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model implicitly assumes the existence of a neutral line in the far tail, the
MHD results show the formation of a near-Mercury neutral line (NMNL)
near 2
R
M
radial distance (right panel of Fig. 3). In the terrestrial mag-
netosphere, the formation of such a neutral line near the planet is com-
monly associated with substorm activity. It is apparent from Fig. 3 that
the existence of such a NMNL can significantly alter the Na
+
trajectories
and change the ratio of ions that precipitate onto the planet surface. That
is, ions that reach the region of tailward flow in the plasma sheet do not
return to the planet regardless of their
κ
parameter. These results suggest
that precipitation and recycling of planetary ions at Mercury depend upon
the magnetic activity and the large-scale convection pattern.
3. Adiabaticity Breaking Due to Small Temporal Scales
During the Mariner-10 pass of 1974, the magnetic field first pointed tailward
and abruptly turned northward after closest approach of the planet. Inter-
estingly, high-energy (several tens of keVs and above) electron injections
were recorded in conjunction with this rapid change of the magnetic field
orientation, a feature which is reminiscent of that observed during substorm
dipolarization at Earth. This suggests that the hermean magnetotail may
be subjected to substorm cycles as well (e.g., Ref. 14), although Luhmann
et al.
15
put forward that these observations may provide indication of pro-
cesses directly driven by the solar wind.
If substorms occur at Mercury in a like manner to Earth, significant
particle energization is to be expected due to the electric field induced by
relaxation of the magnetic field lines. By means of two-dimensional simu-
lations, Ip
16
actually demonstrated that protons may be accelerated up to
10-15 keV during such reconfiguration events. On the other hand, because of
the short-time scales that characterize the hermean environment, it may be
anticipated that electrons are subjected to significant energization as well.
In a recent study, Delcourt
et al.
17
examined this issue using a rescaled ver-
sion of a time-dependent particle code previously developed to investigate
the dynamics of charged particles during substorms at Earth (see, Ref. 18
for details on the modeling technique).
An example of the results obtained by Delcourt
et al.
17
is presented
in Fig. 4. As mentioned above, characteristic time scales at Mercury are
expected to be smaller than those at Earth by about a factor 30 (e.g.,
Ref. 19), and a short-lived (5 s) dipolarization was accordingly considered
here. The left panels of Fig. 4 show selected electron trajectories during
