ON THE STRUCTURE OF SOLAR AND STELLAR CORONAE - LOOPS AND LOOP HEAT-TRANSPORT

We discuss the principal constraints on mechanisms for structuring and heating the outer atmospheres-the coronae-of stars. We argue that the essential cause of highly localized heating in the coronae of stars l ike the Sun is the spatially intermittent nature of stellar surface ma gnetic fields; and that the spatial scale of the resulting coronal str uctures is related to the spatial structure of the photospheric fields . We show that significant constraints on coronal heating mechanisms d erive from the observed variations in coronal emission; and, in additi on, show that the observed structuring perpendicular to coronal magnet ic fields imposes severe constraints on mechanisms for heat dispersal in the low-beta atmosphere. In particular, we find that most of common ly considered mechanisms for heat dispersal, such as anomalous diffusi on due to plasma turbulence or magnetic field line stochasticity, are much too slow to account for the observed rapid heating of coronal loo ps. The most plausible mechanism appears to be reconnection at the int erface between two adjacent coronal flux bundles. Based on a model inv oking hyperresistivity, we show that such a mechanism naturally leads to (1) dominance of isolated single bright coronal loops; and (2) brig ht coronal plasma structures whose spatial scale transverse to the loc al magnetic field is comparable to observed dimensions of coronal X-ra y loops.