A Transient Heating Model for the Structure and Dynamics of the Solar Transition Region

Abstract

Understanding the structure and dynamics of the Sun's transition region has been a major challenge to scientists since the Skylab era. In particular, the characteristic shape of the emission measure distribution and the Doppler shifts observed in EUV emission lines have thus far resisted all theoretical and modeling efforts to explain their origin. Recent observational advances have revealed a wealth of dynamic fine-scale structure at transition-region temperatures, validating earlier theories about the existence of such cool structure and explaining in part why static models focusing solely on hot, large-scale loops could not match observed conditions. In response to this newly confirmed picture, we have investigated numerically the hydrodynamic behavior of small, cool magnetic loops undergoing transient heating spatially localized near the chromospheric footpoints. For the first time we have successfully reproduced both the observed emission measure distribution over the entire range log T = 4.7-6.1 and the observed temperature dependence of the persistent redshifts. The closest agreement between simulations and observations is obtained with heating timescales of the order of 20 s every 100 s, a length scale of the order of 1 Mm, and energy deposition within the typical range of nanoflares. We conclude that small, cool structures can indeed produce most of the quiet solar EUV output at temperatures below 1 MK.