Influence of Atmospheric Circulation on Orientation of Auroral Arcs.

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, that is, the angle between an arc and the West‐East direction, linearly changed with MLT at the rate 2.2°/hr. 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. Plain Language Summary: Aurora is a manifestation of the precipitation of charged particle of the magnetospheric origin into the Earth's ionosphere. The most prominent auroral forms populating in the auroral oval are relatively narrow multiple auroral arcs extended for many hundred kilometers within the oval. In the past a number of studies were made to understand how auroral arcs are oriented with respect to the East‐West direction in different local time and what may affect this orientation. It is commonly believed that the orientation of the arcs primary depends on the solar wind conditions and geomagnetic activity. However, precise analysis of about 10,000 auroral arcs observed by the optical camera located in the Northern Europe during a 6‐year period of 2016–2021 has revealed anomalous orientations of the arcs which cannot be explained by interplanetary or solar conditions. Unexpectedly we have found that the global atmospheric circulation apparently affects orientation of auroral arcs. This effect, which has been never reported before, is the main subject of the present paper. Key Points: Orientation of nightside auroral arcs may be affected by the neutral windThe effect is noticeable during the spring transition in the global atmospheric circulationThe neutral wind can influence the ionosphere‐magnetosphere system through the ionosphere dynamo processes [ABSTRACT FROM AUTHOR]