Electrodynamic Coupling and Dissipation of Thermospheric Winds

A so far simplified model considers the response of the magnetized ionosphere
to neutral winds and the so created effective electrodynamic coupling in
neutral atmosphere dynamics.  The effect resembels viscosity, but the
geomagnetic field couples winds also over large distances.  As a prominent
example the Sq variations are presented as a system that couples the winds
between hemispheres at magnetically conjugate points. The interaction between
hemispheres tends to force the large scale wind systems towards alignment with
magnetic coordinates and towards mirror symmetry with respect to the magnetic
equator. This is, however, for the Earth's thermosphere, never completed
because the time constant exceeds the 24 hours over which dynamics driven by
the energy input from solar radiation creates new winds.

Wind differences are so reduced and kinetic energy gets dissipated. From
observed magnetic Sq variations we estimate that a typical average dissipation
rate by interhemisphere electrodynamic coupling is roughly 0.1 to 1 % of the
heating rate resulting from the absorption of EUV solar radiation.

The same model applies when a neutral wind varies along the geomagnetic field
within the dynamo layer of the ionosphere, for example due to tides and gravity
waves. As a result such neutral wind variations also tend to get evened out and
Joule heat is produced. At mid and high latitudes so upward propagating gravity
waves get damped when they reach to ionospheric dynamo region.