Abstract
Previous work has shown that ionospheric HF radar backscatter in the noon sector can be used to locate the footprint of the magnetospheric cusp particle precipitation. This has enabled the radar data to be used as a proxy for the location of the polar cap boundary, and hence measure the flow of plasma across it to derive the reconnection electric field in the ionosphere. This work used only single radar data sets with a field of view limited to ~2 h of local time. In this case study using four of the SuperDARN radars, we examine the boundary determined over 6 h of magnetic local time around the noon sector and its relationship to the convection pattern. The variation with longitude of the latitude of the radar scatter with cusp characteristics shows a bay-like feature. It is shown that this feature is shaped by the variation with longitude of the poleward flow component of the ionospheric plasma and may be understood in terms of cusp ion time-of-flight effects. Using this interpretation, we derive the time-of-flight of the cusp ions and find that it is consistent with approximately 1 keV ions injected from a subsolar reconnection site. A method for deriving a more accurate estimate of the location of the open-closed field line boundary from HF radar data is described.Key words: Ionosphere (ionosphereâmagnetosphere interactions; plasma convection) · Magnetospheric physics (magnetopause · cusp · and boundary layers)
Highlights
Determining the location of the open/closed ®eld-line boundary (OCB) in the ionosphere, over a wide longitude sector, is of critical importance for manyA relationship between magnetospheric cusp particle precipitation into the ionosphere and HF radar backscatter from the ionospheric F region has been reported on by a number of workers (Baker et al, 1990, 1995; Rodger et al, 1995; Yeoman et al, 1997; Milan et al, 1999) and has been used as an alternative means of determining the OCB at noon (Baker et al, 1997; Pinnock et al, 1999)
A relationship between magnetospheric cusp particle precipitation into the ionosphere and HF radar backscatter from the ionospheric F region has been reported on by a number of workers (Baker et al, 1990, 1995; Rodger et al, 1995; Yeoman et al, 1997; Milan et al, 1999) and has been used as an alternative means of determining the OCB at noon (Baker et al, 1997; Pinnock et al, 1999)
The ion energies derived compare favourably with those observed in the low-altitude cusp by DMSP satellite, the higher energies being close to those often observed at the equatorward edge of the cusp particle precipitation (Newell and Meng, 1991, see their Fig. 2). This is consistent with the equatorward edge of the high spectral width region mapping to the equatorward edge of the cusp particle precipitation (Baker et al, 1990, 1995) and the sharp increase in Pc1 wave activity associated with the equatorward edge of the cusp particle precipitation (e.g. Erlandson and Anderson, 1996) being the cause of the large Doppler spectral widths (Andre et al, 1999)
Summary
Determining the location of the open/closed ®eld-line boundary (OCB) in the ionosphere, over a wide longitude sector, is of critical importance for many. The equatorward edge of radar scatter showing large Doppler spectral widths has been shown to be coincident with the equatorward edge of the cusp particle precipitation (Baker et al, 1990, 1995) for a southward interplanetary magnetic ®eld (IMF). We assume that the cusp particle precipitation travels at the speed of the ions and not at the electron speed This oset has previously only been considered in one dimension (e.g. along a line of meridian) and has been estimated to be 150 km (Rodger and Pinnock, 1997) near noon. The results rearm the intimate link between radar scatter showing the characteristics de®ned in Baker et al (1995) and the cusp particle precipitation and must be taken in to account when the radar scatter is used as a proxy for the open/ closed ®eld line boundary A method for producing more accurate estimates of the location of the OCB is described
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