scostep 2010 (stp12) - Leibniz-Institut für Atmosphärenphysik an der ...

STP12 Abstracts
Berlin, 12 - 16 July 2010
SCOSTEP Symposium 2010
High Latitude Thermal Cells Induced by Ion Drag Driven Gyres
Walterscheid Richard 1 , Crowley Geoff 2
1 The Aerospace Corporation, 2 ASTRA
Under typical conditions, the ions driven into motion by the high latitude electric field
generates a pair of counter-rotating anticyclonic and cyclonic gyres in the neutral wind
circulation. When viscous and ion-drag forces are not too strong dynamical adjustment of the
atmosphere acts to bring the winds into an approximate gradient wind balance between the
inertial forces (Coriolis and centrifugal) and the pressure gradient. These forces act in the
same direction for cyclonic gyres and in the opposite direction for anticyclonic gyres. When
the flow is not too strong or too curved the Coriolis force is dominant and dynamical
adjustment gives a pair of high (warm) and low (cold) pressure cells, associated respectively
with the anticyclonic and cyclonic gyres. However, when the flow is sufficiently rapid the
centrifugal force dominates and both gyres give relatively cold low pressure cells. The
thermal changes induced by dynamics are adiabatic and are in addition to the changes induced
by the diabatic heat sources.
We have examined the balance of forces with a 3-D general circulation model of the upper
atmosphere (TIME-GCM). We infer the balance of forces normal to the motion (centrifugal,
Coriolis and pressure gradient) forces. We have examined the balance of forces and the
pressure and thermal response to these forces during quiet and active times.
The quiet-time structure is radically changed when the atmosphere is spun up into rapid
motion during active periods. During active periods structures form that are more
complicated than the structure of the forcing itself. Centers of relative low density air are
found on both the dawn and dusk sides with a trough of low density air over the pole
connecting them. The intrusion of low density air over the pole splits the region of high
density air that exists under quiet conditions giving two high density centers, one toward the
midnight side and other toward the noon side. This gives the four cell pattern simulated by
Crowley et al., 1989, 1996. We find that this structure evolves when the flow is too rapid to
sustain the quiet-time warm high-pressure anticyclonic cells.
Crowley, G., B. A. Emery, R. G. Roble, H. C. Carlson, and D. J. Knipp (1989),
Thermospheric dynamics during September 18 – 19, 1984: 1. Model simulations, J. Geophys.
Res., 94, 16,925– 16,944.
Crowley, G., J. Schoendorf, R. G. Roble, and F. A. Marcos (1996), Cellular structures in the
high-latitude thermosphere, J. Geophys. Res., 101, 211–223, doi:10.1029/95JA02584.