Evolution of Coronal Loops Observed by GOES-SXI

LÓPEZ FUENTES, M.C.; MANDRINI, C.H.; KLIMCHUK, J.A.

dENVER

Congreso; American Astronomical Society Meeting 204; 2004

American Astronomical Society

Several years ago, Porter and Klimchuk (PK, 1995) studied a collection of coronal loops observed by the SXT instrument on Yohkoh and found that they have evolutionary timescales of typically 104 to 105 s. These very long timescales are roughly one or two orders of magnitude greater than the corresponding cooling times and therefore have important implications for loop heating. However, PK observed each loop for only a single spacecraft orbit ( 1 hr), and it has remained a question whether loops actually persist for as long as the ''instantaneous'' evolutionary timescales suggest. Perhaps loops turn on suddenly, remain nearly constant for a modest period, then decay on a cooling timescale. We have begun to address the general issue of loop evolution using images from the Soft X-ray Imager (SXI) on board the GOES-12 satellite. This instrument has a coarser spatial resolution than TRACE or SXT, but has the advantage of continuous temporal coverage. Since GOES-12 is in geosynchronous orbit, there is no spacecraft night. We present here the light curves of several loops and compare them with the radiative cooling times inferred from the observed temperature and emission measure. We discuss heating rate requirements related to the observed turn on and maintenance of the loops, and we consider the implications for nanoflare versus steady heating scenarios. This work is funded by NASA and the Office of Naval Research. Porter, L. J., and Klimchuk, J. A. 1995, ApJ, 454, 499