Numerical simulations of thermospheric dynamics: divergence as a proxy for vertical winds

Abstract

Abstract.A local scale, time dependent three-dimensional model of the neutral thermosphere was used to test the applicability of two previously published empirical relations between thermospheric vertical wind and velocity divergence, i.e., those due to Burnside et al. (1981) and Brekke (1997). The model self-consistently solves for vertical winds driven by heat and momentum deposited into the neutral atmosphere by high latitude ion convection. The Brekke condition accurately mimicked the overall "shape" of the three-dimensional model vertical wind field although, as written, it consistently overestimated the vertical wind magnitude by a factor of approximately 5/3, for the heating scenarios that we considered. This same general behavior was observed regardless of whether the forcing was static or rapidly changing with time. We discuss the likely reason for the Brekke condition overestimating the magnitude of our vertical winds, and suggest an alternative condition that should better describe vertical winds that are driven by local heating. The applicability of the Burnside condition was, by contrast, quite variable. During static heating, both the magnitude and the sign of the model vertical winds were predicted reliably at heights above those of maximum energy and momentum deposition per unit mass. However, below the thermal forcing, the Burnside condition predicted vertical winds of the wrong sign. It also introduced significant artefacts into the predicted vertical wind field when the forcing changed suddenly with time. If these results are of general applicability (which seems likely, given the way these relations are derived) then the Burnside condition could usually be used safely at altitudes above h m F 2 . But it should be avoided below this height at all times, and even at high altitudes during periods of dynamic forcing. While the Brekke condition (or our modified version of it) could likely be used in all circumCorrespondence to: S. L. Cooper (sl7cooper@students.latrobe.edu.au) stances, there are few experimental scenarios for which this would be useful. This is because evaluation of the Brekke condition would not usually be possible unless the vertical wind was already known in advance.

Importance of capturing heliospheric variability for studies of thermospheric vertical winds

“…Similarly, other studies have found that the Burnside relation can, in fact, break down under strong disturbances in the high‐latitudes [ Crickmore , 1993; Smith and Hernandez , 1995]. For instance, using a time‐dependent three‐dimensional local model of the thermosphere, Cooper et al [2009] have investigated the applicability of the Burnside et al [1981] relation. They have found that although the Burnside et al [1981] condition is usable under slowly varying conditions above the F region peak, it is applicable to a limited extent when the high‐latitude forcing is varying rapidly in time.…”

“…A number of authors have studied the relationship between the vertical winds and the divergence of the horizontal winds [ Burnside et al , 1981; Biondi , 1984; Crickmore , 1993; Cooper and Conde , 2006; Cooper et al , 2009]. The advantages of a nonhydrostatic model such as GITM in studying thermospheric vertical winds and their variability are, among others, (1) high model time resolution of 2–4 s, (2) the individual vertical momentum flux deposition terms can be investigated in detail, (3) the significance of nonhydrostatic acceleration can be quantified, (4) the temporal variations of the horizontal wind divergence can be compared with that of the vertical winds and other thermospheric parameters.…”

Spatial sampling of the thermospheric vertical wind field at auroral latitudes

“…[46] Cooper et al [2009] tested the validity of equation (5) using a local-scale, time-dependent, three-dimensional model of the neutral atmosphere and found that it was generally only applicable above the height of maximum energy and momentum deposition per unit mass, and at times when the forcing did not change rapidly with time. Below the height of maximum forcing the Burnside condition predicted the incorrect sign for the vertical wind.…”

Interplanetary magnetic field and solar cycle dependence of Northern Hemisphere F region joule heating

“…Numerical models of localized heating are widely used in case studies. For example, Cooper et al [] included Joule heating in a local‐scale model of vertical thermospheric winds. Davies et al [] compared ISR measurements with results from the Sheffield University Plasmasphere and Ionosphere Model, calculating parameters along a single magnetic field line.…”