The X-ray observations from the Yohkoh satellite, a small ISAS spacecraft launched in August 1991, provided the greatest step forward in our understanding of the solar corona in nearly two decades. Yohkoh was conceived as a flare mission, but also proved capable of pioneering observations of the active non-flare corona. Among the major advances brought about by Yohkoh SXT data is the observation of dynamic structures which appear to be caused by MHD instabilities and by reconnection of magnetic fields in the corona.
However, the SXT observations have also raised unexpected difficulties to understanding the causes of variability and dynamics in the solar atmosphere. The corona is seen to consist of two fundamental components: high-temperature (5--10 MK) transient sources [1] and low-temperature (1--5 MK) persistent sources [2]. The transient components have clear loop or cusp structures, while the persistent components have more diffuse (or unresolved) structures. There is essentially no correlation between the X-ray intensity and the derived temperature [3, 4], with high temperature material also observed well outside of active regions, in the ``quiet corona.''
An X-ray telescope for Solar-B needs to have a wide temperature sensitivity that covers all of these component structures in order to understand the coronal heating problem, but must also maintain temperature discrimination capability in order to differentiate the components. X-ray loops are seen to vary on a wide range of time scales. Transient brightenings with durations of a few minutes have been observed in active regions [5]. These brightenings show a great variety in X-ray morphology, often involving multiple loops [6]. Short-timescale variability of emission is seen almost everywhere in the active-region corona [7], and this variability is believed to be a manifestation of coronal heating by numerous nanoflare events[1]. Falconer [8] examined the magnetic structures underlying the cores of active regions and found that persistent bright coronal features are rooted in strongly sheared magnetic fields near the polarity inversion line. This suggests that the heating may be due to low-lying reconnection accompanying flux cancellation at the inversion line.
| Level 1 Science Requirements | ||
|---|---|---|
| Topic | Definition/Questions | General Instrument Impact |
| Coronal Mass Ejections | 1. How are they triggered? | High time resolution |
| 2. What is their relation to the magnetic structures? | High spatial resolution | |
| 3. What is the relation between large scale instabilities and and the dynamics of small structures? | Large FOV | |
| Broad temperature coverage | ||
| Coronal Heating | 1. How do coronal structures brighten? | High time resolution |
| 2. What are the wave contributions? | High spatial resolution | |
| 3. Do loop-loop interactions cause heating? | Large FOV | |
| Broad temperature coverage | ||
| Reconnection and Jets | 1. Where and how does reconnection occur? | High time resolution |
| 2. What are the relations to the local magnetic field? | High spatial resolution | |
| Broad temperature coverage | ||
| Co-ordinated observing EIS/SOT | ||
| Flare Energetics | 1. Where and how do flares occur? | High time resolution |
| 2. What are the relations to the local magnetic field? | High spatial resolution | |
| Large FOV | ||
| Photospheric-Coronal Coupling | 1. Can a direct connection between coronal and photospheric events be established? | High time resolution |
| High spatial resolution | ||
| 2. How is energy transferred to the corona? | Large FOV | |
| 3. Does the photosphere determine coronal fine structure? | Broad temperature coverage | |
| Co-ordinated observing with SOT/EIS | ||
The basic goals of a soft X-ray telescope (XRT) for the Solar-B mission are to facilitate the study of the dynamics of fine scale coronal phenomena, such as magnetic reconnection and coronal heating mechanisms, while at the same time recording the large scale global phenomena, such as coronal mass ejections. In order to meet these objectives, the XRT will work closely with the focal plane instruments of the optical telescope (OT) and with the EUV imaging spectrometer (EIS). The XRT on Solar-B is expected to observe and quantify the coronal response to changes in the photospheric. These range from splitting and rearrangement of the intergranular flux elements leading to tangential discontinuities and energy dissipation in the corona, to large-scale magnetic shear leading to global magnetic field rearrangements. These objectives imply that the instrument used must be capable of observing the fairly low-T (<3 MK) pre-event plasma, as well as the higher-T (>5 MK) heated or activated plasma and of coordinating those observations with data from the Solar-B optical telescope.