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Wave–Mean Flow Interaction in the Storm-Time Thermosphere: A Two-Dimensional Model Simulation

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  • 1 Department of Atmospheric Sciences, University of California, Los Angeles, California
  • | 2 Space and Environment Technology Center, The Aerospace Corporation, El Segundo, California
  • | 3 High Altitude Observatory, National Center for Atmospheric Research, Boulder, Colorado
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Abstract

A two-dimensional pole-to-pole numerical model with background solstitial winds has been used to study the global dynamical response of the thermosphere to high-latitude energy inputs associated with a model geomagnetic storm. This model storm has four distinct pulses of heat input over a 12-h period.

The thermospheric wave response to the sustained part of the storm heat input consists in the establishment of a global meridional circulation that is initiated in about 3 to 4 hours after storm commencement and never quite reaches steady state in the simulation.

The main purpose of this study is to investigate the interaction between the disturbances and the mean meridional flow associated with the storm. It is shown that this interaction can be represented in terms of an induced circulation. This induced circulation is forced by the transient nature of the eddy flux convergences (divergences) of heat and momentum. The equivalent temperature changes due to the induced circulation are one-third to one-fourth of the changes due to the mean meridional circulation at altitudes above 150 km in the equatorial region.

Spatial and temporal variations of the storm-time winds are responsible for the differences between Lagrangian and mean Eulerian trajectories of individual fluid elements. Such trajectory calculations show that the material transport of fluid does not occur all the way from the source regions to the equator. However, storm-generated waves, reaching the equator within 3 hours of storm onset, initiate fluid motions at low latitudes.

Abstract

A two-dimensional pole-to-pole numerical model with background solstitial winds has been used to study the global dynamical response of the thermosphere to high-latitude energy inputs associated with a model geomagnetic storm. This model storm has four distinct pulses of heat input over a 12-h period.

The thermospheric wave response to the sustained part of the storm heat input consists in the establishment of a global meridional circulation that is initiated in about 3 to 4 hours after storm commencement and never quite reaches steady state in the simulation.

The main purpose of this study is to investigate the interaction between the disturbances and the mean meridional flow associated with the storm. It is shown that this interaction can be represented in terms of an induced circulation. This induced circulation is forced by the transient nature of the eddy flux convergences (divergences) of heat and momentum. The equivalent temperature changes due to the induced circulation are one-third to one-fourth of the changes due to the mean meridional circulation at altitudes above 150 km in the equatorial region.

Spatial and temporal variations of the storm-time winds are responsible for the differences between Lagrangian and mean Eulerian trajectories of individual fluid elements. Such trajectory calculations show that the material transport of fluid does not occur all the way from the source regions to the equator. However, storm-generated waves, reaching the equator within 3 hours of storm onset, initiate fluid motions at low latitudes.

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