Martian Magnetic Anomalies
Magnetic Field Model
The anomalous magnetic field model was created by
Cain et. al.
(JGR, vol. 108, 2003), generating a 90 term spherical harmonic expansion.
Below is a map of the magnitude of the magnetic field at 400 km above the
surface and 100 km above the simulation boundary. The dark line indicates
the noon meridian.
Simulation Results
The anomalous magnetic field is both enhanced and eroded by the IMF (here in
the Northward direction). Where the anomalous magnetic field is enhanced,
the ionosphere is protected from weathering by the solar wind. Thus the
scale height is larger in that region, leading to a higher ion density at a
given radius, than regions nearby. In regions where the IMF reconnects to
the anomalous magnetic field, the solar wind is funneled to the surface,
compressing the atmosphere and leading to smaller scale heights and lower
densities. Fly through the magnetic field.
The simulations can predict the location of the bow shock and the magnetic
pileup boundary (MPB). The average locations of the bow shock and MPB as
calculated from over 400 crossings by Vignes et al.
(GRL, vol. 27, pg 49-52, 2000) are shown in the black and white curves below.
Cuts in the pressure at the noon meridean indicate that the magnetic anomalies
can lead to the formation of a mini-magnetopause.
The MPB can not be seen in the pressure data but can be seen in the magnetic
and electric field. The figure below shows the magnitude of the magnetic
field at the noon meridean and the electric field vecotrs in that plane for
southward IMF. The component of the electric field out of the page has been
set to zero for clarity. The white curves again indicate the positions of
the bow shock and MPB as measured by Vignes et al. At the MPB the magnetic
field increases by an order of magnitude. The electric field indicates that
more that IMF pileup is occuring at the MPB. The electric field is generated
by the Hall term in Ohm's Law, thus the ions are becoming demagnetized at the
MPB through some mechanism, like collisions.
eharnett at ess.washington.edu
Last modified: Tue Jun 22 14:21:07 PDT 2004
