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Abstract

We investigated statistically the orientation of about 10,000 auroral arcs observed during 2016-2021 in Abisko (68.36°N, 18.81°E, Sweden) in the equatorward part of the nightside auroral oval. The observations were made between 19 and 06 magnetic local time (MLT). On average, the orientation of the arcs, i.e., the angle between an arc and the West-East direction, linearly changed with MLT at the rate 2.2°/hour. In most cases the mean orientation of the auroral arcs near midnight was parallel to the geomagnetic latitude of Abisko, except a few late winter or spring months. These anomalies cannot be explained by geomagnetic disturbances or interplanetary conditions. They do, however, coincide with spring transitions in the atmospheric circulations from winter- to summer-type. The spring transitions were manifested in the data of a meteor radar in the region of observations as reversals of the zonal wind at altitudes 90-100 km from eastward to westward. We propose that these spring transition changes in the global atmospheric circulation couple also the overlying thermosphere at 100-150 km where neutral winds may affect the ionosphere and subsequently the whole ionosphere-magnetosphere system. Then, changes in the coupled ionosphere-magnetosphere system are manifested in changes of the orientation of auroral arcs.