Smooth transition of plasmasphere to plasma trough as observed by IMAGE RPI 

J. Tu, P. Song, and B. W. Reinisch 


Upper hybrid resonance noise and continuum emissions are routinely detected by the 
radio plasma imager (RPI) onboard the IMAGE satellite when IMAGE transits the 
plasmasphere and plasma trough regions. The lower frequency cutoffs of those nar- 
row emission bands provide an estimate of the electron plasma frequency along the 
satellite orbit. A steep density gradient, which defines the plasmapause location, is 
commonly seen when IMAGE transits from the plasmasphere to the plasma trough. It 
appears, however, that at many occasions the transition from plasmaspheric densities 
to the plasma trough is smooth without a signature of the plasmapause. Such smooth 
transitions occur at various magnetic local times, and they can be observed during pe- 
riods of both disturbed and quiet geomagnetic activities. While this smooth transition 
may be associated with a plasmaspheric plum during disturbed times, it is not well 
understood why it happens during magnetically quiet periods when there are no plas- 
maspheric plums. We report the IMAGE/RPI observations of smooth transitions of the 
plasmasphere to the plasma trough and discuss the implications to plasma dynamics 
related to the plasmapause formation mechanisms. 

_______________
36th COSPAR Scientific Assembly, Beijing, China, 16Ð23 July 2006