Derivation of the solar wind dynamic pressure at Mars using Mars Global Surveyor data

There is no upstream solar wind monitor at Mars1.  This data set is a proxy for the solar wind dynamic pressure derived from Mars Global Surveyor (MGS) Magnetometer (MAG) measurements.  The rationale behind the proxy is that pressure is conserved in the solar wind interaction with Mars.  At altitudes from ~400-800 km, the dominant pressure term is magnetic field pressure.  We measure the magnetic field with MAG, calculate the pressure (B2/2µo) in that altitude range2 and assume that it balances the incident solar wind pressure.  This proxy is reliable for determining relative solar wind pressure at Mars.  Absolute values may deviate from the proxy.  

The pressure does drop with solar zenith angle (SZA).  This affect is accounted for by fitting a function of SZA3 to the dayside, northern hemisphere4 data from an MGS pass.  The number of points included in the fit is provided in the file.  The standard deviation from the fit is also provided.

Because we do a fit to one orbit's worth of dayside, northern hemisphere data, we obtain one value for PSW every orbit, which has a period of ~2 hr.  The data used for the proxy only come from at most 0.5 hour of that 2 hr period.  The time listed is the time the spacecraft passed through the lowest SZA of the orbit.  The spacecraft continues up through the northern hemisphere after this time. So the time is more of a start time than a center time or stop time5.

The data are from the time period March 8, 1999-June 22, 2004.  Data from Sept. 13, 1997-March 7, 1999 are coming soon.  Data after June 22, 2004 will be posted periodically after the MGS data is released by the PDS.

Each file has 5 columns:
	year
	day of year (with fraction of day after the decimal)
	solar wind pressure (in nPa)
	number of points used in fit
	standard deviation

Please see Crider et al. [2003]6 for a more complete description and verification of the solar wind proxy method.  Data were provided by Mario Acu–a and the MGS MAG/ER team.
 
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Notes:
1	Mars Express is currently in orbit around Mars and can provide solar wind data when it is outside of the Martian bow shock; however, it only arrived in Dec. 2003.
2	From day 67, 1999 all data are from the MGS mapping orbit and, therefore, were acquired near 400 km altitude.  Prior to day 67, 1999, data were acquired at a range of altitudes from 400-800 km.
3	The actual function is B2/cos2(_), where _ is the angle the solar wind ram direction makes to the solar wind obstacle's normal.  We take that obstacle to be the magnetic pileup boundary and use the nominal shape of the MPB given by Vignes et al (GRL 2000) to define the angle _.  On the dayside _ is slightly less than SZA.  On the nightside, _ is much less than SZA.
4	Strong magnetic fields originating in the Martian crust exist and are prevalent in the southern hemisphere.  This technique omits data from both the entire southern hemisphere and the known crustal anomalies in the northern hemisphere.
5	Owing to the obliquity of Mars (25¡), this SZA minimum can occur at Mars latitudes 25¡S < lat < 25¡N.  The data from the southern hemisphere are not used in this proxy.
6	Crider, D. H., D. Vignes, A. M. Krymskii, T. K. Breus, N. F. Ness, D. L. Mitchell, J. A. Slavin, and M. H. Acu–a. A proxy for determining solar wind dynamic pressure at Mars using Mars Global Surveyor data, J. Geophys. Res. 108 (A12): 1461, 10.1029/2003JA009875, 2003.