A THEORETICAL-STUDY OF THERMOSPHERIC COMPOSITION PERTURBATIONS DURING AN IMPULSIVE GEOMAGNETIC STORM

The compositional response of the neutral thermosphere to an impulsive geomagnetic storm has been investigated using a numerical simulation made with the National Center for Atmospheric Research thermospheric general circulation model (NCAR-TGCM). Calculated time-dependent changes in neutral thermospheric composition have been studied, together with detailed neutral parcel trajectories and other diagnostic information from the model, to gain a greater understanding of the physical mechanisms responsible for composition variability during geomagnetic storms and, in particular, to investigate the causes of the positive and negative ionospheric storm effects. The following principal results were obtained from this study. (1) Calculated perturbations in thermospheric composition following the onset of an impulsive geomagnetic storm were found to be in good qualitative agreement with the previous experimental statistical study of storm time thermospheric morphology by Prolss (1981). (2) During the initial (onset) phase of the simulated storm, upward vertical winds occurred in the auroral zone and downward winds occurred in the central magnetic polar cap. (3) The largest perturbations in mass mixing ratio of nitrogen (PSI-N2) at F region altitudes were found to be associated with parcels of neutral gas that travelled through the cusp region and with parcels that were trapped within the auroral zone for a long time. (4) Storm time enhancements in PSI-N2 were found to occur in the midnight and early morning sectors both within and equatorward of the auroral zone, and these were determined to be associated with the advective effects of the large antisunward polar cap neutral winds. The physical size of the region of enhanced PSI-N2 values was determined to be related to the time taken for individual parcels of air to corotate out of the large day-to-night thermospheric wind jet, and thus to the magnitudes of the antisunward polar cap winds. (5) Decreases in PSI-N2 were found to occur in regions of downward vertical winds and in regions into which relatively nitrogen-poor air was advected by the horizontal wind. (6) Calculated compositional perturbations following the onset of the storm were found to propagate to mid-latitudes due to the advective effects of equatorward winds, leading to the mid-latitude positive ionospheric storm effects. This last result is in disagreement with the earlier study of Rishbeth et al. (1985), who found the compositional disturbance to be largely restricted to the auroral zone on a constant-pressure surface. However, it is consistent with the more recent study undertaken by Fuller-Rowell et al. (1991), who demonstrated that composition changes were seen well beyond the auroral oval.