More than four years of data taken from Mars Global Surveyor during its Mapping Phase Orbits (360--420 km altitude) over low field regions were examined. The nightside magnetic field data were binned according to a proxy solar wind pressure calculated from the dayside measurements. When the crustal field contribution calculated from the internal field model (FSU90) is removed, the distribution of residuals is bi-valued in the sunward component. Pass-by-pass inspections of the data sometimes show a sudden reversal of field, which occur on successive passes. Analysis indicates that for these orbits MGS traverses a current sheet that separates the two lobes of Mars' magnetotail. These results indicate that on the nightside the major contributor to the external field is the draping of the Interplanetary Magnetic Field about the planet and that care must be taken when utilizing such data for modeling Mars' internal field.