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- Title
Gradient Winds and Neutral Flow Dawn-Dusk Asymmetry in the Auroral Oval During Geomagnetically Disturbed Conditions.
- Authors
Larsen, M. F.; Pfaff, R. F.; Mesquita, R.; Kaeppler, S. R.
- Abstract
The Pedersen component of the Lorentz force produces an acceleration that is generally in the zonal direction in much of the dawn and dusk sectors in the auroral oval. During geomagnetically disturbed conditions, as the neutral flow begins to accelerate through the ion drag force and the flow speeds increase, a balance develops in the meridional direction between the Coriolis, curvature, and pressure gradient forces, which are dominant in the lower thermosphere. The gradient wind equation that describes this balance predicts that the cyclonic flow on the dawn side is limited to the so-called regular solution, which has a maximum value of twice the geostrophic wind speed. The anticyclonic flow on the dusk side, on the other hand, can satisfy either the regular or anomalous solution with a transition at twice the geostrophic wind speed. The anomalous flow solutions have wind speeds significantly greater than the transition value, but are limited by the inertial wind value, that is, the value that corresponds to a balance between the curvature and Coriolis forces. The analysis is carried out to show this result, which indicates that a significant quantitative asymmetry is expected between the dawn- and dusk-side flow, as is observed and has been shown in both observations and a number of numerical modeling studies. Implications for the wind distribution of perturbed pressure gradients and inertial instability are discussed. Plain Language Summary The ion drag "push" in the lower ionosphere in the auroral oval accelerates the winds in the direction that is generally westward on the dusk side of the auroral oval and generally eastward on the dawn side. Forces perpendicular to the ion push include the Coriolis force, the curvature (centrifugal) force, and the pressure gradient, which set up a balance in the north-south direction that ideally keeps the flow in the acceleration channel. On the evening side the balance of forces is very effective in doing that, but it is shown that on the morning side the balance is limited to a range of wind speeds between zero and twice the geostrophic value. The balance of forces on the evening side can accommodate much larger wind speeds, but even there, the maximum flow speed is limited by the inertial value, which is the wind speed that corresponds to a balance between the centrifugal and Coriolis forces with a value for typical parameters of approximatel 350 ms-1. The maximum flow speeds on both the dusk and dawn side are ultimately limited by inertial instability in the flow, which develops if the limits are exceeded.
- Subjects
GRADIENT winds; AURORAL electrons; GEOMAGNETISM; THERMOSPHERE; WIND speed
- Publication
Journal of Geophysical Research. Space Physics, 2022, Vol 127, Issue 1, p1
- ISSN
2169-9380
- Publication type
Article
- DOI
10.1029/2021JA029936