An improved parameterization of thermal electron heating by photoelectrons, with application to an X17 flare

Global ionospheric models typically rely on parameterizations to account for heating of the plasma by photoelectrons. We compare the most commonly used parameterization to a rigorous photoelectron model and find the parameterization under-predicts the thermal electron volume heating rate by 20-30% under nominal solar conditions. When applied to a large solar flare, the parameterized heating rate is more than an order of magnitude smaller than the peak rate calculated with the physical model. To remedy this, we develop a new parameterization of electron heating that eliminates these differences; the resulting heating rates are within 5-15% of the physical model for nominal and solar flare conditions. The new algorithm is incorporated into a 1-D Global Average Ionosphere/Thermosphere (GAIT) model and used to investigate the response to the X17 flare of 28 October 2003. Electron temperatures calculated with the new parameterization are up to 10% higher prior to the flare, and 15% higher at the flare peak. The revised parameterization also leads to a 2% increase in neutral exospheric temperatures in the coupled model. The flare response of the global-average thermosphere is described and found to be similar to the satellite drag results reported by Sutton et al.