The effects of multiple propagating plasma patches on the polar thermosphere

Polar cap patches are the largest plasma structures in the high-latitude io nosphere. They have horizontal dimensions of hundreds to thousands of kilom eters. The plasma density in the patches is similar to the density on the d ayside, which is up to an order of magnitude higher than the local backgrou nd density in the polar cap. Frequently, multiple propagating plasma patche s are observed, which are elongated in a direction perpendicular to the gen erally antisunward motion. Because of the large-scale size, the presence an d motion of polar cap patches may have a significant effect on the thermosp here. Previous studies of single patches have shown that the effect of a po lar cap patch is significant. The direct effect on the neutral density is a snowplow effect. A plasma patch steepens the neutral density gradient in f ront 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 c reates a propagating neutral hot spot. In the present work, we used a time- dependent, 3-D thermospheric circulation model, with a high spatial resolut ion, to study the effects of multiple propagating patches on the thermosphe re. A sequence of plasma patches, with horizontal dimensions of 200 km x 20 00 km, were simulated for different solar conditions and different patch-to -background density ratios. From these simulations, we found that both indi vidual and collective effects of the patches on the thermosphere are eviden t. The most general effects are a neutral density depletion and heating. Th e density perturbations can be as large as 30%, and the temperature increas es can reach about 400 K depending on the conditions. The individual effect s show up as localized neutral density depletions and heating associated wi th each patch. Structures are created in the neutral density and temperatur e distributions. A sequence of patches also acts as one large structure. Th e 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 st udy with different conditions, we found that increasing the patch-to-backgr ound density ratio and the cross-cap potential both act to enhance the stre ngth of the perturbation, although it is not in a linear proportion. The ne utral density and temperature changes in absolute values, due to the plasma patches, are higher at solar maximum than at solar minimum. However, the r elative 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 p ointed out that representative patches are used in the simulations and the feedback from the modification of the thermosphere is not included. This th eoretical study is not intended for a direct comparison to particular obser vations. Instead, it is to understand the physical process and to predict t he result for future measurements. (C) 2001 Elsevier Science Ltd. All right s reserved.