Effects of Thermospheric Motions on the Polar Wind' A Time-Dependent Numerical Study

This paper presents the results of a numerical study investigating the effects of low-altitude shortduration plasma heating episodes t .... h a• ho,-iztmta! frictional heating) ,,,, t .... i•nt heavy i,,,, outflows from the polar ionosphere using a time-dependent model. In a previous set of calculations, where the effects of high-altitude transient heat sources were investigated, the low-altitude neutral atmosphere (z < 800 km) acted as a very efficient heat sink, absorbing most of the energy conducted to this region and thus preventing large differences between the ion and neutral temperatures. One purpoqe of the present study was to investigate whether realistic, specified low-altitude frictional heating rates, based on published experimental data, can result in elevated ion temperatures and lift a part of the heavy ion population over the gravitational barrier before the extra energy is lost to the neutral atmosphere. In our model calculations the specified heating generated an upflowing O + disturbance (with a normalized peak flux of • 108 cm -2 s -•) moving upward along the open magnetic field line with a velocity of •2 km/s. The spatial extent of the disturbance was approximately equal to the disturbance velocity times the duration of the heat source. The model results provide a possible mechanism to explain recent European Incoherent Scatter observations of high-latitude O + upwellings.