Evidence for impulsive solar wind plasma penetration through the dayside magnetopause

R Lundin,C Carlson,H Rème,I Dandouras,M Dunlop,J.-A Sauvaud,V Formisano,J M Bosqued,E Möbius,M Mccarthy,L Eliasson,A Korth,B Lavraud,E Amata,A Balogh,G K Parks,L M Kistler,B Klecker

doi:10.5194/angeo-21-457-2003

Abstract

Abstract. This paper presents in situ observational evidence from the Cluster Ion Spectrometer (CIS) on Cluster of injected solar wind "plasma clouds" protruding into the day-side high-latitude magnetopause. The plasma clouds, presumably injected by a transient process through the day-side magnetopause, show characteristics implying a generation mechanism denoted impulsive penetration (Lemaire and Roth, 1978). The injected plasma clouds, hereafter termed "plasma transfer events", (PTEs), (Woch and Lundin, 1991), are temporal in nature and relatively limited in size. They are initially moving inward with a high velocity and a magnetic signature that makes them essentially indistinguishable from regular magnetosheath encounters. Once inside the magnetosphere, however, PTEs are more easily distinguished from magnetopause encounters. The PTEs may still be moving while embedded in an isotropic background of energetic trapped particles but, once inside the magnetosphere, they expand along magnetic field lines. However, they frequently have a significant transverse drift component as well. The drift is localised, thus constituting an excess momentum/motional emf generating electric fields and currents. The induced emf also acts locally, accelerating a pre-existing cold plasma (e.g. Sauvaud et al., 2001). Observations of PTE-signatures range from "active" (strong transverse flow, magnetic turbulence, electric current, local plasma acceleration) to "evanescent" (weak flow, weak current signature). PTEs appear to occur independently of Interplanetary Magnetic Field (IMF) Bz in the vicinity of the polar cusp region, which is consistent with observations of transient plasma injections observed with mid- and high-altitude satellites (e.g. Woch and Lundin, 1992; Stenuit et al., 2001). However the characteristics of PTEs in the magnetosphere boundary layer differ for southward and northward IMF. The Cluster data available up to now indicate that PTEs penetrate deeper into the magnetosphere for northward IMF than for southward IMF. This may or may not mark a difference in nature between PTEs observed for southward and northward IMF. Considering that flux transfer events (FTEs), (Russell and Elphic, 1979), are observed for southward IMF or when the IMF is oriented such that antiparallel merging may occur, it seems likely that PTEs observed for southward IMF are related to FTEs.Key words. Magnetospheric physics (magnetopause, cusp, and boundary layers; magnetosphere-ionosphere interactions; solar-wind magnetosphere interactions)

Highlights

The entry of magnetosheath plasma through the dayside magnetopause and the associated transfer of energy into the magnetosphere remains a key problem in magnetospheric physics
No matter how tempting it may be to consider “ideal” conditions, such as “frozen in flux tubes”, such oversimplifications may become misleading, in particular since observations tell us that the transfer of energy from the solar wind into the magnetosphere includes a number of key parameters: for example, the solar wind plasma dynamics, the magnetosphere plasma dynamics, the ionospheric plasma acceleration and outflow, the generation of magnetospheric currents and fields interconnecting dynamo and load regions are all embedded in ambient electric- and magnetic fields generated remotely but projecting onto the locus of observation
We have analyzed a set of Cluster observations of magnetosheath plasma transfer events, PTEs, through the dayside magnetopause, an analysis that leads to the following conclusions:

Summary

Introduction

The entry of magnetosheath plasma through the dayside magnetopause and the associated transfer of energy into the magnetosphere remains a key problem in magnetospheric physics. PTEs relate from an observational point of view more to impulsive plasma penetration introduced by Lemaire and Roth (1978): i.e. plasma intrusions through the magnetopause may induce current signatures similar to those associated with FTEs (Lundin, 1989). The ambient magnetic field frozen into the plasma (except at the x-line) gives the topology and provides the conditions for transfer or the topology and conditions for transfer are defined by the intrinsic properties of the plasma, inducing local perturbation fields and currents These aspects require careful consideration on basis of observational data. The cases selected here are less ambiguous from the point of view of separating magnetopause encounters from PTEs. The events represent “blobs” of streaming magnetosheath plasma embedded in magnetospheric plasma, injections that may protrude deep into the magnetosphere on closed magnetic field lines. Frequent PTEs associated with local/cold plasma acceleration may become an important source of magnetospheric plasma as described by Sauvaud et al (2001)

Observations

22 Mar 2001 01:00–02:45 -1 7 2

Findings

Discussion and conclusions