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Yohkoh and the mysterious solar flares.

A satellite's X-ray eyes provide remarkable view of violent eruptions on the Sun.

EVERY SO OFTEN a space mission delivers images so beautiful that they jolt our imagination. The Yohkoh (Japanese for "sunbeam") spacecraft does just that. Now two-thirds of the way through its planned three-year mission, this Japanese-American-British venture is providing new and dazzling X-ray views of the Sun's seething corona, or atmosphere, and the solar flares that flash through it.

Since 1859, when English amateur Richard Carrington discovered flares visually (they are rarely seen in white light), physicists have tried to figure out exactly what causes these sudden brightenings. The first X-ray studies were carried out with sounding rockets in 1949, when it was discovered that the Sun is a bright emitter at these wavelengths. In subsequent years X-ray telescopes aboard such satellites as the Orbiting Solar Observatory series, Skylab, Hinotori, and the Solar Maximum Mission have given researchers a much better understanding of the corona and, in particular, solar flares.

The rarefied outer atmosphere of the Sun ranges in temperature from 1 million to 5 million degrees Kelvin and has a very low particle density, about that of a high-quality laboratory vacuum on Earth. The temperatures and densities associated with solar flares exceed those of their surroundings by 30 and 1,000 times, respectively. At temperatures above a million degrees copious X-rays are produced, and solar flares are frequently and conspicuously recorded at these wavelengths. Thus, to understand what happens when a flare occurs, scientists must look at them with spacecraft like Yohkoh.

Yohkoh's tantalizing views of the corona and pinpoint examinations of flares are delivered from a spacecraft that is only the size of an office desk. Its instruments probe violent solar activity that produces both soft (low-energy) and hard (high-energy) X-rays as well as gamma rays.

Millions of vivid images from Yohkoh provide compelling demonstrations of astrophysical processes that are hard to reproduce in Earthbound laboratories. Within months of its August 1991 launch, Yohkoh's X-ray images of the Sun -- the first taken from orbit since the Skylab missions in 1973 -- were already meeting the highest expectations of the experimenters. Last December the Yohkoh science teams published 30 papers detailing their initial mission results. These focused on everything from the spacecraft's performance, to new looks at the role "transient brightenings" play in heating the corona, to in-depth studies of individual flares.

Previous X-ray missions clearly indicated that solar physicists needed better angular resolution to advance their understanding of flares, especially the mechanisms that release their tremendous energies. This knowledge is important because the same physical processes take place in solar flares as in controlled nuclear fusion, which may be a vital, future energy source for our planet. The Sun provides a remarkably versatile laboratory where theories can be tested with different variables and at scales that are not possible on Earth.

Solar research also attempts to predict when flares will occur, which for Earth-dwellers is important for a wide variety of reasons. For example, flares can emit enormous quantities of ionizing radiation and energetic particles that may disrupt shortwave radio communication and cause storms in Earth's magnetic field. Often we can see the effects of flares manifested in auroral displays. Auroras look beautiful and delicate, but the dark side of large solar flares is that they can cause severe damage to ground-based facilities and high-flying aircraft.

In 1982 a solar flare knocked out part of the Canadian power grid. Effects of solar-flare radiation can corrode pipelines and injure humans who fly polar routes outside the protection of Earth's magnetic field. If planners of long-term space missions don't take flares into account when designing spacecraft, people could be exposed to lethal radiation.

A GLIMPSE AT SOLAR FLARES

Yohkoh's primary objective is to give much more detailed information on the location and structure of flares as well as the "pace" of high-energy solar phenomena on both local and global scales. The spacecraft's instruments detail the evolution of soft and hard X-ray emission from flares and reveal how energy release and particle acceleration take place.

Flares occur within active regions of the solar atmosphere surrounding sunspots. The latter are cool depressions in the solar photosphere, the "surface" of the Sun. Sunspot formation is closely related to the magnetic fields that loop across the face of our star.

In an active region each loop is anchored at a so-called footpoint. The magnetic fields arch up from a footpoint of one polarity and then descend to another of opposite polarity -- sometimes tens of thousands of kilometers away. Ongoing Yohkoh observations are focusing on these footpoints and associated X-ray phenomena that appear just before and after a flare.

In a flare hot coronal gases are "bottled" within the loops, creating giant magnetic flux tubes. Visible in X-rays, these are the dominant structures not only in the flares themselves but in the surrounding corona. Magnetic-field lines are buffeted constantly by rotational and convective forces in the photosphere and below.

The easiest way to visualize this buffeting is to twist a rubber band. The more you twist, the more kinks and bends you create. When the magnetic-field "rubber bands" on the Sun are buffeted, so-called lateral displacements travel along the field lines. These are similar to waves in an oscillating rope, and the disturbances may play a role in the still-mysterious means by which the Sun's corona is heated to temperatures vastly higher than those in the photosphere. Twisting the field lines stores energy in the field just as in a rubber band. It also causes tremendous electric currents to flow along the loops.

At some point in the evolution of the field something happens and, in a manner of speaking, the magnetic field snaps. As with a twisted rubber band, this action releases stored energy. The rubber-band analogy is somewhat misleading because the laws of physics do not allow an open break in a magnetic field. What solar physicists believe actually happens is that the magnetic fields rapidly rearrange themselves into a simpler, less twisted configuration with lower stored energy. The energy released in this process goes into heating and accelerating nearby matter, thereby producing the sudden brightening and other effects of a flare.

Yohkoh data support the idea that the energy released during a flare comes from twisted or sheared magnetic fields. As a flare progressed on February 6, 1992, its accompanying loop system was seen to relax into an obviously simpler configuration. At the same time a marked decrease in the electric current flowing in the loops was detected by a ground-based magnetograph in Mitaka, Japan. Other flares have exhibited similar behavior.

What is certain from Yohkoh's observations is that flares heat the coronal plasma and boost subatomic charged particles to very high velocities. As these particles stream along the magnetic-field lines -- sometimes into the denser, lower layers of the corona -- they produce radiation across the entire electromagnetic spectrum. In just a few minutes a large flare releases energy equivalent to a billion one-megaton thermonuclear explosions. Up to half of this energy may appear in interplanetary space as fast-moving particles and ejected bits of corona.

It's obvious that a study of solar flares can have practical benefits, particularly if this work leads to some method for predicting when flares will occur. Yet prediction cannot take place without understanding the physics of the flare process itself. That understanding is being fostered by the tremendous data from Yohkoh. Let's look at a few of the mission's findings.

YOHKOH'S ONGOING LEGACY

One of the most vivid early results of the mission is a dramatic movie of the Sun's X-ray corona. In 10 minutes of time-lapse video footage, Yohkoh scientists present a view of the Sun that reveals an active and ever-changing corona throughout a full solar rotation. While it has been known for decades that the corona is dynamic, this movie -- with better continuity and resolution than the data from Skylab -- provides unprecedented insight into the nature of the corona's brisk changes.

Large regions undergo restructuring over very short periods, often less than an hour. Smaller areas change almost continuously, such as plasma loops that form or drain within minutes. Yohkoh revealed that the X-ray loops are often twisted (recall the rubber-band analogy) or shaped like a wishbone with a cusp at the top. The twists indicate massive electric currents along the length of the loops, and the cusps are prime candidates for locations where magnetic breaks and reconnections take place.

Yohkoh sees an X-ray corona that is continually brightening, fading, and reconstructing itself -- like a forest fire that dies down in one place only to burst forth furiously in another. Large-scale coronal loops that connect widely separated regions may remain relatively unchanged for days. However, these loops can light up suddenly, being energized by a tiny flare (called an X-ray bright point) at one of their footpoints.

Powerful eruptions at the limb of the Sun sometimes evolve into helmet-shaped streamers that in X-rays are clearly visible more than one solar radius (one million km) out. Enormous areas involving more than a quadrant of the solar disk will brighten, fade, or completely change their shape under the influence of a single active region.

Activity in one location can often quickly affect activity in another. In some images, an X-ray brightening at one footpoint of a solar flare triggers a nearly simultaneous brightening of another thousands of kilometers away. Yohkoh has revealed a complex and beautiful solar corona, wreathed with delicate loops that seem to flash and dance before our eyes.

Of particular interest is how a solar flare commences. Scientists often think of a flare as stemming from the interaction site of two or more loops -- magnetic flux tubes filled with plasma. Perhaps one has just emerged and is small, driven toward the interaction by magnetic-field changes below the solar surface. When the loops press against one another, a magnetic reconnection occurs and a burst of energy pours forth.

Another model envisions an inherently unstable magnetic-field configuration: stressed field lines are pressed down by the weight of the gases they carry and by overlying material. Gases supported in this way are called filaments; they are relatively cool and can be seen in the red light of the hydrogen-alpha spectral line. As the magnetic field supporting and surrounding the filament evolves, the filament becomes unstable and moves upward to collide with higher magnetic arches. It's almost like having two wires short-circuit against each other -- currents flow and sparks fly!

To study the evolution of flare loops, Yohkoh scientists have focused on the association between soft X-ray loops and flare "kernels" observed in red hydrogen light. X-ray images reveal the evolution of the loops, while ground-based hydrogen-alpha observations help pinpoint where the flare energy is deposited in the chromosphere of the Sun -- the layer between the photosphere and the corona.

Yohkoh's visible-light camera allows data from space and the ground to be accurately overlaid. By comparing the two it has been proven, at last, that energetic flare electrons often beam downward into the chromosphere to produce the kind of white-light flare Carrington first observed so long ago.

The Soft X-ray Telescope onboard Yohkoh has observed the short-lived transient brightenings. These are compact loop brightenings that take place as often as once every three minutes or so in highly spirited active regions. Although they occur in the same regions as larger solar flares, they do not appear to be especially good predictors of major outbursts.

Soft X-ray studies of the corona above some active regions reveal a continual expansion or apparent outward flow of magnetic fields and matter. This outflow, unexpected before the Yohkoh observations, may affect some very basic ideas about mass loss from the Sun and similar stars. Since the discovery of coronal holes (cool, low-density regions in the solar atmosphere), it has been believed that the Sun loses mass by means of the solar wind blowing outward along the open coronal-hole field lines. The much denser plasmas around active regions were believed to be fully contained by closed magnetic structures. Yohkoh's observations suggest that this simple picture may need to be reconsidered.

The Yohkoh science teams have joined ground-based astronomers to study a number of flares in detail. One such event occurred on November 15, 1991. Observations were coordinated between the spacecraft and the Mees Solar Observatory in Hawaii. Yohkoh's telescopes took images and spectra, in both soft and hard X-rays, while ground-based instruments measured the associated magnetic fields and hydrogen-alpha emissions.

X-ray and hydrogen-alpha activity, accompanied by powerful mass motions, preceded the hard X-ray impulsive phase of the flare. Regions emitting X-rays and hydrogen-alpha light erupted close to each other in what appeared to be twisted or helical loops. The dark umbra of a small sunspot moved about 1,500 km as the flare peaked, implying a significant magnetic restructuring in its vicinity.

LOOKING AHEAD

What's the future of solar flare X-ray research? Will scientists ever be able to predict flares?

Whether or not Yohkoh advances the possibility of flare prediction, it is certainly advancing our understanding of flare physics. Totally reliable flare predictions may never happen, given the chaotic nature of some flare processes. Nonetheless, solar researchers continue to gain understanding, and predictions should improve apace.

Even now, Yohkoh scientists are considering more powerful telescopes to investigate questions unanswered by the present set of instruments. Yet these have already painted new portraits of high-energy solar phenomena and unveiled a beautiful and compelling facet of our familiar and seemingly placid yellow star.

Carolyn Collins Petersen, a graduate research assistant at the University of Colorado, is also an accomplished science writer. Her scripts have been used at more than 300 planetariums worldwide. Marilyn Bruner studies solar physics at the Lockheed Palo Alto Research Laboratory and was instrument architect for Yohkoh's Soft X-ray Telescope. Loren Acton, now at Montana State University, is the United States' principal investigator for that instrument. He conducted solar research aboard NASA's Spacelab 2 mission in 1985. Yoshiaki Ogawara, at Japan's Institute for Space and Astronautical Science, serves as program manager for the Yohkoh mission.

Instruments and Infrastructure

YOHKOH observes the Sun through a suite of telescopes and spectrometers produced in Japan, the United States, and the United Kingdom. The mission is managed, and the data collection and analysis carried out, by the Institute for Space and Astronautical Science (ISAS) in Japan.

Lockheed Palo Alto Research Laboratory built the Soft X-Ray Telescope (SXT) in cooperation with the National Astronomical Observatory of Japan and Tokyo University. Every two seconds the SXT takes pictures in the 0.2- to 4-kiloelectronvolt (60- to 4-angstrom) band with a resolution on the Sun of about 2,000 km. It has made a million exposures since Yohkoh's launch on August 30, 1991. Yet SXT had a close call in 1989. During calibration and testing, the Loma Prieta earthquake in California destroyed part of the testing facility. Fortunately the instrument was not damaged, and work went ahead while the laboratory was rebuilt.

The Hard X-Ray Telescope (HXT) has examined more than 200 solar flares from October 1991 to the present. It operates in the 15- to 100-kiloelectronvolt (0.8- to 0.1-angstrom) range and can resolve features 5,000 km wide in as little as a half second. HXT's primary mission is to find out how and where hard X-rays are emitted in the magnetic loops that accompany solar flares.

Like the HXT, the Wide-Band Spectrometer (WBS) was built under the direction of ISAS. It looks at the full disk of the Sun and can take data eight times a second. By splitting the X-ray and gamma-ray wavelengths of light into very fine intervals, the WBS should answer questions about particle accelerations in flares and how their energy is stored and released.

The Bragg Crystal Spectrometer (BCS) reveals gas motions in flares across the entire solar disk with a time resolution equal to the WBS. Providing very high spectral resolution in four soft X-ray bands, the BCS was built in England jointly by the Mullard Space Science Laboratory and the Rutherford Appleton Laboratory in cooperation with the U.S. Naval Research Laboratory and the National Institutes for Standards and Technology.

Yohkoh studies are combined with ground-based observations from the Canary Islands, China, Europe, Hawaii, Japan, and the continental United States. With more than 50 researchers scattered around the globe, great emphasis has to be placed on timely and accurate communications. Electronic mail and fax machines make it possible.

One of the ingredients that sets this mission apart is its unique observational database and analysis software. Early in the planning stages, it was agreed that data from all Yohkoh experiments belonged to everyone on the team. Thus there is unprecedented ease in accessing this information: as of this writing analysis software has been installed at 21 institutions. The observational database, modern computer workstations, and impressive suite of analysis software are as crucial to the scientific return from the mission as the spacecraft instruments themselves.
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Title Annotation:includes related article; Japanese-American-British spacecraft
Author:Petersen, Carolyn Collins; Bruner, Marilyn; Acton, Loren; Ogawara, Yoshiaki
Publication:Sky & Telescope
Date:Sep 1, 1993
Words:2825
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