Joint observations from satellite and the ground
X-ray observations of the corona are possible only by space vehicles such as YOHKOH, whereas observations of the magnetic field on the solar surface have been made from the ground by measuring the absorption lines in the visible light region. For this reason, joint observations by observatories on the ground and satellites such as YOHKOH are frequently carried out across the world to capture the same region and same phenomenon simultaneously. Fig. 4 shows the result from one of joint observations, catching the site where a micro-flare occurred just after a small magnetic field went up from the solar surface.
Since it is important to link coronal heating/activity with the activities of magnetic elements at the accuracy higher than arc second (i.e., sub arc-second), high resolution is required for the visible light observations. A few ground observatories abroad have recently succeeded to obtain snapshot images in sub arc-second, though not so many, by owing to observatories’ locations where supreme atmospheric seeing may be expected and/or the adoption of data-correction techniques for compensating seeing fluctuations. In the beginning of July 2005, the author conducted an international joint observation at observatories in the Canary Islands, Spain, which are the observatories mentioned above, with X-ray satellites observation. I was surprised that momentary resolution obtained there was greatly improved. However, to study the corona’s dynamic activity and heating, it is essential to measure changes in the magnetic field continuously, stably, and precisely with sub arc-second for a long period. In this respect, ground observations are greatly limited.
Expectations for SOLAR-B satellite
Under the circumstances, it is expected that SOLAR-B satellite (Fig. 5) will play a key role. One of the important research themes for SOLAR-B is to understand magnetic coupling between the corona and the lower atmosphere. To conduct simultaneous observations of the corona and the solar surface’s magnetic field, SOLAR-B will carry three unique telescopes. An X-Ray Telescope (XRT) and an Extreme-ultraviolet Imaging Spectrometer (EIS) will observe the corona while a Solar Optical Telescope (SOT) will observe the solar surface. The SOT is a visible-light telescope with an aperture diameter of 50cm and a resolution capability of 0.2 to 0.3 arc second. It will be the largest-aperture telescope in the world among on-orbit telescopes designed exclusively for observation of the Sun. With regard to observational functions, a filter-imaging instrument will be used to watch dynamic temporal changes and a Stokes polarimeter (spectrometer to measure polarization) to measure in detail physical values including the magnetic field.
The development of onboard telescopes is completed and the satellite’s final integration tests are now on going in ISAS. We are excited to imagine that, after its launch in 2006, the satellite’s telescopes will obtain revolutionary observation data that will give impacts to solar physics and its associated sciences. We are now concentrating on satellite tests to ensure the mission’s success.