On the Response of the Upper Atmosphere to Solar Flares.

Abstract

Over the past several decades, modern civilizations have become increasingly dependent on spacecraft that reside in the near-Earth space environment. For this reason, scientists and engineers have been interested in understanding the causes of perturbations to the background state of the Earth’s upper atmosphere, and to quantify the impact of these events. As a result of the states of the thermosphere and ionosphere being directly dependent on the incident radiation from the sun, it is expected that sudden changes in the solar radiative output should cause significant changes in the upper atmosphere. Such dynamics are investigated in this study, specifically the manner in which solar flares affect the density, circulation, and temperature of the Earth’s thermosphere and ionosphere. A global model of this region is used to examine how the upper atmosphere responds to such transient events. In order to quantify the response, the model is run during realistic events in order to understand the magnitudes of the resulting perturbations to the global ionosphere-thermosphere system. In the thermosphere, density perturbations of approximately 15% are found to occur on the dayside within 1.5 hours after the start of a solar flare. The addition of solar energy to the dayside launches a traveling atmospheric disturbance which propagates towards the night-side at the local sound speed plus the background velocity. As the disturbance converges on itself near the midnight sector, density enhancements almost as large as those seen on the day-side can occur. Furthermore, these nightside neutral perturbations cause both enhancements and depletions in the night-side electron density.