Procedure: thm_part_slice2d_getmag Purpose: Helper function for thm_part_slice2d. Retrieves an average of the b-field vectors over the slice's time range either from the particle structures or a user specified tplot variable. Input: ds: (pointer) Pointer array to particle distribution structures. trange: (double) Two element time range. mag_data: (string) Name of tplot variable containing b-field data. Output: return value: (float) Average magnetic field vector or -1 on error. Notes: $LastChangedBy: aaflores $ $LastChangedDate: 2014-07-31 11:16:59 -0700 (Thu, 31 Jul 2014) $ $LastChangedRevision: 15632 $ $URL: svn+ssh://thmsvn@ambrosia.ssl.berkeley.edu/repos/spdsoft/trunk/projects/themis/spacecraft/particles/slices/thm_part_slice2d_getmag.pro $