Role of variability in determining the vertical wind speeds and structure

Abstract

[1] The University of Michigan's Global Ionosphere Thermosphere Model (GITM) is used to study the temporal and spatial variability of high-latitude vertical winds during the December solstice. Underlying mechanisms controlling the high-latitude distribution and the magnitude of vertical winds in the thermosphere are investigated in a suite of systematic model simulations. First, in a series of six model simulations GITM longitude × latitude resolution is gradually increased from 5° × 5° to 2.5° × 0.3125°, imposing constant moderate solar and low magnetospheric activity in all simulations. Analysis of the high-latitude mean parameters shows that polar distributions of vertical winds and Joule heating demonstrate localized enhancements at high spatial resolution that are not well captured at coarse grid resolution. Second, in simulations with fixed spatial resolution of 2.5° × 2.5°, the impact of temporally variable magnetospheric conditions on the morphology of the high-latitude vertical winds has been investigated. For this, hemispheric power and the cross polar cap potential values are modified in a series of systematic simulations. The magnitude and temporal variations of ion flows significantly impact the high-latitude Joule heating, which in turn dramatically effects the vertical wind structure. It is demonstrated that variability in the vertical winds are driven by variability in the ion flows and that sudden step-like changes in ion flows can cause the largest enhancements in the vertical winds, in particular, via nonhydrostatic effects. The vertical wind variability is seen to be the largest in the summer Southern Hemisphere with much larger vertical wind magnitudes.