Geoscience Reference
In-Depth Information
scale, such a non-adiabatic heating favors ions with large mass-to-charge
ratios because of their large gyration periods. This process may be respon-
sible for the species-dependent energization observed in the storm-time ring
current (e.g., Ref. 23). At Mercury, one may expect this non-adiabatic heat-
ing to affect plasma sheet H
+
populations due to the short-time scale of
magnetic transitions.
This is illustrated in Fig. 5 that shows model H
+
trajectories in a
5-second dipolarization identical to that in Fig. 4. Here, test protons were
initialized with 100 eV energy and 90
◦
pitch angle at different (grey-coded)
positions along the dipolarizing field line. It is clearly apparent from Fig. 5
that most of the test H
+
are subjected to prominent magnetic moment
enhancement and energization (up to several tens of keV), with the excep-
tion of the ions initialized close to the planet surface that have small Larmor
radii and small gyration periods. One expects that a number of these
non-adiabatically accelerated ions will be lost at the dusk magnetopause
because of their large Larmor radii after dipolarization.
In this regard, note that ions at Mercury generally do not conserve
their magnetic moment even in the steady state case (see, e.g., Fig. 1) so
Fig. 5. Model H
+
orbits during a 5-second dipolarization of the magnetic field lines:
(
left
) trajectory projections in noon-midnight and equatorial planes, (
right
)energy,and
magnetic moment versus time.
