Abstract
Abstract. Conjunctions in the cusp between the four Cluster spacecraft and SuperDARN ground-based radars offer unique opportunities to compare the signatures of transient plasma injections simultaneously in the high-altitude dayside magnetosphere and in the ionosphere. We report here on such observations on 17 March 2001, when the IMF initially northward and duskward, turns southward and dawnward for a short period. The changes in the convection direction at Cluster are well correlated with the interplanetary magnetic field (IMF) By variations. Moreover, the changes in the ionosphere follow those in the magnetosphere, with a 2–3min delay. When mapped into the ionosphere, the convection velocity at Cluster is about 1.5 times larger than measured by SuperDARN. In the high-altitude cusp, field and particle observations by Cluster display the characteristic signatures of plasma injections into the magnetosphere suggestive of Flux Transfer Events (FTEs). Simultaneous impulsive and localized convection plasma flows are observed in the ionospheric cusp by the HF radars. A clear one-to-one correlation is observed for three successive injections, with a 2–3min delay between the magnetospheric and ionospheric observations. For each event, the drift velocity of reconnected flux tubes (phase velocity) has been compared in the magnetosphere and in the ionosphere. The drift velocity measured at Cluster is of the order of 400–600ms–1 when mapped into the ionosphere, in qualitative agreement with SuperDARN observations. Finally, the reconnected flux tubes are elongated in the north-south direction, with an east-west dimension of 30–60km in the ionosphere from mapped Cluster observations, which is consistent with SuperDARN observations, although slightly smaller. Key words. Ionosphere (plasma convection) – Magnetospheric physics (magnetopause, cusp, and boundary layers; magnetosphere-ionosphere interactions)
Highlights
Plasma entry from the solar wind into the magnetosphere occurs mainly through the dayside magnetopause
The drift velocity measured at Cluster is of the order of 400–600 m s−1 when mapped into the ionosphere, in qualitative agreement with SuperDARN observations
The convection velocity measured at Cluster after mapping into the ionosphere, is systematically about 1.5 times larger than at SuperDARN, but the relative variations are at both locations very consistent
Summary
Plasma entry from the solar wind into the magnetosphere occurs mainly through the dayside magnetopause. Three main regions just inside the magnetopause with different plasma characteristics, have been recognized since the initial survey by Heos-2 (Haerendel et al, 1978): the low-latitude boundary layer (LLBL), the high-altitude cusp and the plasma mantle Across these boundary regions, a competition exists between diffusion and reconnection for mass, momentum and energy transfer from the solar wind to the magnetosphere. Elphic et al (1990) showed a clear association between an FTE, identified near the magnetopause by its magnetic bipolar signature observed by the ISEE2 satellite, and poleward-moving ionospheric flow bursts and auroral forms observed by the EISCAT radar and photometers. Characteristic FTE signatures are observed by Cluster in the magnetosphere, and duskward flow bursts are simultaneously observed by the CUTLASS radars in the ionosphere, at the magnetic footprint of Cluster. A detailed comparison for three successive injections shows that the local plasma convection velocity and the drift velocity of the flux tubes, evaluated in the ionosphere and at 7–8 RE altitude (and mapped into ionosphere) are fully consistent
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