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Today's science of the sun.


The past several years have seen big changes in our understanding of the Sun. Powerful solar observatories in space such as Yohkoh, the Solar and Heliospheric Observatory, (SOHO), and the Transition Region and Coronal Explorer (TRACE), along with new techniques in helioseismology, are giving astronomers the best-ever views of our home star. Last month Part 1 of this two-part article focused on the Sun's internal processes and the importance of the magnetic field pervading the solar interior. This second part focuses on the Sun's surface and extended atmosphere.

Pivotal to our understanding of the Sun is its magnetic field. The complex, ever-changing field influences nearly all solar processes, from sunspot formation to temperature, to the solar-wind outflow.

The gas that makes up the Sun and its atmosphere is hot enough to be significantly ionized, meaning that many atoms have been stripped of at least one electron. The gas thereby becomes an electrical conductor, which ties it together with the magnetic field--when one moves, it drags the other with it wherever the field is strong enough. Only by closely observing both the magnetic field and the gas motions can we begin to fully comprehend the dynamics of our star.

The Photosphere

When a strand of the Sun's magnetic field rises from the interior and breaks through the Sun's visible surface (the photosphere), it forms a pair of adjacent patches having opposite magnetic polarity. These patches are connected by great arches of magnetic-field lines that rise high into the Sun's outer atmosphere. The surface patches, or active regions, are often marked by dark sunspots, tiny dark pores, and an abundance of bright, compact features known as faculae.

Inside any dense bundle of magnetic-field lines, the solar gas is more tenuous than in the surrounding, relatively nonmagnetic area. As a consequence, magnetic regions in the depths of the Sun are buoyant. They straighten themselves to stand nearly vertically, with one end anchored to the field's deep source. The other end floats up to the surface amid the turbulent, convecting gas, and shreds of it ultimately find themselves carried to one of the many narrow downflow channels arranged around the broad, central upwellings.

Within hours to days of its emergence, the field begins to disperse across the solar surface in what appears to be an unconstrained random walk. This dispersal can be modeled by a diffusive process driven both by convective motions and by large-scale flows--such as the Sun's differential rotation and the slow, poleward "meridional" circulation. This model is one of the major successes in the study of solar activity; it explains a variety of phenomena and has been used to predict the long-term, large-scale behavior of the Sun's field. But why it works so well is still a puzzle--models are only beginning to explain how the connection to the deep field is severed or seriously weakened.

On a much smaller scale, magnetic fields are subject to fragmentations and mergers. Fields disappear from the surface in collisions with fields of opposite polarity and are most likely subducted back into the Sun. Meanwhile, new fields emerge from below, most frequently in small-scale bipolar regions. The result is a quasi-steady state in which the details constantly change but the general picture remains the same--it is a balance in which about as much field emerges as disappears.

In regions on the solar surface showing this mildly fluctuating background, the time scale for complete field replacement is only two days. For areas covered with the decay products of large bipolar regions, the time scale is longer, around 10 days. So, although some large field patterns are known to survive up to a year or more, any given field line is severed and reconnected near the surface surprisingly quickly.

Whether similar processes occur below the surface remains to be seen. If they do, this may explain the apparently unconstrained random walk of the surface field--the frequent cutting and reconnecting would look the same as if the field had no connection to longer-lasting structures below.

The surface magnetic fields hamper surface convection and, therefore, the transport of energy to the top. This is why sunspots are cooler and darker than their surroundings: they are pervaded by strong magnetic fields that suppress convection almost entirely. Energy radiated away from the surface cannot be replaced fast enough to keep the temperature as high as in the surroundings (see the article on starspots on page 42). In addition, sunspots appear to be depressed a little below the surrounding solar surface, so that spots seen near the Sun's limb appear dish-shaped--the "Wilson effect" long known to telescopic observers. We see this effect because the gas in a spot's intense field is more tenuous than the gas outside. Therefore the gas is more transparent, which allows us to look deeper.

Where a field is too weak to make a true sunspot, the slight dimming that it causes is overcompensated by light coming into the depression sideways from hot layers just beneath the outer surface that are exposed in the depression's wall. If the region is small enough, light from the bright wall scatters into the interior of the magnetic-field region, causing it to appear bright. These regions are the faculae.

For the Sun as a whole, the bright faculae overcompensate for the darkness of the spots and pores. So the Sun is actually slightly brighter when it's heavily spotted. Thus we expect the Sun to be dimmer when it is inactive for a long period. The so-called Maunder Minimum lasting from about 1645 to 1715, when the Sun remained nearly spotless, was such a period. This coincided with "the Little Ice Age," during which European summers were cool and winters severe. In fact, there is evidence that similar correlations occurred earlier. For example, an extended period of unusually strong activity, the Grand Maximum, corresponded to warm weather in the 12th century. These are strong indications that solar magnetic weather affects Earth's climate. However, the Earth's atmospheric system is complex, and the effects of society and Sun are difficult to disentangle.

Additionally, different wavelengths of solar radiation affect different parts of Earth's atmosphere. X-rays and ultraviolet radiation ionize the outermost parts of our atmosphere to form the ionosphere. Visible light affects the troposphere, hugging the Earth's surface where clouds and weather form. Accurate spectroscopy, detailed comparisons of irradiance with magnetograms, and the application of models such as the aforementioned field-dispersal model should allow us to recreate the solar magnetic field and the solar irradiance over time and compare it with historical weather records. Once this work is completed, we can unambiguously establish how strongly solar activity affects life on Earth.

The Solar Atmosphere

The magnetic field extends from the solar surface throughout interplanetary space. At a height of just a few thousand kilometers, less than 1 percent of the solar radius, the field causes the Sun's tenuous outer atmosphere to be heated to much more than 1 million degrees Kelvin --hundreds of times hotter than the surface. Exactly how this heating occurs is still not known, but various candidate mechanisms are under investigation. We know that drastically lower surface heat alone cannot cause such a hot atmosphere. Instead, it appears that electric currents and explosive shock waves are involved, each acting in different parts of the atmosphere.

On the Sun, as on Earth, pressure decreases with height--it's why breathing is harder on mountaintops than at sea level. But in especially hot areas, pressure decreases much more slowly with height. Thus the density contrast between hotter and cooler regions can become substantial. At any given height, hotter parts are generally denser and glow brighter. Therefore, the heat that the magnetic field deposits in the atmosphere causes the magnetic areas to stand out brightly when observed at the appropriate wavelengths.

The Sun's atmospheric heating fluctuates markedly both in time and place. Furthermore, a property of hot, ionized gas is that it can move only along, not perpendicular to, a strong magnetic field--as if the atoms of gas were strung on the magnetic field lines like beads on a wire. Consequently hot, dense domains can persist side by side with cooler, more tenuous regions down to below the 700-kilometer resolution of the best solar telescopes.

Surprisingly, recent high-resolution observations reveal that on scales of approximately 1,000 km, the field lines along which the 3-million-degree corona is heated are not necessarily the same lines as those heated lower down: the bases of hot coronal loops can stand on areas that are hardly heated at all, and relatively cold loops can come down on strongly heated regions. But averaged over time, heating is most intense where the field is strongest anywhere in the atmosphere.

The hottest part of the outer atmosphere, the corona, radiates at extreme-ultraviolet and X-ray wavelengths. Space-based telescopes, such as the Soft X-ray Telescope on the Yohkoh satellite, the Extreme-ultraviolet Imaging Telescope (EIT) on the Solar and Heliospheric Observatory (SOHO), and the telescope on the Transition Region and Coronal Explorer (TRACE), have revealed the intricately structured and remarkably dynamic nature of this domain.

As expected for ionized gas, the coronal structures are entirely dominated by the magnetic field, with bright loops outlining the field lines over bipolar surface regions. But the intrinsic fickleness of the heating results in rapid changes. The loops seen in TRACE images typically live for only a few hours, while new loops constantly appear, as density and temperature evolve in response to the changing heating.

A temperature map of the corona over a bipolar region is remarkably intricate. One newly discovered pattern shows a trend with loop height. While the low-lying, short loops show up in soft X-rays at 3 to 5 million degrees, the outer, higher-arching loops tend to be cooler, emitting mostly in the extreme ultraviolet at 1 to 2 million degrees Kelvin.

Between such quiescent loops, others fleetingly expand the temperature range. Above a flare, for example, the temperature can reach tens of millions of degrees K. Whenever the heating is interrupted --which happens frequently--the temperature drops all the way to a few tens of thousands of degrees K and the gas, no longer supported by its own pressure, rains back onto the solar surface. Filaments, surges, and fibrils are transient phenomena that temporarily throw dense, cool material up into the hot corona. We see these as dark incursions against the brighter background.

One of the most surprising properties of the tangled coronal magnetic field is that field lines readily connect with their neighbors having the opposite magnetic polarity. Within hours or days, field lines that initially were connected to one region on the surface will connect to other neighboring regions or even to more distant counterparts. This is expected for a field in vacuum but not for the solar corona, where electric currents should inhibit such changes. The traditional theory states that virtually all observable field changes in the corona should occur because of field distortions and almost never because of reconnections. The observations tell us otherwise.

In recent years, solar physicists have substantially revised their understanding of magnetic reconnections in the corona. This was brought about by two developments: new visualization techniques to better understand the geometry of three-dimensional magnetic fields, and increased computing capability to show that the coronal field rapidly becomes turbulent in response to driving by chaotic convection at the solar surface. Large-scale developments spawn small-scale, intense electric currents that essentially short-circuit a coronal field. These currents rapidly dissipate their energy as heat--the cause of solar flares--and as the currents fade, the field reconnects.

At the surface, the bases of the magnetic-field bundles move about in the granular convection with a scale of approximately 1,000 km. This movement causes braiding of the field lines higher up in the corona, which would lead to a confused tangle if there were little reconnection to straighten things out. Computer experiments suggest that braiding field lines, on average, wrap around each other only once. Further braiding is inhibited by electrical dissipation and magnetic reconnection. The simulations show how reconnection can occur much more easily than previously thought. With new data from space-based telescopes, the results from the computer experiments can be tested observationally for the first time; the resolution of TRACE is slightly smaller than the 1,000-km scale of the driver of the braiding. TRACE observations show no significant wrapping of field lines beyond one-half to one full turn at most, consistent with the computer models.

Another interesting phenomenon relevant to reconnections was recently discovered. Sometimes coronal loops oscillate like plucked guitar strings, in response to explosive flares at the surface. These oscillations are very rapidly damped. They die out so fast that if the damping indeed happens in the corona rather than lower down, either the corona's viscosity or its electrical resistance must be a billion times larger than expected. That would be a huge surprise, but if correct, it would explain the efficient dissipation of electric currents, and therefore field reconnection.

The electrical dissipation associated with reconnection is likely responsible for much of the heating of the solar corona. But a different energy source, strong waves physically shaking the magnetized plasma, may play an important role as well. We still don't know which process dominates.

Even the answer to the simpler question of where the coronal heating primarily occurs remains elusive, but progress has been made (January issue, page 28). The high thermal conductivity of the hot coronal gas, solar astronomers have believed, meant that energy deposited anywhere along a coronal loop quickly spreads along its entire length, obscuring its point of origin. But TRACE and SOHO's EIT are better suited to find the source of the heating than earlier imaging instruments; they have found that even though conduction is indeed very efficient, it leaves a telltale signature of the heating. Apparently, heating must occur primarily in the same place where most of the heat is observed to radiate away--in the lowest 10,000 to 20,000 km of the corona, or within about 2 percent of the solar radius from the surface.

Recent data have unveiled other surprises. One is that the chemical composition of the solar corona differs from that of the solar interior. Iron, silicon, and magnesium are four times more abundant in the corona than on the solar surface. On the other hand, carbon, nitrogen, and oxygen have the normal solar abundance. What these atoms have in common is the temperature at which their first electron is removed--either above or below about 10,000[degrees] K. We are therefore looking for an explanation for the chemical oddity in a region around that temperature--between 3,000 and 6,000 km above the surface. This, along with the discovery that the coronal heating originates low in the photosphere, implies that the lower part of the corona is where most of the action is.

Other surprises raise more fundamental problems. We generally assume that the temperature at any given place and time applies to all the material there. Not so in the corona. Whereas coronal electrons and ions have the same temperature in gas of 1.3 to 3 million degrees Kelvin, this is not true for temperatures around a half million degrees K, where the ions can be up to three times hotter than the electrons. It is surprising that electrons and ions in the same place would have different temperatures, and we are not sure what that difference is telling us; perhaps the heating is changing so rapidly that the ionized gas never quite catches up by reaching the same state for all its components.

Higher in the corona, the concept of a single temperature breaks down even further. It seems that temperature there has a direction, being different when measured along a magnetic field rather than perpendicular to it. This tells us that some of the complicated processes in magnetized, ionized gas are selective. This information, we hope, will lead us to the nature of the coronal heating.

Living with a Star

The Sun's magnetic activity affects Earth in many ways. This is not surprising, because the Earth is literally embedded in the solar atmosphere--movies from SOHO show that the corona is continuously spewing out material together with embedded magnetic field. That field interacts with the Earth's field to produce sometimes damaging effects, such as geomagnetic storms. The imaging capabilities of Yohkoh, SOHO, and TRACE, combined with experiments that measure solar-wind particles streaming past these satellites, form an impressive array of tools with which we have made much headway.

For the future, NASA's Sun-Earth Connection program envisions a fleet of dedicated instruments and a complementary analysis and modeling program to learn as much as we can about "living with a star." By observing the Sun, Earth, and the heliosphere in between, these instruments will improve on even the best images in spatial resolution, time resolution, and field of view. In addition, future missions will rapidly accelerate our understanding of the Sun's magnetic activity and its impact on humanity.

Some Web Pages on the Sun and Solar Physics NASA's Office of Space Sciences, Sun-Earth Connections: a strategic plan for understanding the Sun and its influence on Earth and other bodies in the solar system. NASA/Goddard Space Flight Center home pages for the "Living with a Star" initiative. Links to solar-physics sites, ranging from current images to educational sites. The Solar and Heliospheric Observatory (SOHO). The Transition Region and Coronal Explorer (TRACE), with links to images and movies. Yohkoh's Soft X-ray Telescope. homepage/coolstarimages.html Many illustrations related to the Sun, solar activity, and solar-terrestrial effects.

CAROLUS SCHRIJVER is a staff physicist at the Lockheed Martin Advanced Technology Center, where he studies the Sun's corona with the TRACE spacecraft and is helping to develop NASA's new Living with a Star initiative. ALAN TITLE is a solar astrophysicist at the Lockheed Martin Solar and Astrophysics Laboratory, codirector of the Stanford Lockheed Institute for Solar Research, and principal investigator for the TRACE mission.
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Title Annotation:part 2
Author:Schrijver, Carolus J.; Title, Alan M.
Publication:Sky & Telescope
Date:Mar 1, 2001
Previous Article:Mission update.
Next Article:Starspots.

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