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Solar Storms

TRACE satellite image of the Sun

The Sun is considered a typical star. Its spectral classification is G2 V. G2 basically means it's a yellow-white star, and the roman numeral V means it's a main sequence dwarf star, the most common type, as opposed to supergiant or sub-dwarf. The sun dominates the gravitational field of the solar system and contains about 99.85% of the solar system's mass.

The Sun's axis is tilted about 7.25 degrees to the axis of the Earth's orbit, so we see a little more of the sun's northern polar region each September and more of its southern region in March. The sun rotates once on its axis within a period of approximately 28 days at its equator. Because the sun is a gaseous body, not all its material rotates together. Solar matter at very high latitudes takes over 30 days to complete a rotation while matter near the equator goes around in about 24 days. The Suns output varies slightly over an 11-year cycle, during which the number of sunspots changes. We are currently in Solar Max, the middle of the cycle.

The Sun serves an important role in helping us to understand the rest of the astronomical universe. It is the only star close enough to us to reveal details about its surface. Without the Sun we would not have easily guessed that other stars also have spots and hot outer atmospheres. The Sun is the key to understanding other stars. We know the Sun's age, radius, mass, and luminosity (brightness) and we have also learned detailed information about its interior and atmosphere. This information is crucial for our understanding of other stars and how they evolve. Many physical processes that occur elsewhere in the universe can be examined in detail on the Sun. In this way solar astronomy teaches us much about stars, planetary systems, galaxies, and the universe itself.

The Sun produces its energy by nuclear fusion - four hydrogen nuclei are fused to form single helium nuclei deep within the Sun's core. Scientists have worked for decades to reproduce this process (in a controlled manner) here on Earth. Most of these efforts involve extremely hot plasmas in strong magnetic fields. Plasma a mixture of ions and electrons produced at high temperatures. Much of solar astronomy involves observing and understanding plasmas under similar conditions. There continues to be much interaction between solar astronomers and scientific researchers in this and many other areas.

The solar interior is separated into four regions by the different processes that occur there. Energy is generated in the core. This energy diffuses outward by radiation (mostly gamma-rays and x-rays) through the radiative zone and by convective fluid flows (boiling motion) through the outermost convection zone. The thin interface layer between the radiative zone and the convection zone is where the Sun's magnetic field is thought to be generated.

X-ray image of the sun

The Core

The Sun's core is the central region where nuclear reactions consume hydrogen to form helium. These reactions release the energy that ultimately leaves the surface as visible light. These reactions are highly sensitive to temperature and density. The individual hydrogen nuclei must collide with enough energy to give a reasonable probability of overcoming the repulsive electrical force between these two positively charged particles. The temperature at the very center of the Sun is about 15,000,000° C (27,000,000 ° F) and the density is about 150 g/cm³ (about 10 times the density of gold or lead). Both the temperature and the density decrease as one moves outward from the center of the Sun. The nuclear burning is almost completely shut off beyond the outer edge of the core (about 25% of the distance to the surface or 175,000 km from the center). At that point the temperature is only half its central value and the density drops to about 20 g/cm³.

In the process of fusing hydrogen to form helium, the nuclear reactions also produce elementary particles called neutrinos. These elusive particles pass right through the overlying layers of the Sun and, with some effort, can be detected here on Earth. The number of neutrinos we detect is but a fraction of the number we expect. This problem of the missing neutrinos is one of the great mysteries of solar astronomy.

The Radiative Zone

The radiative zone extends outward from the outer edge of the core to the interface layer at the base of the convection zone (from 25% of the distance to the surface to 70% of that distance). The radiative zone is characterized by the method of energy transport - radiation. The energy generated in the core is carried by light (photons) that bounces from particle to particle through the radiative zone. Although the photons travel at the speed of light, they bounce so many times through this dense material that an individual photon takes about a million years to finally reach the interface layer. The density drops from 20 g/cm³ (about the density of gold) down to only 0.2 g/cm³ (less than the density of water) from the bottom to the top of the radiative zone. The temperature falls from 7,000,000° C to about 2,000,000° C over the same distance.

The Interface Layer

The interface layer lies between the radiative zone and the convective zone. The fluid motions found in the convection zone slowly disappear from the top of this layer to its bottom where the conditions match those of the calm radiative zone. This thin layer has become more interesting in recent years as more details have been discovered about it. It is now believed that the Sun's magnetic field is generated by a magnetic dynamo in this layer. The changes in fluid flow velocities across the layer (shear flows) can stretch magnetic field lines of force and make them stronger. There also appears to be sudden changes in chemical composition across this layer.

The Convection Zone

The convection zone is the outer-most layer. It extends from a depth of about 200,000 km right up to the visible surface. At the base of the convection zone the temperature is about 2,000,000° C. This is "cool" enough for the heavier ions (such as carbon, nitrogen, oxygen, calcium, and iron) to hold onto some of their electrons. This makes the material more opaque so that it is harder for radiation to get through. This traps heat that ultimately makes the fluid unstable and it starts to "boil" or convect. These convective motions carry heat quite rapidly to the surface. The fluid expands and cools as it rises. At the visible surface the temperature has dropped to 5,700° K and the density is only 0.0000002 gm/cm³ (about 1/10,000th the density of air at sea level). The convective motions themselves are visible at the surface as granules and supergranules.

X-ray image of the sun

Coronal mass ejections (or CMEs) are huge bubbles of gas threaded with magnetic field lines that are ejected from the Sun over the course of several hours. Although the Sun's corona has been observed during total eclipses of the Sun for thousands of years, the existence of coronal mass ejections was unrealized until the space age. The earliest evidence of these dynamical events came from observations made with a coronagraph on the 7th Orbiting Solar Observatory (OSO 7) from 1971 to 1973. A coronagraph produces an artificial eclipse of the Sun by placing an "occulting disk" over the image of the Sun. During a natural eclipse of the Sun the corona is only visible for a few minutes at most, too short a period of time to notice any changes in coronal features. With ground based coronagraphs only the innermost corona is visible above the brightness of the sky. From space the corona is visible out to large distances from the Sun and can be viewed continuously.

Coronal mass ejections are often associated with solar flares and prominence eruptions but they can also occur in the absence of either of these processes. The frequency of CMEs varies with the sunspot cycle. At solar minimum we observe about one CME a week. Near solar maximum we observe an average of 2 to 3 CMEs per day. Coronal Mass Ejections disrupt the flow of the solar wind and produce disturbances that strike the Earth with sometimes catastrophic results.

Solar flares are tremendous explosions on the surface of the Sun. In a matter of just a few minutes they heat material to many millions of degrees and release as much energy as a billion megatons of TNT. They occur near sunspots, usually along the dividing line (neutral line) between areas of oppositely directed magnetic fields. Flares release energy in many forms - electro-magnetic (Gamma rays and X-rays), energetic particles (protons and electrons), and mass flows. Flares are characterized by their brightness in X-rays (X-Ray flux). The biggest flares are X-Class flares. M-Class flares have a tenth the energy and C-Class flares have a tenth of the X-ray flux seen in M-Class flares.

Magnetism, or magnetic field, is produced on the Sun by the flow of electrically charged ions and electrons. Sunspots are places where very intense magnetic lines of force break through the Sun's surface. The sunspot cycle results from the recycling of magnetic fields by the flow of material in the interior. The prominences seen floating above the surface of the Sun are supported, and threaded through, with magnetic fields. The streamers and loops seen in the corona are shaped by magnetic fields. Magnetic fields are at the root of virtually all of the features we see on and above the Sun.

The key to understanding and predicting solar flares is the structure of the magnetic field around sunspots. If this structure becomes twisted and sheared then magnetic field lines can cross and reconnect with the explosive release of energy. In the image to the left the blue lines represent the neutral lines between areas of oppositely directed magnetic fields. Normally the magnetic field would loop directly across these lines from positive (outward pointing magnetic field) to negative (inward pointing magnetic field ) regions. The small line segments show the strength and direction of the magnetic field measured with the MSFC Vector Magnetograph. These lines and line segments overlie an image of a group of sunspots with a flaring region. The flare (the bright area) lies along a section of a neutral line where the magnetic field is twisted (or sheared) to point along the neutral line instead of across it. Scientists have found that this shear is a key ingredient in the production of solar flares.

Coronal loops

Magnetic forces change the direction of motion of moving charged particles like electrons. Because of this, electrons that orbit around a nucleus in one direction will have more energy than electrons that orbit about the nucleus in the opposite direction. This allows us to remotely measure the Sun's magnetic field by observing the difference in the energy of the light emitted as these electrons jump from orbit to orbit. With the proper instrumentation we can determine both the strength and the direction of the magnetic field all across the surface of the Sun.

A better understanding of the Sun's magnetic field and its behavior will allow us to make better predictions of space weather. Observations of magnetic fields associated with solar flares show that flares are likely to occur when the magnetic field lines linking two sunspots become sheared or twisted. Observations of the Sun's magnetic field over the last 20 years illustrates its behavior over two sunspot cycles. However, predicting long-range behavior, such as the size of the sunspot cycle, is still based on observing trends and patterns. We hope that in the near future we will understand the Sun well enough to make these predictions based on current conditions and past history using a mathematical model of the actual processes.

Magnetic field lines loop through the solar atmosphere and interior to form a complicated web of magnetic structures. Many of these structures are visible in the chromosphere and corona, the outermost layers of the Sun's atmosphere. However, we usually measure the magnetic field itself in the photosphere, the innermost layer of the Sun's atmosphere. Techniques can be used to mathematically map these magnetic field lines into the outer layers where they can be compared with the observed structures.

Coronal loops are found around sunspots and in active regions. These structures are associated with the closed magnetic field lines that connect magnetic regions on the solar surface. Many coronal loops last for days or weeks. Some loops, however, are associated with solar flares and are visible for much shorter periods. These loops contain denser material than their surroundings. The three-dimensional structure and the dynamics of these loops is an area of active research. Within the magnetic confines of these loops the material is somewhat isolated from the million degree corona and can cool to much lower temperatures. These particular loops are of interest because they include a set of "bent-over" loops that figure prominently in theoretical models for some flares.

The velocity of the material flowing in these loops can be determined using the Doppler Effect. The light from material moving toward us is shifted toward the blue end of the spectrum while light from material moving away from us is shifted toward the red end. Regardless of the frequency of a source of electromagnetic waves, they are subject to the Doppler effect. The Doppler effect causes the observed frequency of a source to differ from the radiated frequency of the source if there is motion that is increasing or decreasing the distance between the source and the observer. The same effect is readily observable as variation in the pitch of sound between a moving source and a stationary observer.

Coronal holes, or Sun Spots, are regions where the corona is dark. These features were discovered when X-ray telescopes were first flown above the earth's atmosphere to reveal the structure of the corona across the solar disc. Coronal holes are associated with open magnetic field lines and are often found at the Sun's poles. The high-speed solar wind is known to originate in coronal holes.

Sunspots and loop

The solar wind is a flow of gases from the Sun that streams past the Earth at speeds of more than 500 km per second (a million miles per hour). Disturbances in the solar wind shake the Earth's magnetic field and pump energy into the radiation belts. Regions on the surface of the Sun often flare and give off ultraviolet light and x-rays that heat up the Earth's upper atmosphere. This Space Weather can change the orbits of satellites and shorten mission lifetimes. The excess radiation can physically damage satellites and pose a threat to astronauts. Shaking the Earth's magnetic field can also cause current surges in power lines that destroy equipment and knock out power over large areas. As we become more dependent upon satellites in space we will increasingly feel the effects of space weather and need to predict it.

The chemical composition of the solar wind has several interesting aspects that hint at physical processes occuring in the solar wind source regions. The solar wind composition is different from the composition of the solar surface and shows variations that are associated with solar activity and solar features. The solar wind is not uniform. Although it is always directed away from the Sun, it changes speed and carries with it magnetic clouds, interacting regions where high speed wind catches up with slow speed wind, and composition variations. Magnetic Clouds are produced in the solar wind when solar eruptions (flares and coronal mass ejections) carry material off of the Sun along with embedded magnetic fields. These magnetic clouds can be detected in the solar wind through observations of the solar wind characteristics - wind speed, density, and magnetic field strength and direction.

The temperature of the corona gases are so high that the Sun's gravity cannot hold on to it. Although we understand why this happens scientists do not understand the details about how and where the coronal gases are accelerated to these high velocities. This question is related to the question of coronal heating.The solar wind speed is high (800 km/s) over coronal holes and low (300 km/s) over streamers. These high and low speed streams interact with each other and alternately pass by the Earth as the Sun rotates. These wind speed variations buffet the Earth's magnetic field and can produce storms in the Earth's magnetosphere.

The solar wind inflates a bubble, called the heliosphere, in the surrounding interstellar medium (ISM).The boundary at which the solar wind meets the ISM, containing the collective solar wind from other local stars in our galaxy, is called the heliopause. This is where the solar wind and the sun's magnetic field stop. The boundary is theorized to be roughly teardrop-shaped, because it gets "blown back" to form a heliotail, as the sun moves through the ISM. The sun's relative motion may also create an advance bow shock, analogous to that of a moving boat. This is a matter of debate and depends partly on the strength of the interstellar magnetic field. Before it gets out to the heliopause, the solar wind is thought to slow to subsonic speeds, creating a termination shock. Its actual shape, whether roughly spherical or teardrop, depends on magnetic field strengths, as yet unknown. The solar wind changes with the 11-year solar cycle, and the interstellar medium is not homogeneous, so the shape and size of the heliosphere probably fluctuate.

The solar magnetic field is the dominating magnetic field within the heliosphere, except in the immediate environment of planets which have their own magnetic fields. It can be measured by spacecraft throughout the solar system, but not here on earth, where we are shielded by our planet's own magnetic field. The actual properties of the interstellar medium (outside the heliosphere), including the strength and orientation of its magnetic field, are important in determining the size and shape of the heliopause.

The Advanced Composition Explorer (ACE) satellite was launched in August of 1997 and placed into an orbit about the L1 point between the Earth and the Sun. The L1 point is one of several points in space where the gravitational attraction of the Sun and Earth are equal and opposite. This particular point is located about 1.5 million km (1 million miles) from the Earth in the direction of the Sun. ACE has a number of instruments that monitor the solar wind and the spacecraft team provides real-time information on solar wind conditions at the spacecraft.

Images and data courtesy of NASA and the TRACE and ACE satellites