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significant plasma supply to Mercury's magnetosphere. This has motivated
several numerical studies using a variety of modeling approaches (MHD,
single-particle, hybrid) (e.g., Refs. 1-3). The purpose of this paper is to
review some results obtained from single-particle simulations. These sim-
ulations were performed using either an analytical model of the hermean
magnetosphere (adapted from Ref. 4) or results of MHD simulations. Sec-
tion 2 is dedicated to consequences of small spatial scales, whereas Sec. 3
focuses on consequences of small temporal scales.
2. Adiabaticity Breaking Due to Small Spatial Scales
Because of the weak intrinsic magnetic field of Mercury and of the enhanced
dynamical pressure of the solar wind, the hermean magnetosphere exhibits
spatial scales that are much smaller (by about a factor 7) than those of the
terrestrial magnetosphere. This raises questions for the non-linear dynamics
of charged particles since their Larmor radii must be small compared to
the characteristic scale of magnetic field variations for their motion to be
adiabatic (equivalently, for the guiding center approximation to be valid).
As a matter of fact, a simple calculation of the adiabaticity parameter κ
defined as the square root of the minimum curvature radius to maximum
Larmor radius ratio 5 reveals that this parameter likely is smaller than 3
throughout most of the magnetotail for keV electrons and ions (e.g., Ref. 2);
hence, prominent non-adiabatic features.
In a study intended to examine the entry of solar wind ions in Mer-
cury's magnetosphere, Delcourt and Leblanc 6 showed that access of these
ions to the inner magnetosphere is restricted to a limited domain of the
phase space. This is illustrated in the right panels of Fig. 1 that show the
color-coded altitude, energy, and time of flight of solar wind protons as a
function of initial latitude and longitude at the magnetopause. Here, test
H + were initialized with 100 eV and 90 pitch angle. Test H + launched with
small pitch angles or large energies (
1 keV) were found to escape into the
magnetotail due to transit times significantly smaller than the convection
time scale (see, e.g., Fig. 1 of Ref. 6). In the right panels of Fig. 1, white
areas correspond to test ions that reach the model boundary at 6 R M radial
distance without intercepting the magnetopause or the planet surface. It is
clearly apparent from these panels that the solar wind H + that precipitate
onto Mercury's surface originate from a limited longitudinal interval in the
dawn sector (note that longitudes of 0 and -90 correspond to local times
of 1200 and 0600, respectively).
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