Modeling and Consequences of Supersonic Winds in the Thermosphere 
 
H. G. Demars and R. W. Schunk 
 
Center for Atmospheric & Space Sciences, Utah State University, Logan, Utah 
84322-4405 
 
Supersonic flow has been observed in the high-latitude thermosphere via DE-2 
satellite measurements.  The supersonic flow events were observed to occur most 
frequently in the dawn sector at high magnetic latitudes and in the altitude 
range from about 300-600 km.  To gain theoretical insight into these supersonic 
flow events, we used a high-resolution model of the global thermosphere to study 
the geophysical conditions that give rise to supersonic flow.  In particular, we 
considered the dependence of the neutral wind on IMF orientation, magnetic 
activity level, convection pattern shape, solar activity level, season, and 
universal time.  We also studied the extent to which narrow plasma convection 
channels can induce supersonic neutral winds.  We found that the predominant 
factor controlling the thermospheric wind speed at high latitudes is the 
magnitude of the cross-polar-cap potential, indicating that interaction with the 
convecting ionosphere can drive the thermosphere to supersonic speeds. Supersonic 
neutral winds occur at and above 300 km within 3 hours for cross-polar-cap potentials 
greater than about 140 kV. The region of supersonic flow predicted by the model 
corresponds closely to the region where supersonic flow events were most frequently 
observed by the DE satellite.