2.5D Particle and MPD simulations
2.5D particle simulations of the solar wind interaction
with the magnetized regions on the surface of
the Moon confirm the earlier 2D MHD result that mini-magnetospheres can form
around the magnetic anomalies. A dipole buried 100 km below the
surface with a field strength equal to
50 nT at the surface and 10 nT at 100 km above the
surface held the solar wind off of the surface and caused a bow shock
and a magnetopause to form. But the boundary separating the bow shock
and magnetopause becomes ambiguous as the two structures merge due to
the small scale size of the mini-magnetospheres.
Acceleration of solar wind particles occurs at the shock.
Inside the magnetopause, the Lunar electrons remained highly
magnetized and exhibited fluid-like behavior. The Lunar ions, on the other
hand, become demagnetized. Outside of the magnetopause,
the solar wind ions and electrons exhibited fluid-like behavior.
Small scale and non-ideal MHD effects can be included into
fluid simulations by adding Hall and pressure gradient terms in
Ohm's Law, creating a magnetoplasma dynamics (MPD) model.
The small scale effects allow for field-aligned currents
and electric fields which look qualitatively similar to those in
the particle simulations, but do not appear to change the overall
shape of the mini-magnetosphere. The extra components of the electric
field indicate the presence of charge separation at the shock surface,
due to the momentum difference between ions and electrons,
and the near the Lunar surface, due to non-ideal
MHD behavior inside the mini-magnetosphere. The 2.5D MPD model can
replicate the ion demagnetization seen in the 2.5D particle simulations.
2.5D Particle Simulations
The surface magnetic field is compressed by the solar wind but can
still hold the solar wind particles off the surface, creating
a boundary that resembles both a bow shock and a magnetopause.
Both regions of enhanced Lunar electron density (c) correspond to
density voids in the Lunar ion population (d).
The tick marks on the axes of the density plots equal 104.4 km (and 9 grid
points) while the tick marks on the axes of the
magnetic field line plot are 34.8 km
apart. The three spots, and labels, in (e) refer to positions
discussed in particle distributions.
(From Harnett and Winglee, JGR, vol. 107, 2002).
Particle acceleration occurs, but in the mini-magnetosphere, two
separate acceleration regions form, one at the sub-solar point,
primarily for electrons and the second in the downstream region,
primarily for ions.
(From Harnett and Winglee, JGR, vol. 107, 2002).
The total electron distributions show that the bulk flow in the upstream
region is slowed while the plasma is heated. The locations of the distributions
in plots (a), (b), and (c) are noted in the first figure.
Close to the mini-magnetosphere, the velocity distributions
show significant heating, both isotropic and anisotropic heating.
(From Harnett and Winglee, JGR, vol. 107, 2002).
Decreasing the bulk speed of the solar wind decreases the ion
gyroradius, and translates to an increase in the magnitude
of the magnetic field on the
Lunar surface. This leads to inflation of the mini-magnetosphere.
(From Harnett and Winglee, JGR, vol. 107, 2002).
Varying the ion to electron mass ratio for the surface ionosphere, on
the other hand, had
little effect on the size and shape of the mini-magnetosphere. (From Harnett and Winglee, JGR, vol. 107, 2002).
2.5D MPD Simulations
When the IMF is anti-parallel to the dipole moment, the anomalous surface
field is parallel. The resulting mini-magnetosphere is rounded in shape as
the IMF drapes over the surface field. The size and shape is identical to
the MHD simulations.
(From Harnett and Winglee, JGR, vol. 107, 2002).
The IMF is parallel to the dipole moment and therefore anti-parallel to the
surface field. Reconnection erodes the surface field leading to a
mini-magnetosphere that is much smaller and elongated. Large currents in the
cusp regions indicate electron precipitation.
(From Harnett and Winglee, JGR, vol. 107, 2002).
The pressure gradient term indicates the presence of charge seperation.
Charge seperation is most prominate at the shock surface where ions can
penetrate further into the mini-magnetosphere due to their larger momentum.
This is further supported by the presence of an electric field in this region
that points out from the shock surface.
(From Harnett and Winglee, JGR, vol. 107, 2002).
The Hall term indicates the presence of non-ideal MHD behaviour. In the
region that the Hall term is non-zero, the electric field points towards the
surface indicating a deficit of ions relative to the electrons near the
surface. Momentum effects would lead to a electric field in the opposite
direction, therefore the MPD simulations also indicate that the ions
become demagnetized near the surface.
The small size of the the parallel mini-magnetosphere
suggests that it is in the regime where ideal MHD breaks down. The Hall term
has the largest magnitude for this case but it is non-zero over a smaller
region leading to little change in the size and shape of the mini-magnetosphere
from the ideal MHD case.
(From Harnett and Winglee, JGR, vol. 107, 2002).
The case with no IMF to compare with the the particle simulations.
(From Harnett and Winglee, JGR, vol. 107, 2002).
eharnett at ess.washington.edu
Last modified: Tue Jun 22 14:29:13 PDT 2004
