Abstract
Abstract. Pulsed ionospheric flows (PIFs) in the cusp foot-print have been observed by the SuperDARN radars with periods between a few minutes and several tens of minutes. PIFs are believed to be a consequence of the interplanetary magnetic field (IMF) reconnection with the magnetospheric magnetic field on the dayside magnetopause, ionospheric signatures of flux transfer events (FTEs). The quasiperiodic PIFs are correlated with Alfvénic fluctuations observed in the upstream solar wind. It is concluded that on these occasions, the FTEs were driven by Alfvén waves coupling to the day-side magnetosphere. Case studies are presented in which the dawn-dusk component of the Alfvén wave electric field modulates the reconnection rate as evidenced by the radar observations of the ionospheric cusp flows. The arrival of the IMF southward turning at the magnetopause is determined from multipoint solar wind magnetic field and/or plasma measurements, assuming plane phase fronts in solar wind. The cross-correlation lag between the solar wind data and ground magnetograms that were obtained near the cusp footprint exceeded the estimated spacecraft-to-magnetopause propagation time by up to several minutes. The difference can account for and/or exceeds the Alfvén propagation time between the magnetopause and ionosphere. For the case of short period ( < 13 min) PIFs, the onset times of the flow transients appear to be further delayed by at most a few more minutes after the IMF southward turning arrived at the magnetopause. For the case of long period (30 – 40 min) PIFs, the observed additional delays were 10–20 min. We interpret the excess delay in terms of an intrinsic time scale for reconnection (Russell et al., 1997) which can be explained by the surface-wave induced magnetic reconnection mechanism (Uberoi et al., 1999). Here, surface waves with wavelengths larger than the thickness of the neutral layer induce a tearing-mode instability whose rise time explains the observed delay of the reconnection onset. The compressional fluctuations in solar wind and those generated in the magnetosheath through the interaction between the solar wind Alfvén waves and the bow shock were the source of magnetopause surface waves inducing reconnection.Key words. Interplanetary physics (MHD waves and turbulence) – Magnetospheric physics (magnetosphere-ionosphere interactions; solar wind-magnetosphere interactions)
Highlights
Ionospheric dynamics near the cusp footprint attest to processes at the dayside magnetopause and in particular, to pulsed magnetic reconnection (Cowley et al, 1991; Lockwood et al, 1993)
We note that the 17 September, 1979 solar wind data discussed by Le et al (1993; their Fig. 2) seem to indicate Alfvenic fluctuations that we associate with the flux transfer events (FTEs) that were observed every 5–6 min by ISEE-1 spacecraft in the magnetosheath
By adopting the propagation time determined by Russell et al (1997), there appears to be a delay of a few minutes between the expected arrival times of the southward turnings of the solar wind BZ component of the Alfven wave and the FTEs observed by ISEE-1
Summary
Ionospheric dynamics near the cusp footprint attest to processes at the dayside magnetopause and in particular, to pulsed magnetic reconnection (Cowley et al, 1991; Lockwood et al, 1993). While the early observations by ISEE satellites provided evidence for quasi-steady dayside reconnection (Paschmann et al, 1979), impulsive reconnection of the magnetosheath and magnetospheric fields is regarded as a primary mechanism for magnetic flux transfer from the solar wind to the magnetosphere (Russell and Elphic, 1978, 1979) Episodes of such flux transfer, referred to as flux transfer events (FTEs), occur with separation times between successive FTEs, ranging from a few minutes to several tens of minutes (Lockwood et al, 1989; Lockwood and Wild, 1993; Kuo et al, 1995, McWilliams et al, 2000). The observed solar wind-to-ground magnetic field cross-correlation lags are compared with estimated propagation times between the spacecraft and the subsolar magnetopause and with the onset of the ionospheric signatures of FTEs observed by the radar
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