External fields on the nightside of Mars at Mars Global Surveyor mapping altitudes

Abstract

[1] 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. Citation:

Mars Global Surveyor Measurements of the Martian Solar Wind Interaction

“…Comparison of MGS premapping MPB crossings to a model for the boundary shape by Verigin et al (2004) suggests that crustal magnetic fields act to make the tail boundary (the nightside extension of the MPB) thicker by up to 1000 km, so that both intrinsic and induced fields influence the tail structure. MGS mapping data revealed a magnetic flux asymmetry between the two lobes of the induced magnetotail (Ferguson et al, 2005). More detailed study of nightside current sheet crossings (inferred from field reversals) shows that the thin current sheets measured at the 400 km mapping altitude of MGS (shown in Figure 12) have locations and variability consistent with reconnection of the draped IMF to crustal magnetic fields .…”

“…Therefore, a negative net flux at the 2am orbit may be compensated by a net positive flux at other local times. Ferguson et al (2005) also observed an asymmetry in the sunward component of the nightside magnetic field, after a spherical harmonic crustal field model was subtracted (see Figure 16). Further, the asymmetry between the number of observations with B toward the Sun/planet vs. away grew with upstream solar wind pressure.…”

“…One unexplained feature of this map is a ∼ (20%) reduction in average field strengths on the day side relative to the night side in some regions of strong horizontal crustal magnetic field. Other variability in magnetic field demonstrated using MGS data includes the influence of upstream pressure and IMF direction on the night side (Ferguson et al, 2005;Brain et al, 2006). These analyses show that external influences have not been entirely removed from the data also used to construct models for the crustal magnetic fields (Purucker et al, 2000;Arkani-Hamed, 2001;Cain et al, 2003;Langlais et al, 2004).…”

“…Histograms of the sunward component of magnetic field on the Martian night side, after the Cain et al (2003) crustal magnetic field model has been subtracted, separated according to upstream pressure. From Ferguson et al (2005Ferguson et al ( ). (2006 showed an asymmetry in the nightside radial field component controlled by IMF direction.…”

The magnetic field draping direction at Mars from April 1999 through August 2004

“…It contains the night side plasma sheet (Dubinin et al, 1991;Ferguson et al, 2005), a steady outflow of escaping oxygen ions (Kallio et al, 1995), and is the region used as the primary source of information about unperturbed crustal magnetic fields, since external fields are weaker and the plasma environment is less turbulent than the day side . While external fields are nearly entirely horizontal on the day side of Mars, the radial field component comprises a larger fraction of the total field on the night side in MGS data (Brain et al, 2003).…”