Modeling Thermospheric Dynamics Under all Solar, Geomagnetic, and Lower Atmosphere Conditions

Abstract:

During very active geomagnetic conditions, neutral thermospheric dynamics is dominated by magnetospheric momentum and energy input. Mid and high latitude horizontal winds can exceed many hundreds of meters per second, vertical winds many tens of meters per second, and the impulsive energy input can drive global wave surges of one to two hundred meters per second. The latitudinal change in global temperature and pressure from the energy input drives an altered circulation, neutral composition structure, and the disturbance dynamo, all of which have dramatic impact on the ionosphere. During more typical solar and geomagnetic activity conditions the balance between these solar and magnetospheric drivers, and forcing from the lower atmosphere, is not so clear. A whole spectrum of waves, including migrating and non-migrating tides and gravity waves, propagate from sources in the lower atmosphere, increase in amplitude, and drive winds throughout the thermosphere and at all latitudes. Some of these waves break in the mesosphere and lower thermosphere and mix the constituents. Some penetrate further into the thermosphere driving steep vertical gradients, and as they dissipate by molecular viscosity they deposit momentum. Much of the variability in electrodynamics is driven by the larger-scale tidal spectrum of waves driving neutral winds in the lower thermosphere dynamo region. The shorter period and smaller scale spectrum of gravity waves also drive neutral winds, and their impact is readily apparent in the ionosphere from incoherent scatter and dynasonde observations. The still sparse direct observations of neutral winds means that sometimes it is the impact on the ionosphere that must be used to indicate the presence of neutral atmospheric winds and waves.