of Sun's atmosphere

This false-color temperature map shows solar active region AR 10923, observed close to center of the Sun's disk. Blue regions indicate plasma near 10 million degrees K. Reale, et al.

August 17, 2009

Solar physicists at NASA have confirmed that small, sudden bursts of heat and energy — called nanoflares — cause temperatures in the thin, translucent gas of the Sun's atmosphere to reach millions of degrees.

The Sun's outer atmosphere, or corona, is made up of loops of hot gas that arch high above the surface. These loops are comprised of bundles of smaller, individual magnetic tubes or strands that can have temperatures reaching several million degrees Kelvin (K), even though the Sun's surface is only 5,700 degrees K.

Nanoflares are small, sudden bursts of energy that happen within these thin magnetic tubes in the corona. Unlike solar flares that can be viewed through satellites and ground-based telescopes, nanoflares are too small to be resolved individually. We only see the combined effect of many of them occurring at about the same time.

"Coronal loops are the fundamental building blocks of the corona," said James Klimchuk, an astrophysicist at the Goddard Space Flight Center in Greenbelt, Maryland. "Their shape is defined by the magnetic field, which guides the hot flowing gases called plasma. The magnetic field is also the source of the nanoflare energy. We believe that stresses in the field are released when thin sheets of electric current become unstable."

Klimchuk and colleagues have constructed a theoretical model to explain how nanoflares occur and how plasma within the tubes responds to the skyrocketing temperatures. "We simulate bursts of heating and predict what the loop should look like when observed with a variety of instruments," said Klimchuk.

To test their model, the team observed gas emissions in the solar corona using the NASA-funded X-Ray Telescope and Extreme Ultraviolet Imaging Spectrometer on Japan's Hinode spacecraft.

"The 10 million degree temperatures we detected in the corona can only be produced by the impulsive energy bursts," said Klimchuk. The ultra-hot plasma cools very quickly, however, which explains why it is so faint and has been so difficult to detect until now. The energy lost from the cooling conducts down to the relatively cold solar surface. The gas there is heated to about 1 million degrees K and expands upward to become the 1 million degree component of the corona that has been observed for many years.

NASA's upcoming mission to study the Sun, the Solar Dynamics Observatory, will help scientists answer the outstanding questions of coronal heating by observing the coronal plasma at different temperatures with an unprecedented combination of close-up detail and rapid sequences.

Goddard Space Center, Greenbelt, MD

Solar Flare Surprise

The X9-class solar flare of Dec. 5, 2006, observed by the Solar X-Ray Imager aboard NOAA's GOES-13 satellite.

Solar flares are the most powerful explosions in the solar system. Packing a punch equal to a hundred million hydrogen bombs, they obliterate everything in their immediate vicinity. Not a single atom should remain intact.

At least that's how it's supposed to work.

"We've detected a stream of perfectly intact hydrogen atoms shooting out of an X-class solar flare," says Richard Mewaldt of Caltech. "What a surprise! These atoms could be telling us something new about what happens inside flares."

The event occurred on Dec. 5, 2006. A large sunspot rounded the sun's eastern limb and with little warning it exploded. On the "Richter scale" of flares, which ranks X1 as a big event, the blast registered X9, making it one of the strongest flares of the past 30 years.

NASA managers braced themselves. Such a ferocious blast usually produces a blizzard of high-energy particles dangerous to both satellites and astronauts. Indeed, moments after the explosion, radio emissions from a shock wave in the sun's atmosphere signaled that a swarm of particles was on its way.

An hour later they arrived. But they were not the particles researchers expected.

NASA's twin STEREO spacecraft made the discovery: "It was a burst of hydrogen atoms," says Mewaldt. "No other elements were present, not even helium (the sun's second most abundant atomic species). Pure hydrogen streamed past the spacecraft for a full 90 minutes."

Next came more than 30 minutes of quiet. The burst subsided and STEREO's particle counters returned to low levels. The event seemed to be over when a second wave of particles enveloped the spacecraft. These were the "broken atoms" that flares are supposed to produce—protons and heavier ions such as helium, oxygen and iron. "Better late than never," he says.



STEREO particle counts on Dec. 5, 2006. The vertical axis measures the angle to the sun. Note how the initial hydrogen burst arrived from a narrow angle while the ions that followed swarmed in from all directions. The "swarming action" is a result of deflections by the sun's magnetic field--a force not felt by the neutral hydrogen.

At first, this unprecedented sequence of events baffled scientists, but now Mewaldt and colleagues believe they're getting to the bottom of the mystery.

First, how did the hydrogen atoms resist destruction?

"They didn't," says Mewaldt. "We believe they began their journey to Earth in pieces, as protons and electrons. Before they escaped the sun’s atmosphere, however, some of the protons recaptured an electron, forming intact hydrogen atoms. The atoms left the sun in a fast, straight shot before they could be broken apart again." (For experts: The team believes the electrons were recaptured by some combination of radiative recombination and charge exchange.)

Second, what delayed the ions?
"Simple," says Mewaldt. "Ions are electrically charged and they feel the sun's magnetic field. Solar magnetism deflects ions and slows their progress to Earth. Hydrogen atoms, on the other hand, are electrically neutral. They can shoot straight out of the sun without magnetic interference."

Imagine two runners dashing for the finish line. One (the ion) is forced to run in a zig-zag pattern with zigs and zags as wide as the orbit of Mars. The other (the hydrogen atom) runs in a straight line. Who's going to win?

"The hydrogen atoms reached Earth two hours before the ions," says Mewaldt.

Mewaldt believes that all strong flares might emit hydrogen bursts, but they simply haven't been noticed before. He's looking forward to more X-flares now that the two STEREO spacecraft are widely separated on nearly opposite sides of the Sun. (In 2006 they were still together near Earth.) STEREO-A and –B may be able to triangulate future bursts and pinpoint the source of the hydrogen. This would allow the team to test their ideas about the surprising phenomenon.

"All we need now," he says, "is some solar activity."

For more information about this research, look for the article "STEREO Observations of Energetic Neutral Atoms during the 5 December 2006 Solar Flare" by R. A. Mewaldt et al, in a future issue of the Astrophysical Journal Letters.

Sun is not a perfect Sphere

photo: "Cantaloupe ridges" on the sun. The glowing white magnetic network is what gives the sun its extra oblateness during times of high solar activity. Amateur astronomer Gary Palmer took the picture in July 2005 using a violet calcium-K filter.

“The sun is the biggest and therefore smoothest object in the solar system, perfect at the 0.001% level because of its extremely strong gravity,” says study co-author Hugh Hudson of UC Berkeley. “Measuring its exact shape is no easy task.”

The team accomplished the task by analyzing data from the Reuven Ramaty High-Energy Solar Spectroscopic Imager, RHESSI for short, an x-ray/gamma-ray space telescope launched in 2002 on a mission to study solar flares. Although RHESSI was never intended to measure the roundness of the sun, it has turned out ideal for the purpose. RHESSI observes the solar disk through a narrow slit and spins at 15 rpm. The spacecraft’s rapid rotation and high data sampling rate (necessary to catch fast solar flares) make it possible for investigators to trace the shape of the sun with systematic errors much less than any previous study. Their technique is particularly sensitive to small differences in polar vs. equatorial radius or “oblateness.”

“We have found that the surface of the sun has rough structure: bright ridges arranged in a network pattern, as on the surface of a cantaloupe but much more subtle,” describes Hudson. During active phases of the solar cycle, these ridges emerge around the sun’s equator, brightening and fattening the “stellar waist.” At the time of RHESSI’s measurements in 2004, ridges increased the sun’s apparent equatorial radius by an angle of 10.77 +- 0.44 milli-arcseconds, or about the same as the width of a human hair viewed one mile away.



photo: In this diagram, the sun's oblateness has been magnified 10,000 times for easy visibility. The blue curve traces the sun's shape averaged over a three month period. The black asterisked curve traces a shorter 10-day average. The "wiggles" in the 10-day curve are real, caused by strong magnetic ridges in the vicinity of sunspots.
Credit: NASA Goddard Space Flight

Scientists using NASA’s RHESSI spacecraft have measured the roundness of the sun with unprecedented precision. They find that it is not a perfect sphere. During years of high solar activity the sun develops a thin “cantaloupe skin” that significantly increases its apparent oblateness: the sun’s equatorial radius becomes slightly larger than its polar radius. Their results appear the Oct. 2nd edition of Science Express.

“That may sound like a very small angle, but it is in fact significant,” says Alexei Pevtsov, RHESSI Program Scientist at NASA Headquarters. Tiny departures from perfect roundness can, for example, affect the sun’s gravitational pull on Mercury and skew tests of Einstein’s theory of relativity that depend on careful measurements of the inner planet’s orbit. Small bulges are also telltale signs of hidden motions inside the sun. For instance, if the sun had a rapidly rotating core left over from early stages of star formation, and if that core were tilted with respect to its outer layers, the result would be surface bulging. “RHESSI’s precision measurements place severe constraints on any such models.”

The “cantaloupe ridges” are magnetic in nature. They outline giant, bubbling convection cells on the surface of the sun called “supergranules.” Supergranules are like bubbles in a pot of boiling water amplified to the scale of a star; on the sun they measure some 30,000 km across (twice as wide as Earth) and are made of seething hot magnetized plasma. Magnetic fields at the center of these bubbles are swept out to the edge where they form ridges of magnetism. The ridges are most prominent during years around Solar Max when the sun’s inner dynamo “revs up” to produce the strongest magnetic fields. Solar physicists have known about supergranules and the magnetic network they produce for many years, but only now has RHESSI revealed their unexpected connection to the sun’s oblateness.

“When we subtract the effect of the magnetic network, we get a ‘true’ measure of the sun’s shape resulting from gravitational forces and motions alone,” says Hudson. “The corrected oblateness of the non-magnetic sun is 8.01 +- 0.14 milli arcseconds, near the value expected from simple rotation.”

Further analysis of RHESSI oblateness data may help researchers detect a long-sought type of seismic wave echoing through the interior of the sun: the gravitational oscillation or “g-mode.” Detecting g-modes would open a new frontier in solar physics—the study of the sun’s internal core.

The paper reporting these results, “A large excess in apparent solar oblateness due to surface magnetism,” was authored by Martin Fivian, Hugh Hudson, Robert Lin and Jabran Zahid, and appears in the Oct. 2nd issue of Science Express.

Date: Saturday, October 04, 2008

Some properties of sun:Some important snapshots

photo: The Sun as it appears through a camera lens from the surface of Earth


photo: The Moon passing in front of the Sun, as taken by the STEREO-B spacecraft on February 25, 2007. Because the satellite is in an Earth-trailing orbit and is further from the Moon than the Earth is, the Moon appears smaller than the Sun.


photo: Solar "fireworks" in sequence as recorded in November 2000 by four instruments onboard the SOHO spacecraft.


photo: The heliospheric current sheet extends to the outer reaches of the Solar System, and results from the influence of the Sun's rotating magnetic field on the plasma in the interplanetary medium.


photo: History of the number of observed sunspots during the last 250 years, which shows the ~11-year solar cycle.


photo: Coronal mass ejections blast filaments and bubbles of magnetic plasma into space as seen in this ultra-violet light picture taken by SOHO.


photo: Measurements of solar cycle variation during the last 30 years.


photo: Taken by Hinode's Solar Optical Telescope on January 12, 2007, this image of the Sun reveals the filamentary nature of the plasma connecting regions of different magnetic polarity.


photo: During a total solar eclipse, the solar corona can be seen with the naked eye.


photo: The effective temperature, or black body temperature, of the Sun (5777 K) is the temperature a black body of the same size must have to yield the same total emissive power.


photo: Structure of the Sun


photo: Cross-section of a solar-type star.


photo: An illustration of the structure of the Sun


Photo: Life-cycle of the Sun; sizes are not drawn to scale.

Sun: The heart of the solar system

Observation Data:

Mean distance
from Earth:1.496×1011 m
(8.31 min at light speed)

Visual brightness (V): −26.74m

Absolute magnitude:4.83m

Spectral classification:G2V

Metallicity:Z = 0.0177

Angular size: 31.6′ - 32.7′

Adjectives:solar

Orbital characteristics:

Mean distance:(from Milky Way core) ~2.5×1020 m
26 000 light-years

Galactic period: (2.25–2.50)×108 a

Velocity: ~2.20×105 m/s
(orbit around the center of the Galaxy)

~2×104 m/s
(relative to average velocity of other stars in stellar neighborhood)

Physical characteristics:

Mean diameter: 1.392×109 m
(109 Earths)

Equatorial radius: 6.955×108 m
(109 × Earth)

Equatorial circumference: 4.379×109 m
(109 × Earth)

Flattening: 9×10−6

Surface area: 6.0877×1018 m²
11 990 × Earth

Volume: 1.4122×1027 m³
1 300 000 Earths

Mass: 1.9891 ×1030 kg
332 946 Earths

Average density :1.408 ×103 kg/m³

Different Densities Core: 1.5×105 kg/m³

lower Photosphere: 2×10-4 kg/m³

lower Cromosphere: 5×10-6 kg/m³

Avg. Corona: 10×10-12kg/m³

Equatorial surface gravity: 274.0 m/s2
27.94 g
28 × Earth surface gravity

Escape velocity:
(from the surface) 617.7 km/s
55 × Earth

Temperature:
of surface (effective) 5 778 K

Temperature:
of corona ~5×106 K

Temperature:
of core ~15.7×106 K

Luminosity (Lsol): 3.846×1026 W
~3.75×1028 lm
~98 lm/W efficacy

Mean Intensity (Isol): 2.009×107 W m-2 sr-1

Rotation characteristics:

Obliquity :7.25°
(to the ecliptic)
67.23°
(to the galactic plane)

Right ascension
of North pole: 286.13°
19 h 4 min 30 s

Declination
of North pole: +63.87°
63°52' North

Sidereal Rotation period:
(at 16° latitude) 25.38 days
25 d 9 h 7 min 13 s
(at equator) 25.05 days
(at poles) 34.3 days

Rotation velocity:7.284 ×103 km/h
(at equator)

Photospheric composition (by mass):

Hydrogen 73.46 %
Helium 24.85 %
Oxygen 0.77 %
Carbon 0.29 %
Iron 0.16 %
Sulfur 0.12 %
Neon 0.12 %
Nitrogen 0.09 %
Silicon 0.07 %
Magnesium 0.05 %



The Sun is the star at the center of the Solar System. The Earth and other matter (including other planets, asteroids, meteoroids, comets, and dust) orbit the Sun, which by itself accounts for about 99.8% of the Solar System's mass. Energy from the Sun, in the form of sunlight, supports almost all life on Earth via photosynthesis, and drives the Earth's climate and weather.The surface of the Sun consists of hydrogen (about 74% of its mass, or 92% of its volume), helium (about 24% of mass, 7% of volume), and trace quantities of other elements, including iron, nickel, oxygen, silicon, sulfur, magnesium, carbon, neon, calcium, and chromium.The Sun has a spectral class of G2V. G2 means that it has a surface temperature of approximately 5,780 K, giving it a white color that often, because of atmospheric scattering, appears yellow when seen from the surface of the Earth. This is a subtractive effect, as the preferential scattering of shorter wavelength light removes enough violet and blue light, leaving a range of frequencies that is perceived by the human eye as yellow. It is this scattering of light at the blue end of the spectrum that gives the surrounding sky its color. When the Sun is low in the sky, even more light is scattered so that the Sun appears orange or even red.The Sun's spectrum contains lines of ionized and neutral metals as well as very weak hydrogen lines. The V (Roman five) in the spectral class indicates that the Sun, like most stars, is a main sequence star. This means that it generates its energy by nuclear fusion of hydrogen nuclei into helium. There are more than 100 million G2 class stars in our galaxy. Once regarded as a small and relatively insignificant star, the Sun is now known to be brighter than 85% of the stars in the galaxy, most of which are red dwarfs.

The Sun orbits the center of the Milky Way galaxy at a distance of approximately 26 000 or 27 000 light-years from the galactic center, moving generally in the direction of Cygnus and completing one revolution in about 225–250 million years. Its orbital speed was thought to be 220 ± 20 km/s, but a new estimate gives 251 km/s. This is equivalent to about one light-year every 1,190 years, and about one AU every 7 days. These measurements of galactic distance and speed are as accurate as we can get given our current knowledge, but may change as we learn more. Since our galaxy is moving with respect to the cosmic microwave background radiation (CMB) in the direction of Hydra with a speed of 550 km/s, the sun's resultant velocity with respect to the CMB is about 370 km/s in the direction of Crater or Leo.

The Sun is currently traveling through the Local Interstellar Cloud in the low-density Local Bubble zone of diffuse high-temperature gas, in the inner rim of the Orion Arm of the Milky Way Galaxy, between the larger Perseus and Sagittarius arms of the galaxy. Of the 50 nearest stellar systems within 17 light-years (1.6×1014 km) from the Earth, the Sun ranks 4th in absolute magnitude as a fourth magnitude star (M=4.83).The Sun is a Population I, or heavy element-rich,star.The formation of the Sun may have been triggered by shockwaves from one or more nearby supernovae. This is suggested by a high abundance of heavy elements such as gold and uranium in the Solar System relative to the abundances of these elements in so-called Population II (heavy element-poor) stars. These elements could most plausibly have been produced by endergonic nuclear reactions during a supernova, or by transmutation via neutron absorption inside a massive second-generation star.

Sunlight is Earth's primary source of energy. The solar constant is the amount of power that the Sun deposits per unit area that is directly exposed to sunlight. The solar constant is equal to approximately 1370 watts per square meter at a distance of one AU from the Sun (that is, on or near Earth). Sunlight on the surface of Earth is attenuated by the Earth's atmosphere so that less power arrives at the surface—closer to 1000 watts per directly exposed square meter in clear conditions when the Sun is near the zenith. This energy can be harnessed via a variety of natural and synthetic processes—photosynthesis by plants captures the energy of sunlight and converts it to chemical form (oxygen and reduced carbon compounds), while direct heating or electrical conversion by solar cells are used by solar power equipment to generate electricity or to do other useful work. The energy stored in petroleum and other fossil fuels was originally converted from sunlight by photosynthesis in the distant past.Ultraviolet light from the Sun has antiseptic properties and can be used to sanitize tools and water. It also causes sunburn, and has other medical effects such as the production of Vitamin D. Ultraviolet light is strongly attenuated by Earth's ozone layer, so that the amount of UV varies greatly with latitude and has been partially responsible for many biological adaptations, including variations in human skin color in different regions of the globe.

Observed from Earth, the Sun's path across the sky varies throughout the year. The shape described by the Sun's position, considered at the same time each day for a complete year, is called the analemma and resembles a figure 8 aligned along a north/south axis. While the most obvious variation in the Sun's apparent position through the year is a north/south swing over 47 degrees of angle (because of the 23.5-degree tilt of the Earth with respect to the Sun), there is an east/west component as well, caused by the acceleration of the Earth as it approaches its perihelion with the Sun, and the reduction in the Earth's speed as it moves away to approach its aphelion. The north/south swing in apparent angle is the main source of seasons on Earth.

A rare optical phenomenon may occur shortly after sunset or before sunrise, known as a green flash. The flash is caused by light from the sun just below the horizon being bent (usually through a temperature inversion) towards the observer. Light of shorter wavelengths (violet, blue, green) is bent more than that of longer wavelengths (yellow, orange, red) but the violet and blue light is scattered more, leaving light that is perceived as green.The Sun is a magnetically active star. It supports a strong, changing magnetic field that varies year-to-year and reverses direction about every eleven years around solar maximum. The Sun's magnetic field gives rise to many effects that are collectively called solar activity, including sunspots on the surface of the Sun, solar flares, and variations in solar wind that carry material through the Solar System. Effects of solar activity on Earth include auroras at moderate to high latitudes, and the disruption of radio communications and electric power. Solar activity is thought to have played a large role in the formation and evolution of the Solar System. Solar activity changes the structure of Earth's outer atmosphere.Although it is the nearest star to Earth and has been intensively studied by scientists, many questions about the Sun remain unanswered. Current topics of scientific inquiry include the Sun's regular cycle of sunspot activity, the physics and origin of flares and prominences, the magnetic interaction between the chromosphere and the corona, and the origin (propulsion source) of solar wind.

Location within the galaxy

The Sun lies close to the inner rim of the Milky Way Galaxy's Orion Arm, in the Local Fluff or the Gould Belt, at a hypothesized distance of 7.62 ± 0.32 kpc from the Galactic Center.The distance between the local arm and the next arm out, the Perseus Arm, is about 6 500 light-years.The Sun, and thus the Solar System, is found in what scientists call the galactic habitable zone.

The Apex of the Sun's Way, or the solar apex, is the direction that the Sun travels through space in the Milky Way. The general direction of the Sun's galactic motion is towards the star Vega near the constellation of Hercules, at an angle of roughly 60 sky degrees to the direction of the Galactic Center. The Sun's orbit around the Galaxy is expected to be roughly elliptical with the addition of perturbations due to the galactic spiral arms and non-uniform mass distributions. In addition the Sun oscillates up and down relative to the galactic plane approximately 2.7 times per orbit. This is very similar to how a simple harmonic oscillator works with no drag force (damping) term. It has been argued that the Sun's passage through the higher density spiral arms often coincides with mass extinctions on Earth, perhaps due to increased impact events.

It takes the Solar System about 225–250 million years to complete one orbit of the galaxy (a galactic year),so it is thought to have completed 20–25 orbits during the lifetime of the Sun and 1/1250th of a revolution since the origin of humans. The orbital speed of the Solar System about the center of the Galaxy is approximately 220 km/s. At this speed, it takes around 1400 years for the Solar System to travel a distance of 1 light-year, or 8 days to travel 1 AU.

Life cycle

The Sun's current main sequence age, determined using computer models of stellar evolution and nucleocosmochronology, is thought to be about 4.57 billion years.

It is thought that about 4.59 billion years ago, the rapid collapse of a hydrogen molecular cloud led to the formation of a third generation T Tauri Population I star, the Sun. The nascent star assumed a nearly circular orbit about 26 000 light-years from the center of the Milky Way Galaxy.

The Sun is about halfway through its main-sequence evolution, during which nuclear fusion reactions in its core fuse hydrogen into helium. Each second, more than 4 million tonnes of matter are converted into energy within the Sun's core, producing neutrinos and solar radiation; at this rate, the Sun will have so far converted around 100 Earth-masses of matter into energy. The Sun will spend a total of approximately 10 billion years as a main sequence star.

The Sun does not have enough mass to explode as a supernova. Instead, in about 5 billion years, it will enter a red giant phase, its outer layers expanding as the hydrogen fuel in the core is consumed and the core contracts and heats up. Helium fusion will begin when the core temperature reaches around 100 million kelvin and will produce carbon, entering the asymptotic giant branch phase.Earth's fate is unclear. As a red giant, the Sun will have a maximum radius beyond the Earth's current orbit, 1 AU (1.5×1011 m), 250 times the present radius of the Sun.However, by the time it is an asymptotic giant branch star, the Sun will have lost roughly 30% of its present mass due to a stellar wind, so the orbits of the planets will move outward. If it were only for this, Earth would probably be spared, but new research suggests that Earth will be swallowed by the Sun owing to tidal interactions.Even if Earth escapes incineration in the Sun, its water will be boiled away and most of its atmosphere would escape into space. In fact, even during its life in the main sequence, the Sun is gradually becoming more luminous (about 10% every 1 billion years), and its surface temperature is slowly rising. The increase in solar temperatures is such that in about a billion years, the surface of the Earth will become too hot for liquid water to exist, ending all terrestrial life.

Following the red giant phase, intense thermal pulsations will cause the Sun to throw off its outer layers, forming a planetary nebula. The only object that will remain after the outer layers are ejected is the extremely hot stellar core, which will slowly cool and fade as a white dwarf over many billions of years. This stellar evolution scenario is typical of low- to medium-mass stars.


Structure

The Sun is a yellow main sequence star comprising about 99% of the total mass of the Solar System. It is a near-perfect sphere, with an oblateness estimated at about 9 millionths, which means that its polar diameter differs from its equatorial diameter by only 10 km (6 mi). As the Sun exists in a plasmatic state and is not solid, it rotates faster at its equator than at its poles. This behaviour is known as differential rotation. The period of this actual rotation is approximately 25 days at the equator and 35 days at the poles. However, due to our constantly changing vantage point from the Earth as it orbits the Sun, the apparent rotation of the star at its equator is about 28 days. The centrifugal effect of this slow rotation is 18 million times weaker than the surface gravity at the Sun's equator. The tidal effect of the planets is even weaker, and does not significantly affect the shape of the Sun.

The Sun does not have a definite boundary as rocky planets do, and in its outer parts the density of its gases drops approximately exponentially with increasing distance from its center. Nevertheless, it has a well-defined interior structure, described below. The Sun's radius is measured from its center to the edge of the photosphere. This is simply the layer above which the gases are too cool or too thin to radiate a significant amount of light, and is therefore the surface most readily visible to the naked eye. The solar core comprises 10 percent of its total volume, but 40 percent of its total mass.

The solar interior is not directly observable, and the Sun itself is opaque to electromagnetic radiation. However, just as seismology uses waves generated by earthquakes to reveal the interior structure of the Earth, the discipline of helioseismology makes use of pressure waves (infrasound) traversing the Sun's interior to measure and visualize the star's inner structure. Computer modeling of the Sun is also used as a theoretical tool to investigate its deeper layers.

Core

The core of the Sun is considered to extend from the center to about 0.2 solar radii. It has a density of up to 150 000 kg/m³ (150 times the density of water on Earth) and a temperature of close to 13 600 000 kelvin (by contrast, the surface of the Sun is around 5,800 kelvin). Recent analysis of SOHO mission data favors a faster rotation rate in the core than in the rest of the radiative zone.Through most of the Sun's life, energy is produced by nuclear fusion through a series of steps called the p–p (proton–proton) chain; this process converts hydrogen into helium. The core is the only location in the Sun that produces an appreciable amount of heat via fusion: the rest of the star is heated by energy that is transferred outward from the core. All of the energy produced by fusion in the core must travel through many successive layers to the solar photosphere before it escapes into space as sunlight or kinetic energy of particles.

About 3.4×1038 protons (hydrogen nuclei) are converted into helium nuclei every second (out of ~8.9×1056 total amount of free protons in the Sun), releasing energy at the matter–energy conversion rate of 4.26 million tonnes per second, 383 yottawatts (3.83×1026 W) or 9.15×1010 megatons of TNT per second. This actually corresponds to a surprisingly low rate of energy production in the Sun's core—about 0.3 W/m³ (watts per cubic meter). This is less power than generated by a candle. Power density is about 6 µW/kg of matter. For comparison, the human body produces heat at approximately the rate 1.2 W/kg, roughly a million times greater per unit mass. The use of plasma with similar parameters for energy production on Earth would be completely impractical—even a modest 1 GW fusion power plant would require about 170 billion tonnes of plasma occupying almost one cubic mile. Hence, terrestrial fusion reactors utilize far higher plasma temperatures than those in Sun's interior.

The rate of nuclear fusion depends strongly on density and temperature, so the fusion rate in the core is in a self-correcting equilibrium: a slightly higher rate of fusion would cause the core to heat up more and expand slightly against the weight of the outer layers, reducing the fusion rate and correcting the perturbation; and a slightly lower rate would cause the core to cool and shrink slightly, increasing the fusion rate and again reverting it to its present level.

The high-energy photons (gamma rays) released in fusion reactions are absorbed in only few millimetres of solar plasma and then re-emitted again in random direction (and at slightly lower energy)—so it takes a long time for radiation to reach the Sun's surface. Estimates of the "photon travel time" range between 10 000 and 170 000 years.

After a final trip through the convective outer layer to the transparent "surface" of the photosphere, the photons escape as visible light. Each gamma ray in the Sun's core is converted into several million visible light photons before escaping into space. Neutrinos are also released by the fusion reactions in the core, but unlike photons they rarely interact with matter, so almost all are able to escape the Sun immediately. For many years measurements of the number of neutrinos produced in the Sun were lower than theories predicted by a factor of 3. This discrepancy was recently resolved through the discovery of the effects of neutrino oscillation: the Sun in fact emits the number of neutrinos predicted by the theory, but neutrino detectors were missing 2/3 of them because the neutrinos had changed flavor.

Radiative zone


From about 0.2 to about 0.7 solar radii, solar material is hot and dense enough that thermal radiation is sufficient to transfer the intense heat of the core outward. In this zone there is no thermal convection; while the material grows cooler as altitude increases, this temperature gradient is less than the value of adiabatic lapse rate and hence cannot drive convection. Heat is transferred by radiation—ions of hydrogen and helium emit photons, which travel a brief distance before being reabsorbed by other ions. In this way energy makes its way very slowly (see above) outward.

Between the radiative zone and the convection zone is a transition layer called the tachocline. This is a region where the sharp regime change between the uniform rotation of the radiative zone and the differential rotation of the convection zone results in a large shear -- i.e. a condition where successive vertical layers slide past one another.


Convection zone


In the Sun's outer layer (down to approximately 70% of the solar radius), the solar plasma is not dense enough or hot enough to transfer the heat energy of the interior outward via radiation. As a result, thermal convection occurs as thermal columns carry hot material to the surface (photosphere) of the Sun. Once the material cools off at the surface, it plunges back downward to the base of the convection zone, to receive more heat from the top of the radiative zone. Convective overshoot is thought to occur at the base of the convection zone, carrying turbulent downflows into the outer layers of the radiative zone.The thermal columns in the convection zone form an imprint on the surface of the Sun, in the form of the solar granulation and supergranulation. The turbulent convection of this outer part of the solar interior gives rise to a "small-scale" dynamo that produces magnetic north and south poles all over the surface of the Sun.The Sun's thermal columns are Bénard cells and therefore tend to be hexagonal prisms.

Photosphere

The visible surface of the Sun, the photosphere, is the layer below which the Sun becomes opaque to visible light. Above the photosphere visible sunlight is free to propagate into space, and its energy escapes the Sun entirely. The change in opacity is due to the decreasing amount of H- ions, which absorb visible light easily. Conversely, the visible light we see is produced as electrons react with hydrogen atoms to produce H- ions. The photosphere is actually tens to hundreds of kilometers thick, being slightly less opaque than air on Earth. Because the upper part of the photosphere is cooler than the lower part, an image of the Sun appears brighter in the center than on the edge or limb of the solar disk, in a phenomenon known as limb darkening. Sunlight has approximately a black-body spectrum that indicates its temperature is about 6 000 K, interspersed with atomic absorption lines from the tenuous layers above the photosphere. The photosphere has a particle density of about 1023 m−3 (this is about 1% of the particle density of Earth's atmosphere at sea level).

During early studies of the optical spectrum of the photosphere, some absorption lines were found that did not correspond to any chemical elements then known on Earth. In 1868, Norman Lockyer hypothesized that these absorption lines were because of a new element which he dubbed "helium", after the Greek Sun god Helios. It was not until 25 years later that helium was isolated on Earth.


Atmosphere


The parts of the Sun above the photosphere are referred to collectively as the solar atmosphere. They can be viewed with telescopes operating across the electromagnetic spectrum, from radio through visible light to gamma rays, and comprise five principal zones: the temperature minimum, the chromosphere, the transition region, the corona, and the heliosphere. The heliosphere, which may be considered the tenuous outer atmosphere of the Sun, extends outward past the orbit of Pluto to the heliopause, where it forms a sharp shock front boundary with the interstellar medium. The chromosphere, transition region, and corona are much hotter than the surface of the Sun. The reason why has not been conclusively proven; evidence suggests that Alfvén waves may have enough energy to heat the corona.

The coolest layer of the Sun is a temperature minimum region about 500 km above the photosphere, with a temperature of about 4 000 K. This part of the Sun is cool enough to support simple molecules such as carbon monoxide and water, which can be detected by their absorption spectra.

Above the temperature minimum layer is a thin layer about 2 000 km thick,dominated by a spectrum of emission and absorption lines. It is called the chromosphere from the Greek root chroma, meaning color, because the chromosphere is visible as a colored flash at the beginning and end of total eclipses of the Sun. The temperature in the chromosphere increases gradually with altitude, ranging up to around 100 000 K near the top.Above the chromosphere is a transition region in which the temperature rises rapidly from around 100 000 K to coronal temperatures closer to one million K. The increase is because of a phase transition as helium within the region becomes fully ionized by the high temperatures. The transition region does not occur at a well-defined altitude. Rather, it forms a kind of nimbus around chromospheric features such as spicules and filaments, and is in constant, chaotic motion. The transition region is not easily visible from Earth's surface, but is readily observable from space by instruments sensitive to the far ultraviolet portion of the spectrum.

The corona is the extended outer atmosphere of the Sun, which is much larger in volume than the Sun itself. The corona merges smoothly with the solar wind that fills the Solar System and heliosphere. The low corona, which is very near the surface of the Sun, has a particle density of 1014–1016 m−3. (Earth's atmosphere near sea level has a particle density of about 2×1025 m−3.) The temperature of the corona is several million kelvin. While no complete theory yet exists to account for the temperature of the corona, at least some of its heat is known to be from magnetic reconnection.

The heliosphere extends from approximately 20 solar radii (0.1 AU) to the outer fringes of the Solar System. Its inner boundary is defined as the layer in which the flow of the solar wind becomes superalfvénic—that is, where the flow becomes faster than the speed of Alfvén waves.Turbulence and dynamic forces outside this boundary cannot affect the shape of the solar corona within, because the information can only travel at the speed of Alfvén waves. The solar wind travels outward continuously through the heliosphere, forming the solar magnetic field into a spiral shape, until it impacts the heliopause more than 50 AU from the Sun. In December 2004, the Voyager 1 probe passed through a shock front that is thought to be part of the heliopause. Both of the Voyager probes have recorded higher levels of energetic particles as they approach the boundary.

Chemical composition

The Sun is composed primarily of the chemical elements hydrogen and helium; they account for 74.9% and 23.8% of the mass of the Sun in the photosphere, respectively. All heavier elements, called metals in astronomy, account for less than 2 percent of the mass. The most abundant metals are oxygen (roughly 1% of the Sun's mass), carbon (0.3%), neon (0.2%), and iron (0.2%).

The Sun inherited its chemical composition from the interstellar medium out of which it formed: the hydrogen and helium in the Sun were produced by Big Bang nucleosynthesis. The metals were produced by stellar nucleosynthesis in generations of stars which completed their stellar evolution and returned their material to the interstellar medium prior to the formation of the Sun. The chemical composition of the photosphere is normally considered representative of the composition of the primordial Solar System. However, since the Sun formed, the helium and heavy elements have settled out of the photosphere. Therefore, the photosphere now contains slightly less helium and only 84% of the heavy elements than the protostellar Sun did; the protostellar Sun was 71.1% hydrogen, 27.4% helium, and 1.5% metals.

In the inner portions of the Sun, nuclear fusion has modified the composition by converting hydrogen into helium, so the innermost portion of the Sun is now roughly 60% helium, with the metal abundance unchanged. Because the interior of the Sun is radiative, not convective (see Structure above), none of the fusion products from the core have risen to the photosphere.

The solar heavy-element abundances described above are typically measured both using spectroscopy of the Sun's photosphere and by measuring abundances in meteorites that have never been heated to melting temperatures. These meteorites are thought to retain the composition of the protostellar Sun and thus not affected by settling of heavy elements. The two methods generally agree well.

Singly-ionised iron group elements

In 1970s, much research focused on the abundances of iron group elements in the Sun.

The first largely complete set of oscillator strengths of singly-ionised iron group elements were made available first in the 1960s, and improved oscillator strengths were computed in 1976. In 1978 the abundances of singly-ionised elements of the iron group were derived.

The abundance determination of some iron group elements is difficult because of their hyperfine structures, eg cobalt and manganese.

Solar and planetary mass fractionation relationship

Various authors have considered the existence of a mass fractionation relationship between the isotopic compositions of solar and planetary noble gases,for example correlations between isotopic compositions of planetary and solar Ne and Xe. Nevertheless, the belief that the whole Sun has the same composition as the solar atmosphere was still widespread, at least until 1983.

In 1983, it was claimed that it was the fractionation in the Sun itself that caused the fractionation relationship between the isotopic compositions of planetary and solar wind implanted noble gases.

Solar cycles

Sunspots and the sunspot cycle

When observing the Sun with appropriate filtration, the most immediately visible features are usually its sunspots, which are well-defined surface areas that appear darker than their surroundings because of lower temperatures. Sunspots are regions of intense magnetic activity where convection is inhibited by strong magnetic fields, reducing energy transport from the hot interior to the surface. The magnetic field gives rise to strong heating in the corona, forming active regions that are the source of intense solar flares and coronal mass ejections. The largest sunspots can be tens of thousands of kilometers across.The number of sunspots visible on the Sun is not constant, but varies over an 11-year cycle known as the Solar cycle. At a typical solar minimum, few sunspots are visible, and occasionally none at all can be seen. Those that do appear are at high solar latitudes. As the sunspot cycle progresses, the number of sunspots increases and they move closer to the equator of the Sun, a phenomenon described by Spörer's law. Sunspots usually exist as pairs with opposite magnetic polarity. The magnetic polarity of the leading sunspot alternates every solar cycle, so that it will be a north magnetic pole in one solar cycle and a south magnetic pole in the next.The solar cycle has a great influence on space weather, and is a significant influence on the Earth's climate. Solar activity minima tend to be correlated with colder temperatures, and longer than average solar cycles tend to be correlated with hotter temperatures. In the 17th century, the solar cycle appears to have stopped entirely for several decades; very few sunspots were observed during this period. During this era, which is known as the Maunder minimum or Little Ice Age, Europe experienced very cold temperatures. Earlier extended minima have been discovered through analysis of tree rings and also appear to have coincided with lower-than-average global temperatures.

Possible long term cycle

A recent theory claims that there are magnetic instabilities in the core of the Sun which cause fluctuations with periods of either 41 000 or 100 000 years. These could provide a better explanation of the ice ages than the Milankovitch cycles. Like many theories in astrophysics, this theory cannot be tested directly.

Theoretical problems

Solar neutrino problem

For many years the number of solar electron neutrinos detected on Earth was one third to one half of the number predicted by the standard solar model. This anomalous result was termed the solar neutrino problem. Theories proposed to resolve the problem either tried to reduce the temperature of the Sun's interior to explain the lower neutrino flux, or posited that electron neutrinos could oscillate—that is, change into undetectable tau and muon neutrinos as they traveled between the Sun and the Earth.Several neutrino observatories were built in the 1980s to measure the solar neutrino flux as accurately as possible, including the Sudbury Neutrino Observatory and Kamiokande. Results from these observatories eventually led to the discovery that neutrinos have a very small rest mass and do indeed oscillate. Moreover, in 2001 the Sudbury Neutrino Observatory was able to detect all three types of neutrinos directly, and found that the Sun's total neutrino emission rate agreed with the Standard Solar Model, although depending on the neutrino energy as few as one-third of the neutrinos seen at Earth are of the electron type. This proportion agrees with that predicted by the Mikheyev-Smirnov-Wolfenstein effect (also known as the matter effect), which describes neutrino oscillation in matter. Hence, the problem is now resolved.

Coronal heating problem

The optical surface of the Sun (the photosphere) is known to have a temperature of approximately 6 000 K. Above it lies the solar corona at a temperature of 1 000 000 K. The high temperature of the corona shows that it is heated by something other than direct heat conduction from the photosphere.

It is thought that the energy necessary to heat the corona is provided by turbulent motion in the convection zone below the photosphere, and two main mechanisms have been proposed to explain coronal heating. The first is wave heating, in which sound, gravitational and magnetohydrodynamic waves are produced by turbulence in the convection zone. These waves travel upward and dissipate in the corona, depositing their energy in the ambient gas in the form of heat. The other is magnetic heating, in which magnetic energy is continuously built up by photospheric motion and released through magnetic reconnection in the form of large solar flares and myriad similar but smaller events.

Currently, it is unclear whether waves are an efficient heating mechanism. All waves except Alfvén waves have been found to dissipate or refract before reaching the corona.In addition, Alfvén waves do not easily dissipate in the corona. Current research focus has therefore shifted towards flare heating mechanisms. One possible candidate to explain coronal heating is continuous flaring at small scales, but this remains an open topic of investigation.

Faint young Sun problem

Theoretical models of the Sun's development suggest that 3.8 to 2.5 billion years ago, during the Archean period, the Sun was only about 75% as bright as it is today. Such a weak star would not have been able to sustain liquid water on the Earth's surface, and thus life should not have been able to develop. However, the geological record demonstrates that the Earth has remained at a fairly constant temperature throughout its history, and in fact that the young Earth was somewhat warmer than it is today. The consensus among scientists is that the young Earth's atmosphere contained much larger quantities of greenhouse gases (such as carbon dioxide, methane and/or ammonia) than are present today, which trapped enough heat to compensate for the lesser amount of solar energy reaching the planet.

Magnetic field

All matter in the Sun is in the form of gas and plasma because of its high temperatures. This makes it possible for the Sun to rotate faster at its equator (about 25 days) than it does at higher latitudes (about 35 days near its poles). The differential rotation of the Sun's latitudes causes its magnetic field lines to become twisted together over time, causing magnetic field loops to erupt from the Sun's surface and trigger the formation of the Sun's dramatic sunspots and solar prominences (see magnetic reconnection). This twisting action gives rise to the solar dynamo and an 11-year solar cycle of magnetic activity as the Sun's magnetic field reverses itself about every 11 years.

The influence of the Sun's rotating magnetic field on the plasma in the interplanetary medium creates the heliospheric current sheet, which separates regions with magnetic fields pointing in different directions. The plasma in the interplanetary medium is also responsible for the strength of the Sun's magnetic field at the orbit of the Earth. If space were a vacuum, then the Sun's 10-4 tesla magnetic dipole field would reduce with the cube of the distance to about 10-11 tesla. But satellite observations show that it is about 100 times greater at around 10-9 tesla. The dipole field of the sun is roughly the same as the earth's magnetic field, but it extends over a vastly greater volume of space. Magnetohydrodynamic (MHD) theory predicts that the motion of a conducting fluid (such as the interplanetary medium) in a magnetic field induces electric currents, which in turn generate magnetic fields, and in this respect it behaves like an MHD dynamo.

solar space missions:

The first satellites designed to observe the Sun were NASA's Pioneers 5, 6, 7, 8 and 9, which were launched between 1959 and 1968. These probes orbited the Sun at a distance similar to that of the Earth, and made the first detailed measurements of the solar wind and the solar magnetic field. Pioneer 9 operated for a particularly long period of time, transmitting data until 1987.

In the 1970s, Helios 1 and the Skylab Apollo Telescope Mount provided scientists with significant new data on solar wind and the solar corona. The Helios 1 satellite was a joint U.S.-German probe that studied the solar wind from an orbit carrying the spacecraft inside Mercury's orbit at perihelion. The Skylab space station, launched by NASA in 1973, included a solar observatory module called the Apollo Telescope Mount that was operated by astronauts resident on the station. Skylab made the first time-resolved observations of the solar transition region and of ultraviolet emissions from the solar corona. Discoveries included the first observations of coronal mass ejections, then called "coronal transients", and of coronal holes, now known to be intimately associated with the solar wind.

In 1980, the Solar Maximum Mission was launched by NASA. This spacecraft was designed to observe gamma rays, X-rays and UV radiation from solar flares during a time of high solar activity. Just a few months after launch, however, an electronics failure caused the probe to go into standby mode, and it spent the next three years in this inactive state. In 1984 Space Shuttle Challenger mission STS-41C retrieved the satellite and repaired its electronics before re-releasing it into orbit. The Solar Maximum Mission subsequently acquired thousands of images of the solar corona before re-entering the Earth's atmosphere in June 1989.

Japan's Yohkoh (Sunbeam) satellite, launched in 1991, observed solar flares at X-ray wavelengths. Mission data allowed scientists to identify several different types of flares, and also demonstrated that the corona away from regions of peak activity was much more dynamic and active than had previously been supposed. Yohkoh observed an entire solar cycle but went into standby mode when an annular eclipse in 2001 caused it to lose its lock on the Sun. It was destroyed by atmospheric reentry in 2005.

One of the most important solar missions to date has been the Solar and Heliospheric Observatory, jointly built by the European Space Agency and NASA and launched on December 2, 1995. Originally a two-year mission, SOHO has now operated for over ten years (as of 2007). It has proved so useful that a follow-on mission, the Solar Dynamics Observatory, is planned for launch in 2008. Situated at the Lagrangian point between the Earth and the Sun (at which the gravitational pull from both is equal), SOHO has provided a constant view of the Sun at many wavelengths since its launch. In addition to its direct solar observation, SOHO has enabled the discovery of large numbers of comets, mostly very tiny sungrazing comets which incinerate as they pass the Sun.

All these satellites have observed the Sun from the plane of the ecliptic, and so have only observed its equatorial regions in detail. The Ulysses probe was launched in 1990 to study the Sun's polar regions. It first traveled to Jupiter, to 'slingshot' past the planet into an orbit which would take it far above the plane of the ecliptic. Serendipitously, it was well-placed to observe the collision of Comet Shoemaker-Levy 9 with Jupiter in 1994. Once Ulysses was in its scheduled orbit, it began observing the solar wind and magnetic field strength at high solar latitudes, finding that the solar wind from high latitudes was moving at about 750 km/s which was slower than expected, and that there were large magnetic waves emerging from high latitudes which scattered galactic cosmic rays.

Elemental abundances in the photosphere are well known from spectroscopic studies, but the composition of the interior of the Sun is more poorly understood. A solar wind sample return mission, Genesis, was designed to allow astronomers to directly measure the composition of solar material. Genesis returned to Earth in 2004 but was damaged by a crash landing after its parachute failed to deploy on reentry into Earth's atmosphere. Despite severe damage, some usable samples have been recovered from the spacecraft's sample return module and are undergoing analysis.

The Solar Terrestrial Relations Observatory (STEREO) mission was launched in October 2006. Two identical spacecraft were launched into orbits that cause them to (respectively) pull further ahead of and fall gradually behind the Earth. This enables stereoscopic imaging of the Sun and solar phenomena, such as coronal mass ejections.If one were to observe it from Alpha Centauri, the closest star system, the Sun would appear to be in the constellation Cassiopeia.

Sun and global warming: A cosmic connection?

In February 2007, depending on what newspaper you read, you might have seen an article detailing a "controversial new theory" of global warming.The idea was that variations in cosmic rays penetrating the Earth's atmosphere would change the amount of cloud cover, in turn changing our planet's reflectivity, and so the temperature at its surface.This, it was said, could be the reason why temperatures have been seen to be varying so much over the Earth's history, and why they are rising now.The theory was detailed in a book, The Chilling Stars, written by Danish scientist Henrik Svensmark and British science writer Nigel Calder, which appeared on the shelves a week after the Intergovernmental Panel on Climate Change (IPCC) had published its landmark report concluding it was more than 90% likely that humankind's emissions of greenhouse gases were warming the planet.In truth, the theory was not new; Dr Svensmark's team had proposed it a decade earlier, while the idea of a cosmic ray influence on weather dates back to 1959 and US researcher Edward Ney.The bigger question is whether it amounts to a theory of global warming at all.

Small change

Over the course of the Earth's history, the main factor driving changes in its climate has been that the amount of energy from the Sun varies, either because of wobbles in the Earth's orbit or because the Sun's power output changes.Most noticeably, it changes with the 11-year solar cycle, first identified in the mid-1800s by astronomers who noticed periodic variations in the number of sunspots.If it varied enough, it could change the Earth's surface temperature markedly. So is it? "Across the solar cycle, the Sun's energy output varies only by about 0.1%," says Sami Solanki from the Max-Planck Institute for Solar System Research in Germany."When you look across much longer timescales, you also see changes only of about 0.1%. So just considering directly variations in energy coming from the Sun, this is not enough to explain the climatic changes we have seen and are seeing now."This is why scientists have been investigating mechanisms which could amplify the changes in solar output, scaling up the 0.1% variation into an effect that could explain the temperature rise of almost half a degree Celsius that we have seen at the Earth's surface in just the last few decades.One is Joanna Haigh from Imperial College, London, UK. She realised that although the Sun's overall energy output changes by 0.1%, it changes much more in the ultraviolet part of the spectrum."The changes in the UV are much larger, between 1% and 10%," she says."And that primarily has an impact in the stratosphere (the upper atmosphere) - UV is absorbed by ozone in the stratosphere and also produces ozone, and this warms the air."Using computer models of climate, Dr Haigh's team showed that warming in the stratosphere could change the way energy is distributed across the troposphere, the lower atmosphere, changing wind and weather patterns. But not by much."We found it might raise temperatures by a maximum of half to one Celsius in certain regions," she says. "But in terms of an impact on the global average temperature, it's small, maybe about 0.2C."Which is not enough to explain the warming that has occurred since the late 1970s.

Crash test

Henrik Svensmark and his collaborators at the Danish National Space Center (DNSC) believe the missing link between small solar variations and large temperature changes on Earth are cosmic rays."I think the Sun is the major driver of climate change," he says, "and the reason I'm saying that is that if you look at historical temperature data and then solar activity and cosmic ray activity, it actually fits very beautifully."If CO2 is a very important climate driver then you would expect to see its effect on all timescales; and for example when you look at the last 500 million years, or the last 10,000 years, the correlation between changes in CO2 and climate are very poor."When hugely energetic galactic cosmic rays - actually particles - crash into the top of the atmosphere, they set in train a sequence of events which leads to the production of ions in the lower atmosphere.The theory is that this encourages the growth of tiny aerosol particles around which water vapour can condense, eventually aiding the formation of clouds.And the link to the Sun? It is because cosmic rays are partially deflected by the solar wind, the stream of charged particles rushing away from the Sun, and the magnetic field it carries. A weaker solar wind means more cosmic rays penetrating the atmosphere, hence more clouds and a cooler Earth.

Maximum power

The theory makes some intuitive sense because over the last century the Sun has been unusually active - which means fewer cosmic rays, and a warmer climate on Earth."We reconstructed solar activity going back 11,000 years," relates Sami Solanki."And across this period, the level of activity we are seeing now is very high - we coined the term 'grand maximum' to describe it. We still have the 11-year modulation on top of the long-term trend, but on average the Sun has been brighter and the cosmic ray flux lower."There is evidence too that cosmic rays and climate have been intertwined over timescales of millennia in the Earth's past. And the theory received some experimental backing when in October 2006, Henrik Svensmark's team published laboratory research showing that as the concentration of negative ions rose in air, so did the concentration of particles which could eventually become condensation nuclei.Other scientists, meanwhile, had started putting the idea to the test in the real world.

Seeing the light

In 1947, British meteorologists began deploying instruments in various sites across the country to measure sunlight.Whether through foresight or luck, they included one feature which was to prove very useful; the capacity to measure the relative amounts of direct and diffuse light.It is the difference between a sunny day, when light streams directly from above, and a cloudy day, when it seems to struggle in from everywhere, and photographers give up and go home.Giles Harrison from Reading University realised that the UK Met Office's record of hourly readings from its sunlight stations could be used to plot the extent of cloud cover over a period going back more than 50 years; the larger the ratio of diffuse to direct light, the cloudier the skies.

By chance, cosmic rays have been recorded continuously over almost exactly the same period. So Dr Harrison's team compared the two records, looking for a correlation between more intense cosmic rays and more clouds."We concluded that there is an effect, but that it is small - 'small but significant' was how we described it," he recalls."It varied UK cloud cover only by about 2%, although we suggested it would have a larger effect on centennial timescales; and it's difficult to assess what effect this would have on global surface temperature."He concludes it would be premature to lay global warming at the door of cosmic rays. Perhaps surprisingly, you will find no references to his work in The Chilling Stars.

Cosmic flaw

In July, Mike Lockwood from the UK's Rutherford-Appleton Laboratory attempted a definitive answer to the question with what appeared to be a simple method. He simply looked at the changing cosmic ray activity over the last 30 years, and asked whether it could explain the rising temperatures.His conclusion was that it could not. Since about 1985, he found, the cosmic ray count had been increasing, which should have led to a temperature fall if the theory is correct - instead, the Earth has been warming."This should settle the debate," he told me at the time.It has not. Last month Dr Svensmark posted a paper on the DNSC website that claimed to be a comprehensive rebuttal."The argument that Mike Lockwood put forward was that they didn't see any solar signal in the surface temperature data," he says."And when you look at [temperatures in] the troposphere or the oceans, then you do see a solar signal, it's very clear."

Dr Lockwood disagrees; he says he has re-analysed the issue using atmospheric temperatures, and his previous conclusion stands. And he thinks the Svensmark team has been guilty of poor practice by not publishing their argument in a peer-reviewed scientific journal."Lots of people have been asking me how I respond to it; but how should I respond to something which is just posted on a research institute's website?" he asks."This isn't on, because the report title says it is a 'comprehensive rebuttal'; if it were that, then it would be his duty to publish it in a scientific journal and clean up the literature - that's how science filters out what is incorrect, and how it comes to a consensus view as to what is correct."

Droplets of doubt

This dispute presumably has some distance to run.But Mike Lockwood's larger conclusion that current warming has nothing to do with solar changes is backed up by others - notably the IPCC, which concluded earlier this year that since temperatures began rising rapidly in the 1970s, the contribution of humankind's greenhouse gas emissions has outweighed that of the Sun by a factor of about 13 to one.Even though misguided journalists have sometimes mistaken his work as implying a solar cause to modern-day warming, Sami Solanki agrees with the IPCC verdict."Since 1970, the cosmic ray flux has not changed markedly while the global temperature has shown a rapid rise," he says. "And that lack of correlation is proof that the Sun doesn't cause the warming we are seeing now."Even to prove that the link between cosmic rays and cloud cover matters in the real world needs a lot more work, observes Joanna Haigh.

"You need to demonstrate a whole long chain of events - that the atmosphere is ionised, then that the ionised particles act to nucleate the condensation of water vapour, then that you form droplets, and then that you get clouds; and you have to show it's important in comparison to other sources of nucleation."And that hasn't been demonstrated. Proponents of this mechanism have tended to extrapolate their results beyond what is reasonable from the evidence."And Giles Harrison believes climate sceptics need to apply the same scepticism to the cosmic ray theory as they do to greenhouse warming - particularly those who say there are too many holes in our understanding of how clouds behave in the man-made greenhouse."There is some double-speak going on, as uncertainties apply to many aspects of clouds," he says."If clouds have to be understood better to understand greenhouse warming, then, as we have only an emerging understanding of the electrical aspects of aerosols and non-thunderstorm clouds, that is probably also true of any effect of cosmic rays on clouds."Dr Svensmark agrees it would be wrong for anyone to claim the case has been proved."If anyone said that there is proof that the Sun or greenhouse gases alone are responsible for the present-day warming, then that would be a wrong statement because we don't really have proofs as such in the natural sciences," he says.

Waned world

Two events loom on the horizon that might settle the issue once and for all; one shaped by human hands, one entirely natural.At Cern, the giant European physics facility, an experiment called Cloud is being constructed which will research the notion that cosmic rays can stimulate the formation of droplets and clouds. There may be some results within three or four years.By then, observations suggest that the Sun's output may have started to wane from its "grand maximum".If it does, and if Henrik Svensmark is right, we should then see cosmic rays increase and global temperatures start to fall; if that happens, he can expect to see a Nobel Prize and thousands of red-faced former IPCC members queuing up to hand back the one they have just received.But if the Sun wanes and temperatures on our planet continue to rise, as the vast majority of scientists in the field believe, the solar-cosmic ray concept of global warming can be laid to eternal rest.And if humankind has done nothing to stem the rise in greenhouse gas emissions by then, it will be even harder to begin the task.

Solar wind blows at 50-year low

[The Sun is a variable star; activity rises and falls in cycles]

The solar wind - the stream of charged particles billowing away from the Sun - is at its weakest for 50 years.

Scientists made the assessment after studying 18 years of data from the Ulysses satellite which has sampled the space environment all around our star.

They expect the reduced output to have effects right across the Solar System.

Indeed, one impact is to diminish slightly the influence the Sun has over its local environment which extends billions of kilometres into space.Confirmation of that prediction should come from the far-distant Voyager spacecraft which were launched in the 1970s and are now bearing down on the edge of the heliosphere - the great "bubble" of wind material that surrounds the Sun.

Scientists now predict the Voyagers will hit the edge and cross over into interstellar space - that region considered to be "between the stars" - sooner than anticipated.

Space age

The solar wind, which originates in the Sun's hot outer atmosphere known as the corona, gusts and calms with the star's familiar 11-year cycle of activity (but also over its less well known longer cycles, too).

Calmer wind conditions would be expected to prevail right now, but the Ulysses data indicates circumstances unprecedented in recent times.

"This is a whole Sun phenomenon," said Dave McComas, Ulysses solar wind instrument principal investigator, from Southwest Research Institute, San Antonio, US.

"The entire Sun is blowing significantly less hard - about 20-25% less hard - than it was during the last solar minimum 10-15 years ago.

"That's a very significant change. In fact, the solar wind we're seeing now is blowing the least hard we've see it for a prolonged time, since the start of those observations in the 1960s at the start of the space age."

In addition to being calmer, the wind measured at Ulysses is 13% cooler.

However, judging from Sun activity data collected by non-satellite methods over the past 200 years, the current behaviour is thought to be well within the long-term norm.

Nonetheless, scientists expect the weakened wind to have a wide range of impacts.

Energetic rays

The charged wind particles also carry with them the Sun's magnetic field, and this has a protective role in limiting the number of high-energy cosmic rays that can enter the Solar System.

More of them will probably now make their way through.

Many of these rays, which include electrons and atomic nuclei, originate in exploding stars and at black holes, and move at colossal speeds.

They pose no major risk to people on Earth because our atmosphere also works to reduce their intensity; but they are a consideration for space operations.

The rays can damage satellite electronics, and if current solar wind conditions persist, engineers would have to take this into account when deciding how to "harden" their spacecraft. Astronauts, too, are at risk from the higher doses of radiation associated with cosmic rays.

"The Sun also puts out cosmic rays in the form of bursts and these bursts are much less frequent at solar minimum. However, when they do occur at solar minimum, they are more lethal, so this is not a good time to be travelling in space owning to both kinds of cosmic rays," explained Professor Nancy Crooker, from Boston University, Massachusetts, US.

"Reduced solar activity also leads to the cooling of Earth's upper atmosphere and if Earth's upper atmosphere is cooler then there is less drag up there on satellites and this means we are left with much more debris up there - which is also something astronauts have to look out for."

Some researchers have attempted to link the intensity of cosmic rays at Earth to cloudiness and climate change. Current conditions may be a good opportunity to test these ideas further.

The Ulysses mission is a co-operative venture between the US space agency and the European Space Agency (Esa). Launched by the shuttle in 1990, it was the first satellite to study the space environment above and below the Sun's poles.

It samples the solar wind and solar magnetic field as it circles the star in a six-year orbit that also carries it out to Jupiter and back.

But the harsh conditions of space are now slowly taking their toll on the spacecraft.

Ulysses' main transmitter no longer works and it is struggling to put enough power into its heating systems. With the satellite currently moving away from the Sun, it is gradually getting colder; and engineers expect the hydrazine fuel used in its thrusters to freeze very soon.

When this happens, Ulysses will no longer be able to orientate itself and its antenna, and contact will be lost with Earth.

"Even though the end is now in sight, every day's worth of new data is adding to our knowledge of the Sun and its environment; and it's been a great and exciting mission," said Richard Marsden, Esa's Ulysses project scientist and mission manager.