Magnetic storm response of lower thermosphere density

Physics

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Meteorology And Atmospheric Dynamics: Thermospheric Dynamics, Atmospheric Composition And Structure: Thermosphere-Energy Deposition

Scientific paper

Measurements of atmospheric density near 200 km from the Satellite Electrostatic Triaxial Accelerometer (SETA) experiment are used to delineate the temporal, seasonal-latitudinal, and day/night dependences of the response to magnetic storm-related perturbations in high-latitude energetic inputs. Five periods of geomagnetic activity are analyzed and yield consistent results which can be interpreted within the framework of recent thermosphere-ionosphere simulations by Fuller-Rowell et al. [1995]: In response to a change in magnetic activity level from quiet (Kp~1-2) to active (Kp~4-7) conditions, an increase in daytime (1030 LT) density of order 50-70% occurs between 60 and 80° geographic latitude in the summer hemisphere, with about half the maximum response in the winter hemisphere. This difference is mainly due to the difference in ionization/conductivity levels (and hence joule heating rates) between the hemispheres. On the dayside, penetration of the disturbance at about the 50% intensity level is realized at the equator, whereas in the winter hemisphere equatorward penetration is much weaker. These effects are connected with the prevailing solar-driven circulation; the net summer-to-winter meridional flow facilitates equatorward advection of the disturbance bulge in the summer hemisphere but hinders advection in the winter hemisphere. In both hemispheres the daytime component of the solar-driven diurnal circulation tends to oppose equatorward penetration to the same degree. However, on the nightside (2230 LT) penetration at nearly the 100% level of both summer and winter disturbance bulges are realized to within 20° of the geographic equator. This behavior is associated with the equatorward advection in both hemispheres consistent with the nighttime component of the solar-driven circulation. Comparisons with the MSISE90 model [Hedin, 1991] show it to capture the salient features of the daytime behavior but exhibits little day/night asymmetry, in contrast to the experimental results.

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