A new enthalpy-based approach to the transition region in an impulsively heated corona

Abstract

Observations of the solar corona reveal persistent and ubiquitous redshifts, which correspond to bulk downflows. For an impulsively heated corona (e.g., by nanoflares), this indicates that a majority of the component loop structures are in the radiatively cooling phase of their lifecycle, and these motions should not be used to verify the predictions of any proposed theory of heating. However, the nature of the bulk downflows raises the possibility that enthalpy may play a key role in the energy balance of the loops and in particular that it powers the transition region radiation. In this Letter, we use one-dimensional hydrodynamic simulations of loop cooling to show that enthalpy losses from the corona are easily sufficient to power the transition region radiation. This contrasts with the long-held view that downward thermal conduction powers the transition region. The traditional distinction between the transition region and the corona in terms of temperature alone is then a grossly unphysical simplification, and a proper definition of the interface between these atmospheric layers requires a detailed knowledge of their energy balance. To this end, we propose a robust new definition of the transition region. © 2010. The American Astronomical Society. All rights reserved.