Storm Response

Geomagnetic disturbances cause global changes in thermospheric circulation and composition. Thermospheric heating at high latitudes leads to upward expansion, creating horizontal pressure gradients that cause neutral air to flow away from the heated area. At middle latitudes, meridional pressure gradient from a high-latitude source at daytime contradicts the meridional pressure gradient from a solar heating source, while the situation is opposite in nighttime. This results in a weaker daytime poleward wind and a stronger nighttime equatorward wind. Co-existence of two heating sources, solar and high-latitude, leads to complicated dependence of thermospheric parameters (temperature, density, wind flow) from universal time.

Variations in thermospheric circulation cause changes in neutral density due to both diffusion and transport. As heated neutral air moves upward and towards cooler regions, at a fixed altitude over heated region total neutral density increases, while relative fraction of lighter components decreases. As a result, heated regions, i.e. high and middle latitudes, have higher content of molecular components O2 and N2, while cooler regions, i.e. lower latitudes, have higher content of O.

As both the heat sources and the thermospheric composition have complex spatial and temporal variations acting on different scales, predicting the state of the thermosphere for any practical case remains a complicated and challenging task.

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