Bouncing ion clusters in the plasma sheet boundary layer observed by Cluster-CIS

A. Keiling, G. K. Parks, H. Reme, I. Dandouras, J.M. Bosqued, M. Wilber, 
M. McCarthy, C. Mouikis, E. Amata, B. Klecker, A. Korth, R. Lundin, H.U. Frey 


We report on ion beams injected into the plasma sheet boundary layer (at or near
the separatrix) at distances >39 R E and up to 169 R E that bounced several
times back and forth (up to three echoes) while remaining in coherent bunches
before thermalizing in the central plasma sheet. These bouncing ion clusters
(BIC) interacted with the far-tail current sheet with a possible curvature
parameter, kappa, of less than 2. The existence of these BIC shows that ion
beams can interact several times nonadiabatically with the far-tail current
sheet and still remain coherent. Owing to the large-scale E X B drift, echoes
also appeared in the central plasma sheet (CPS) after several bounces. The
echoes had higher energies compared with the initially injected ion cluster
which can be attributed to additional nonadiabatic acceleration during their
second and third interaction with the tail current sheet. After multiple
bounces, the ion cluster became thermalized isotropic plasma mixing with the
CPS. The three BIC events presented here were identified on the basis of the
energy dispersion slopes associated with the ions. Simple model calculations
showed, however, that in the case of these far-tail ion injections the
1:3:5:etc.-ratios of travel distances for echoes, used as diagnostics for
near-Earth adiabatic BIC, are not valid. This is largely due to a significant
shortening of the tail field lines, caused by earthward convection, during the
large ion travel times. The model calculations also reproduced newly observed
properties such as concave dispersion slopes for the echoes. Furthermore, we
argue here that the energy dispersion of the BIC was dominated by a
time-of-flight effect. The injection region for the three BIC events, determined
on the basis of this time-of-flight interpretation, covered broad ranges of
delta X (GSE) = 26Ð40 R E. Two BIC events occurred during the substorm recovery
phase; the other BIC event occurred during quiet geomagnetic activity. For two
BIC events, UV images were available showing that they were magnetically
connected to the poleward arc of the double oval. One BIC event was also
conjugate to a small active region inside the poleward arc. We conclude that
these nonadiabatic BIC are different from the adiabatic BIC that are routinely
reported in the CPS. 

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Journal of Geophysical Research, 110, A09207, doi:10.1029/2004JA010497, 2005