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latitudes since particles behave in a non-adiabatic manner throughout most
of the magnetotail. As for the poleward boundary, it can be seen in the
bottom left panel of Fig. 2 that it does not map into the far tail but at
distances of a few planetary radii and that it is due to the fact that ions
intercept the dusk magnetopause in the course of their interaction with the
current sheet. That is, in contrast to the situation prevailing at Earth, the
poleward boundary of precipitation in the right panels of Fig. 2 directly
follows from the finite ion Larmor radius. At large energies and/or large
mass-to-charge ratios, these Larmor radii become comparable to or larger
than the magnetotail width, in which case ions do not execute a full Speiser
orbit and are not reflected toward the planet. Here, it may actually be
suspected that this finite Larmor radius boundary at the poleward edge
of precipitation depends upon ion species (with a cutoff for heavy ions
occurring at lower latitudes than for H
+
) and provides information of the
magnetotail structure, in a like manner to IB.
The results displayed in Figs. 1 and 2 were obtained using a modi-
fied version of the Luhmann and Friesen
4
model together with a two-cell
pattern of magnetospheric convection. For comparison, Fig. 3 shows the
results obtained in the study of Seki
et al.
,
13
that examines the transport
of exospheric Na
+
in the electric and magnetic fields derived from MHD
simulations of the Mercury-solar wind interaction, assuming a southward
orientation of the interplanetary magnetic field (IMF). While the analytical
Fig. 3. Identical to the left panels of Fig. 2 but using electric and magnetic fields derived
from MHD simulations (adapted from Ref. 13).
