Cluster observations of a structured magnetospheric cusp

Abstract

Abstract. On 18 April 2002 the Cluster spacecraft crossed through the northern outer magnetospheric cusp region during 16:25-17:55 UT when the solar wind dynamic pressure was rather low (<2 nPa) and IMF B z was more negative than IMF B y . The Cluster data from the FGM, CIS, PEACE, EFW, WHISPER and STAFF instruments reveal that the cusp is structured with three anti-sunward ion flow events of durations ≈1.5, 17.5 and 19.0 min, with bulk plasma flow roughly parallel to the magnetopause toward north. The ion and electron densities within the events are much greater than those outside. The zonal electric field in the ion flow events turns eastward as expected from V ×B effect. The sharp inward boundaries of the ion flow events cross the four spacecraft in one time sequence, and the outward boundaries of the events cross the spacecraft in the reverse time sequence. The observations studied using magnetosphere and magnetopause models suggest that the structured cusp is a temporal feature that arises due to three inward and outward movements of the magnetopause by about 1.5 R E so that Cluster, while crossing through the cusp, happened to be in the magnetosheath (ion flow event) and cusp alternately. The magnetopause moved due to the changes in the solar wind dynamic pressure by up to 100%.

Multiple cusps during an extended northward IMF period with a significant By component

“…[8] Previous triple cusp event was observed by Cluster during southward IMF [Zong et al, 2004;Balan et al, 2006]. In this paper, we report observations of fourfold cusp event found during northward IMF on 21-22 March 2001.…”

Magnetosphere–ionosphere coupling during the CME events of 07–12 November 2004

“…Though the CMEs started with a weak front end, its impact compressed and deformed the magnetosphere such that Cluster, which was in the southern magnetospheric lobe, suddenly found itself in the magnetosheath. Such compressions of the magnetopause by comparatively small changes in the solar wind pressure have been reported earlier (e.g., Cargill et al, 2005;Balan et al, 2006). While Cluster was crossing the compressed magnetosheath under steady solar wind pressure and steady velocity components, IMF B y became zero for about 1.5 h when the magnetosphere shifted back to the Cluster position.…”

Response of the magnetosheath‐cusp region to a coronal mass ejection

“…The magnetopause and cusp move back and forth during periods of high and low solar wind pressure, and dawnward/duskward during periods of dawnward/duskward solar wind velocity component [ Lundin et al , 2001]. Since the exterior cusps are regions where plasma pressure and magnetic field pressure balance [ Cargill et al , 2004], a small change in the solar wind dynamic pressure can lead to a large effect in the cusp [ Balan et al , 2006]. The occurrence of double cusp during periods of large IMF B y and small IMF B z [ Wing et al , 2001], and cusp regions that look like multiple cusps have also been reported [ Newell et al , 1989; Zong et al , 2004]; multiple cusp regions are due to magnetospheric (or cusp) oscillations (with respect to observing spacecraft) driven by changes in solar wind pressure, azimuthal flow and IMF B y [e.g., Zong et al , 2004].…”