Subsolar Magnetic Field Strength at 400 km
altitudes at Mars
This page contains information and files related to the determination of the
subsolar magnetic field magnitude at 400 km altitudes above Mars, inferred
from measurements on the Martian dayside by MGS MAG/ER.
Subsolar field strength determination
Subsolar field strength is calculated on an orbit-by-obit basis using MGS
mapping data (taken in a circular ~400 km orbit). For each orbit, we
take all measured field strengths when the spacecraft was at solar zenith
angles less than 110o. We then omit observations taken above
longitudes and latitudes associated with moderate or strong crustal
magnetic fields. If enough observations remain, then we fit the
remaining data to a cos(SZA) function, and extrapolate to the field
magnitude at SZA=0o.
This technique has now been used in a number of published, accepted, and
submitted papers. To my knowledge, the first of these is a manuscript by
Brain et al., Variability of the Altitude of the Martian Sheath,
GRL, doi:10.1029/2005GL023126, 2005 - available here.
Similarities to the solar wind pressure proxy by Crider et al.,
JGR, 2003.
The information presented here is quite similar in concept and result to
the solar wind pressure proxy published by Dana Crider and co-authors in
a manuscript in JGR in 2003 (doi:10.1029/2003JA009875). In this work
magnetic field pressure measured in the Martian `magnetic pileup region'
is used as a proxy for the upstream solar wind dynamic pressure. The
resulting pressure proxy has been made publicly available by Dana Crider
at http://www.vho.nasa.gov/.
These two methods have been compared, and appear to agree.
Key differences include:
The values reported on this webpage are field strengths at the
subsolar point, and not upstream pressures. There are a number of
assumptions inherent in extrapolating to an upstream pressure, and we
chose not to make these assumptions and report only subsolar field
strength. This should be sufficient for distinguishing times of low
upstream dynamic pressure from times of high upstream pressure.
The solar zenith angle cutoff is less restrictive for the
technique reported here.
The fitting funtion is slightly different for the two techniques
(the one here is linear in cosine and includes a constant offset,
while the Crider et al. technique is quadratic in cosine).
The method for exlcuding data above crustal fields is slightly
different in the two techniques.
Important Caveats
Upstream pressure can change on any timescale, from seconds to
minutes to hours to days. Therefore, determination of the
subsolar magnetic field strength may not be reliable over an
entire MGS 2-hour orbital period. Please use appropriate caution
when using the information on this page.
In its ~400 km mapping orbit, MGS spends as much as 20 percent of
its time above the magnetic pileup region (in the sheath),
in a region where magnetic pressure is likely not the dominant
pressure term. Therefore, extrapolation to a specific value of
upstream dynamic pressure is much less reliable during these
times. We recommend that these data be used to distinguish high
pressure periods from low pressure periods.
I use only publicly available MGS data, since those data have
been fully calibrated. Therefore, the timeseries will usually be
several months "behind" the current date. You can always email
me to see whether the page is as up-to-date as possible.
