The plasma properties of the solar upper atmosphere determined from high resolution observations and the nature of the physical processes sustaining it*

Abstract

Half a century ago, it was established that the temperature of the ‘‘quiescent’’ solar corona is ≊106 K (≊100 eV). Wave theories have been used to explain the means by which the Sun keeps the corona about 200 times hotter than the 5000 K (≊0.5 eV) photosphere. In recent years, it has been suggested that a mechanism other than wave dissipation heats the ‘‘quiescent’’ corona and other transient phenomena that occur in the upper solar atmosphere. It is postulated by some that the solar upper atmosphere is heated by small bursts of energy resulting from magnetic reconnection. In this paper, the plasma properties of the upper solar atmosphere, a domain that, when flares are included, encompasses about three orders of magnitude in temperature (3×104–3×107 K) and four orders of magnitude in electron density (3×108—3×1012 cm−3), are described. It is shown that conventional scenarios based on magnetic reconnections may not adequately explain the observed plasma properties of the solar upper atmosphere. However, a model, consisting of electric currents flowing along magnetic field lines may have a better chance simulating the observed properties. For quiescent solar upper atmosphere phenomena, constant currents are needed to simulate the observations, while for flares, currents supplied by an exponentially decaying storage media are required.