The effects of multiple propagating plasma patches on the polar thermosphere

Abstract Polar cap patches are the largest plasma structures in the high-latitude ionosphere. They have horizontal dimensions of hundreds to thousands of kilometers. The plasma density in the patches is similar to the density on the dayside, which is up to an order of magnitude higher than the local background density in the polar cap. Frequently, multiple propagating plasma patches are observed, which are elongated in a direction perpendicular to the generally antisunward motion. Because of the large-scale size, the presence and motion of polar cap patches may have a significant effect on the thermosphere. Previous studies of single patches have shown that the effect of a polar cap patch is significant. The direct effect on the neutral density is a snowplow effect. A plasma patch steepens the neutral density gradient in front of the patch and decreases the neutral density in and behind the patch. This effect is realized via a localized increase of the wind velocity. In the patch occupied region, the neutral temperature also increases, which creates a propagating neutral hot spot. In the present work, we used a time-dependent, 3-D thermospheric circulation model, with a high spatial resolution, to study the effects of multiple propagating patches on the thermosphere. A sequence of plasma patches, with horizontal dimensions of 200 km ×2000 km , were simulated for different solar conditions and different patch-to-background density ratios. From these simulations, we found that both individual and collective effects of the patches on the thermosphere are evident. The most general effects are a neutral density depletion and heating. The density perturbations can be as large as 30%, and the temperature increases can reach about 400 K depending on the conditions. The individual effects show up as localized neutral density depletions and heating associated with each patch. Structures are created in the neutral density and temperature distributions. A sequence of patches also acts as one large structure. The neutral density enhancement only occurs in front of the first patch. The neutral density depletion and heating also occur in a large region outside the patch occupied regions, particularly in the front. From a systematic study with different conditions, we found that increasing the patch-to-background density ratio and the cross-cap potential both act to enhance the strength of the perturbation, although it is not in a linear proportion. The neutral density and temperature changes in absolute values, due to the plasma patches, are higher at solar maximum than at solar minimum. However, the relative changes (percentage) show the opposite trend. The perturbations in the neutral thermosphere penetrate to lower altitudes, and there is usually a time delay associated with this low-altitude penetration. It should be pointed out that representative patches are used in the simulations and the feedback from the modification of the thermosphere is not included. This theoretical study is not intended for a direct comparison to particular observations. Instead, it is to understand the physical process and to predict the result for future measurements.