Scientific Justification

This program will study high velocity or explosive events simultaneously at temperatures ranging from the lower transition region to the corona.

The term explosive event has been used for a high velocity phenomenon mainly studied in the C IV lines at 1550 A with HRTS. Their properties are described by Dere (1994) and by Moses and Cook (1994) and more thoroughly in number of earlier papers (see references in Kjeldseth--Moe and Cheng 1994). Detailed knowledge is, however, limited to their appearance in the lower transition region around T 10K. Similar high velocity events at higher temperatures extending into the corona was observed with the slitless spectrograph SO82A on Skylab in 1973--74. Quantitative measurements have been performed by Kjeldseth--Moe and Cheng (see Kjeldseth--Moe and Cheng 1994, and references therein. See also the table below.)

The main purpose of the proposed Joint Observing Program is to determine whether the energetic high velocity events seen in the corona and upper transition region is the same phenomenon as the explosive events at T 10K. The observations may furthermore:

• Map the energy and momentum release for high velocity events as functions of location, temperature and time, and
• Reveal whether the high velocity events may be explained by flows from a reconnecting region in a short magnetic loop.

It has been hypothesised that such high velocity events are caused by magnetic reconnection in the solar atmosphere. Whether or not this is the case, they may be important for the energy and momentum input to the upper solar atmosphere. Their unusual properties may furthermore give important clues to understanding the structures and processes in the upper solar atmosphere.

References

Dere, K. P. 1994, Space Science Reviews, 70, 21.

Kjeldseth--Moe, O. and Cheng, C.-C. 1994, Space Science Reviews, 70, 85.

Moses, D. and Cook, J.W. 1994, Space Science Reviews, 70, 81.