Geoscience Reference
In-Depth Information
Buchner and Zelenyi.
5
They contribute to the formation of thin current
sheets and are responsible for the beamlets that are frequently observed in
the Earth's plasma sheet boundary layer (e.g., Refs. 8 and 9).
In Fig. 1, resonant particles and associated beamlets are characterized
by short residence times (upper left panel), whereas quasi-trapped parti-
cles exhibit longer residence times as well as larger energies due to repeated
interactions with the current sheet and larger drift in the
Y
-direction (lower
left panel). It is thus clearly apparent from Fig. 1 that solar wind protons
entering Mercury's magnetosphere exhibit an extreme sensitivity to initial
conditions at the magnetopause. That is, a slight change in injection posi-
tion leads to a large variation in the adiabaticity parameter
κ
, an effect
which is of importance for hybrid simulations and contrasts with the situ-
ation prevailing at Earth.
The small width of Mercury's magnetotail has another important impli-
cation for the dynamics of charged particles. Indeed, in an attempt to esti-
mate the contribution of planetary ions to the magnetospheric populations,
Delcourt
et al.
2
showed that low-energy Na
+
ions sputtered from the planet
surface may gain access to the magnetotail via convection over the polar
cap, in a like manner to low-energy ions upflowing from the topside iono-
sphere at Earth. In the magnetospheric lobes, these ions are subjected to a
prominent parallel energization due to an
E
B
related centrifugal effect
that is more pronounced (because of a larger angular speed of the convect-
ing field lines) than at the Earth. Once they interact with the magnetotail
current sheet, these Na
+
ions are accelerated in a non-adiabatic manner and
may subsequently precipitate onto the planet surface, hence contributing to
regolith sputtering. Delcourt
et al.
2
showed that, on the whole, exospheric
Na
+
form a substantial (several tenths of ions/cm
3
) source of plasma for
the magnetotail. As envisioned in the “geopause” interpretation framework
of Moore and Delcourt,
10
this raises the question of the net magnetospheric
contribution of planetary material versus that of the solar wind, especially
when the planet occupies a large volume of the magnetosphere as is the
case at Mercury.
An example of planetary ion behaviors is shown in Fig. 2 that shows the
trajectories of test Na
+
launched from two distinct latitudes in the dayside
sector (left panels), together with their energy and magnetic moment as a
function of time (center panels). As for the right panels of Fig. 2, they show
the results of systematic Na
+
trajectory computations, with grey-coded flux
and energy of precipitating ions as a function of longitude and latitude. It
is apparent from the right panels of Fig. 2 that, after transport into the
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