Letter to the Editor
A statistical study of the location and motion of the HF radar cusp

Abstract. The large-scale and continuous monitoring of the ionospheric cusp region offered by HF radars has been exploited in order to examine the statistical location and motion of the equatorward edge of the HF radar cusp as a function of the upstream IMF B_Z component. Although a considerable scatter is seen, both parameters have a clear influence from the north-south component of the IMF. Excellent agreement is achieved with previous observations from low altitude spacecraft data. The HF radar cusp region is seen to migrate equatorward at a rate of 0.02° min^-1 nT^-1 under IMF B_Z south conditions, but remains static for IMF B_Z north. The motion of the cusp implies an addition of magnetic flux of ~ 2 × 10⁴ Wbs^-1 nT^-1 under IMF B_Z south conditions, equivalent to a reconnection voltage of 20 kV nT^-1, which is consistent with previous estimates from case studies on both the dayside and nightside regions.

Key words. Ionosphere (auroral ionosphere) – Magnetospheric physics (magnetosphere-ionosphere interaction; solar wind magnetosphere interactions)