Focus

Solar-B

Japan's Institute of Space and Astronauti-cal Science, abbreviated ISAS, is expected to launch its Solar-B Mission in 2006, providing years of detailed study of the root causes of the corona, the solar wind, and exploding solar activity. It is an international collaboration building on the highly successful SolarA, renamed Yohkoh after launch, involving Japan, the United States and the United Kingdom.

Solar-B will investigate the Sun's changing magnetic fields from the photosphere to the low corona using a coordinated set of optical, extreme ultraviolet and X-ray telescopes, "ttey will provide an improved understanding of the mechanisms of solar magnetic vari ability and how this variability modulates the Sun's output.

"tte processes of magnetic reconnection and wave dissipation will be investigated; they are believed to be responsible for the conversion of magnetic energy into coronal heat, ultraviolet and X-ray radiation, and the expanding solar wind. Solar-B will additionally improve our understanding of solar eruptions, from flares to coronal mass ejections, which are triggered and powered by some similar magnetic processes.

"tte ongoing creation and destruction of the Suns magnetic fields will also be investigated, over time scales from seconds to years, revealing mechanisms for low-level modulation of the Sun's luminosity, as well as the internal dynamo process that sustains the ever-changing solar magnetism.

the Sun's center, oxygen ions have the higher outflow velocity, approaching 500 kilometers per second in the holes, while hydrogen moves at about half this speed, ttat violates common sense. It would be something like watching people jogging around a racetrack, with heavier adults running much more rapidly than lighter, slimmer youngsters. Something is unexpectedly and preferentially energizing the heavier particles in coronal holes.

Magnetic waves might accelerate the heavier ions more than lighter ones by pumping up gyrations around the open magnetic fields. More massive ions gyrate with lower frequencies where the magnetic waves are most intense, thereby absorbing more magnetic-wave energy and becoming accelerated to higher speeds. Ulysses has detected magnetic fluctuations, attributed to Alfven waves, blowing further out in the winds far above the Sun's poles, ttese Alfven waves might provide an extra boost that pushes the polar winds to higher speeds.

In another approach, the speed of the solar wind as it blows past the Earth has been tied to deep roots within the chromosphere. NASA's Advanced Composition Explorer, abbreviated ACE, spacecraft measured the wind velocity near Earth, while NASA's Transition Region And Coronal Explorer, or TRACE for short, was used to measure the time sound waves took to travel between two heights in the chromosphere, tte comparison indicated that the speed of the solar wind emerging from a given area of the solar corona could be estimated from the thickness of the underlying chromosphere. It is stretched thin and opened wide in coronal holes, with their open magnetic fields and fast, tenuous solar wind, but the chromosphere is compressed below magnetically closed regions associated with the gusty, slow dense solar wind outflow (Fig 6.16).

An improved understanding of the fundamental causes of coronal heating and the Sun's winds are amongst the several objectives of the Solar-B mission of the Japanese space agency, scheduled for launch in 2006 (Focus 6.2).

FIG. 6.16 Fast and slow winds, open and closed magnetism The solar atmosphere, or corona, is threaded with magnetic fields (yellow lines). Regions with open magnetic fields, known as coronal holes, give rise to fast, low density, solar wind streams (long, solid red arrows). In addition to permanent coronal holes at the Sun's poles (top and bottom), coronal holes can sometimes occur closer to the Sun's equator (center). Areas with closed magnetic fields yield the slow, dense wind (short, dashed red arrows). Comparisons of TRACE images with solar wind ACE data indicate that the speed and composition of the solar wind emerging from a given area have deep roots in the chromosphere. There is a shallow dense chromosphere below the strong, closed magnetic regions with a slow, dense, solar-wind outflow; deep, less dense chromosphere is found below the open magnetic regions with fast, tenuous, solar-wind outflow. This image was taken onll September 2003 with the Extreme-ultraviolet Imaging Telescope, abbreviated EIT, aboard the SOlar and Heliospheric Observatory, or SOHO for short. (Courtesy of the SOHO EIT consortium. SOHO is a project of international collaboration between ESA and NASA.)

FIG. 6.16 Fast and slow winds, open and closed magnetism The solar atmosphere, or corona, is threaded with magnetic fields (yellow lines). Regions with open magnetic fields, known as coronal holes, give rise to fast, low density, solar wind streams (long, solid red arrows). In addition to permanent coronal holes at the Sun's poles (top and bottom), coronal holes can sometimes occur closer to the Sun's equator (center). Areas with closed magnetic fields yield the slow, dense wind (short, dashed red arrows). Comparisons of TRACE images with solar wind ACE data indicate that the speed and composition of the solar wind emerging from a given area have deep roots in the chromosphere. There is a shallow dense chromosphere below the strong, closed magnetic regions with a slow, dense, solar-wind outflow; deep, less dense chromosphere is found below the open magnetic regions with fast, tenuous, solar-wind outflow. This image was taken onll September 2003 with the Extreme-ultraviolet Imaging Telescope, abbreviated EIT, aboard the SOlar and Heliospheric Observatory, or SOHO for short. (Courtesy of the SOHO EIT consortium. SOHO is a project of international collaboration between ESA and NASA.)

But to sum up our current knowledge, winds of different velocities seem to originate in places with different magnetic field configurations, which may be related to phenomena beneath them, tte fastest solar winds are traced back to coronal holes with open magnetic fields, those of intermediate speed to the inactive Sun with moderate magnetism, and the slow wind to streamer stalks and underlying active regions of intense closed magnetic fields. And each of these magnetic regions is rooted in the transition region and chromosphere, even into the photosphere and convective flows just beneath it.

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