Explanation of MAG/ER Drag Pass Results
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                  D.L. Mitchell     19 Aug 1997


During a drag pass, the Electron Reflectometer operates as a Langmuir
probe to measure electron density along the spacecraft trajectory.
MAG/ER drag pass results are summarized in two gif plots, one each for
the inbound and outbound crossings of the ionospheric F1 peak (i.e.,
the altitude of maximum electron density).

The F1 peak altitude is diagnostic for the MTGCM and MarsGRAM and will
be used by the Atmospheric Advisory Group to help assess and monitor
the state of Mars' upper atmosphere during aerobraking.  In the
absence of dust storms, the F1 peak altitude is expected to be in the
range 120-150 km, depending on latitude and solar zenith angle.  At a
given latitude and solar zenith angle, a 5-km increase in the F1 peak
altitude corresponds to a 2-fold increase in the neutral density.
During global dust storms, altitude increases of up to 20 km have been
observed by radio occultation experiments.

The plots show relative electron density (measured by the ER) as a
function of altitude (derived from ephemeris data).  The symbols and
annotation for each plot are as follows:

  solid squares = data used in fit

  open squares = data not used in fit

  solid line = best-fit Chapman layer model near the peak density
 
  box in lower right corner = fit results


For more information, please contact:

David L. Mitchell
Space Sciences Laboratory
University of California
Berkeley, CA  94720

(510) 643-1561

mitchell@ssl.berkeley.edu


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Drag Pass Spacecraft Attitude
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The spacecraft's aerodynamic response during a drag pass results in
attitude oscillations.  The ER is close to ion wakes resulting from
upstream obstructions (the spacecraft body, the -Y solar panel gimbal
and yoke).  As the attitude deviates from the nominal drag pass
orientation, these wakes move relative to the ER.  If the ER is caught
within a wake, it cannot measure ionospheric electrons.  These plots
help to evaluate the case current measurements.

The top panel shows the spacecraft ram velocity relative to Mars.
Below this is the orientation of the velocity vector in payload
coordinates.  The "pitch" is the angle the velocity vector makes with
the -Z direction.  The "roll" is the azimuthal angle of velocity
vector's projection onto the XY plane: 0 degrees points in the +X
direction; +90 degrees points in the +Y direction.  (I am aware that
these are not the usual definitions of pitch and roll.)

The next two panels are the case current, I(Vmax), and the altitude.
(See the two-hour summary plot caption for more information.)