Abstract
Abstract. Previous studies have shown that dayside equatorward edge of coherent HF radar backscatter having broad Doppler spectral width is coincident with the equatorward edge of the cusp particle precipitation. This enables the boundary between broad and narrow spectral width backscatters (spectral width boundary) in the dayside magnetic local time sector to be used as a proxy for the open/closed field line boundary. The present case study employs magnetically conjugate SuperDARN coherent HF radars to make an inter-hemispheric comparison of the location and variation of the spectral width boundaries. Agreement between the magnetic latitudes of the boundaries in both hemispheres is remarkable. Correlation coefficients between the latitudes of the boundaries are larger than 0.70. Temporal variation of the spectral width boundary follows the same equatorward trend in both hemispheres. This is consistent with the accumulation of open flux in the polar cap by dayside low-latitude magnetopause reconnection, expected when IMF Bz is negative. Boundaries in both hemispheres also exhibit short-lived poleward motions superposed on the general equator-ward trend, which follows the onset of substorm expansion phase and a temporary northward excursion of IMF Bz during substorm recovery phase. There is an interhemispheric difference in response time to the substorm occurrence between two hemispheres. The spectral width boundary in the Southern Hemisphere starts to move poleward 10 min earlier than that in the Northern Hemisphere. We discuss this difference in terms of interhemispheric asymmetry of the substorm breakup region in the longitudinal direction associated with the effect of IMF By.Key words. Ionosphere (ionosphere-magnetosphere interactions; plasma convection) – Magnetospheric physics (magnetopause, cusp, boundary layers)
Highlights
It is very important for the study of magnetosphereionosphere coupling processes to identify the location of the open/closed field line boundary (OCFLB) in the ionosphere
The dayside auroral zone provides a good target for HF radars (Milan et al, 1998) and a relationship between the magnetospheric cusp particle precipitation into the ionosphere and the HF radar backscatter from the ionospheric F-region has been reported by a number of authors (Baker et al, 1990, 1995; Rodger et al, 1995; Yeoman et al, 1997)
If we assume that the broad spectral width is caused by the time-varying electric field closely related to the cusp particle precipitation, the offset between spectral width boundary and particle trapping boundary seen in the present observation can be explained by the effect of finite poleward flow suggested by Lockwood (1997)
Summary
It is very important for the study of magnetosphereionosphere coupling processes to identify the location of the open/closed field line boundary (OCFLB) in the ionosphere. The dayside auroral zone provides a good target for HF radars (Milan et al, 1998) and a relationship between the magnetospheric cusp particle precipitation into the ionosphere and the HF radar backscatter from the ionospheric F-region has been reported by a number of authors (Baker et al, 1990, 1995; Rodger et al, 1995; Yeoman et al, 1997). There have been only a few simultaneous observations of the spectral width boundary using data from magnetic conjugate radars (Pinnock et al, 1999; Milan and Lester, 2001) These authors assumed the spectral width boundary to be a proxy for the true OCFLB and estimated the dayside reconnection electric field from the convection flow across the OCFLB. A significant interhemispheric difference in the response time to the occurrence of substorm expansion was identified
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