Energy and Momentum Balance

Abstract

The outwards increase of temperature in the chromosphere and corona is associated with dissipation of mechanical energy presumably carried upwards from the photosphere as wave motion. This mechanical energy, dissipated as heat, ultimately finds its way out of the Sun primarily in the form of radiation. A small percentage of the energy is carried outwards by conduction and convection in the solar wind. This is not to say, however, that convection and conduction are not locally important within the atmosphere. The sharp increases in temperature between temperature plateaus suggest that thermal conduction is important in these regions. In the case of the transition region between the chromosphere and corona where we have been able to form an estimate of the conductive energy flux we find that the flux indeed is large. The average value of T ^5/2 dT/dh for the transition region corresponds to a conductive flux of F _c ≈ 6 x 10⁵ erg cm⁻² s⁻¹ in regions of vertical magnetic field and in a spherically symmetric atmosphere. Radiation flux from the corona amounts to 1−3 x 10⁵ erg cm⁻² s⁻¹ depending on the level of solar activity. Similarly, the radiation flux from the upper chromosphere, which is almost entirely in the Lyman-α line, amounts to 2−4 x 10⁵ erg cm⁻² s⁻¹, again depending on the level of solar activity. The radiation losses from the upper chromosphere and corona occur in emission lines that are produced by collisional excitations. As such, they represent a direct loss of energy from the thermal reservoir and must, therefore, be balanced by an equivalent energy input. As we noted earlier, the basic heat input comes from the mechanical energy carried upwards. It seems clear, however, that thermal conduction through the transition region may represent a strong energy drain on the inner corona and may provide a strong source of energy in the upper chromosphere. The actual value of the conductive flux is uncertain because of the complex magnetic field structure in the transition region.