Energy balance in the chromosphere-corona transition region

The classical picture of the transition region is that of a thin spherically symmetric shell maintained in a steady average thermodynamical state by a balance between conductive heating from the hot overlying corona and radiative losses. The further analysis of existing extreme ultraviolet flux data casts doubt on the correctness of this simple model. It is shown that the downward heat flux between the chromosphere and corona cannot be nearly as large as the value 6 × 10⁵ erg cm^–)= 13.46 - 2.99 sin² s^–1 derived in previous studies by assuming a planar atmosphere, and in fact is insufficient to balance transition-region radiative losses. An alternative picture is developed, consisting of a transition region network covering only a small fraction of the solar disk. The dissipation of mechanical energy, previously neglected in many calculations of theoretical transition region models, appears to play a dominant role in the local energy balance of the network.The National Center for Atmospheric Research is sponsored by the National Science Foundation.