| 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) |