Abstract
Abstract. A statistical study has been performed by using two years of DMSP (Defense Meteorological Satellite Program) plasma observations to investigate the seasonal effect of SAPS (subauroral polarization stream) on the ion upflow in the duskside ionosphere of the Northern Hemisphere. There are obvious upflows occurring in the topside ionosphere around the SAPS region, exceeding 200 m s−1 at winter solstice, indicating an important relationship between SAPS and the local plasma upward motion. Both SAPS and ion upward velocities show similar seasonal variations, largest in winter and smallest in summer, irrespective of geomagnetic activity. A good correlation is found and a linear relationship is derived between SAPS and the ion upflow velocities. During December solstice the average upflow flux can reach about 2 × 108 cm−2 s−1 for more disturbed periods, which is comparable to the typical upflow flux in the dayside cusp region. The depression of the ion temperatures around the peak SAPS region can be understood in terms of the adiabatic cooling. The hot ion cools down when expanding into the low ion concentration region. The electron temperature elevates around the SAPS region because of the reduced Coulomb cooling in the low ion density region. Both the changes of ion and electron temperatures are larger in winter than in summer, however, for Kp < 4 the electron temperatures are almost seasonably independent. The present work highlights the important role of the SAPS-related frictional heating at mid-latitudes on the local formation of the strong upward flow, which might provide a direct ionospheric ion source for the ring current and plasmasphere in the duskside sector.
Highlights
A lot of studies have concentrated on ion upflow at high latitudes, an interesting and important feature at midlatitude, the effect of subauroral polarization stream (SAPS) on ion upflow, has not been investigated statistically
Since the location of SAPS is conjugate to the peak ring current energy density (Yeh et al, 1991; Foster and Vo, 2002), and coincides with the equatorward edge of the ion plasma sheet (Southwood and Wolf, 1978; Anderson et al, 1993; Huang and Foster, 2007), the upward ions in the SAPS region might contribute to the development of the storm time ring current
We have presented the seasonal variations of SAPS and ion upflow around subauroral latitudes by using DMSP observations in the top ionosphere
Summary
The acceleration mechanisms of the ion upflow include parallel electric field, magnetic momentum pumping, wave– particle interaction, auroral plasma cavity, etc., which are generally effective above 1000 km (Sharp et al, 1977; Papadopoulos, 1977; Whalen et al, 1978; Retterer et al, 1986; Singh et al, 1989; Yau and Andre, 1997; Moore et al, 1999). Since the location of SAPS is conjugate to the peak ring current energy density (Yeh et al, 1991; Foster and Vo, 2002), and coincides with the equatorward edge of the ion plasma sheet (Southwood and Wolf, 1978; Anderson et al, 1993; Huang and Foster, 2007), the upward ions in the SAPS region might contribute to the development of the storm time ring current From this aspect a statistical study of SAPS effects on ion upflow might aid understanding the ionosphere–inner magnetosphere coupling. Yeh and Foster (1990) have reported about a large ion upflow event exhibiting a velocity of 3 km s−1 at 1000 km altitude around the SAPS region observed by Millstone Hill during the February 1986 storm. In this paper we show statistical results of the vertical upflow (i.e. drift speed and ion/electron temperature) related to SAPS in the northern subauroral topside ionosphere (∼ 800 km) by using two years of DMSP (Defense Meteorological Satellite Program) observations during 2002 and 2003.
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