On the mechanisms responsible for high‐latitude thermospheric composition variations during the recovery phase of a geomagnetic storm

Abstract

Listen

Translate article

A considerable amount of previous work has been carried out to investigate variations in thermospheric composition that occur in response to the onset and main phase of geomagnetic storms. In this paper we investigate the causal mechanisms responsible for the variations in high‐latitude thermospheric composition that occur during the recovery phase of storms, a problem that has not received as much attention. Runs of the National Center for Atmospheric Research (NCAR) thermosphere general circulation model (TGCM) were made to simulate the global thermosphere during the period of the Equinox Transition Study (September 17–24, 1984). A preliminary version of the new thermosphere/ionosphere general circulation model (TIGCM) was also used to study the compositional response to the ETS period. During this period, two major geomagnetic storms occurred. Because of the comprehensive nature of the experimental coverage for the ETS it has been possible to improve the realism of the high‐latitude model inputs and to perform an extensive and more reliable diagnostic analysis of the model outputs. We present the results of numerical experiments performed to investigate the thermospheric compositional recovery from the first of the two ETS storm events. The individual terms in the single‐species continuity equation for molecular nitrogen are discussed, and their relative importance evaluated. Our results indicate that the thermospheric composition variations that occur during the recovery period are forced less by diffusive processes, as might have been anticipated, than by the effects of vertical winds advecting nitrogen‐poor air downward. Specifically, it is suggested that the storm time enhancements of high‐latitude NO densities that are produced at altitudes of about 120–180 km lead to rapid radiative cooling of the lower thermosphere. This cooling produces strong downward winds that advect nitrogen‐poor air into the regions where storm time effects are observed in ionosonde electron density data (200–500 km).