Factors controlling the location of the Bow Shock at Mars !Vignes D., Acuña M., Connerney J., Crider D., NASA Goddard Space Flight Center, Greenbelt, MD, USA !Rème H., Mazelle C., Centre d'Étude Spatiale des Rayonnements, Toulouse, France
Day of Year 184 − July 3rd, 1998
Abstract: θ
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We found that the high crustal magnetic sources, found in the southern hemisphere, do not seem responsible for the Bow Shock variability. Contrary to many expectations there is no obvious strong one to one correlation between the location of the highest crustal sources and the variability of the shock position.
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The Bow Shock identification inbound and outbound are shaded on the figure with labels BSi and BSo respectively. Bow Shock crossings are identified by an increase of the magnetic field amplitude and a simultaneous increase of electron flux.
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During the first year of the Mars Global Surveyor (MGS) mission, 553 shock crossings have been identified from a total of 363 orbits. Figure 2, on the left, displays the locations of MGS Bow Shock crossings. The Bow Shock position is found highly variable.
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Figure 2: Martian bow shock fits from MGS and Phobos 2.
The IMF orientation can influence the pickup effect and thus the Bow Shock variability. The pick-up effect is at its maximum value when the solar wind flow and the IMF (cone angle) are perpendicular.
However, the results do show a clear tendency for the Martian Bow Shock to be found at slightly greater heights over the southern hemisphere.
Figure 5 shows terminator distance as function of the clock angle. In the northern hemisphere, the mean value of the terminator distance for large cone angle is 13% greater than for small cone angle in the same hemisphere. While, in the southern hemisphere we found a small differences between cases of large and small cone angle. Thus, the shock appears significantly farther from the planet in the hemisphere of locally upward electric field.
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Further, we also need to look to the hemisphere of locally upward convective electric field. The relative clock angle corresponds to the angle between the projection into the terminator plane of the radial vector at the shock crossing location and the IMF. Then, a Bow Shock crossing in the northern hemisphere occurs for clock angles between 00 to 1800.
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Figure 4: MGS Bow Shock positions versus subsolar longitude.
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Figure 6 shows the extrapolated terminator distance as function of the clock angle like in Figure 5. The Bow Shock crossings distance which occur at low solar wind dynamic pressure, displayed in blue, are 10% farthest from Mars than those which occur with high solar wind dynamic pressure. The effect of the solar wind dynamic pressure on the Bow Shock variability appears to be in the same order of the mass loading effect.
Bow shock distance versus clock angle for high (o) and low (.) cone angle 4.5
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Bow shock distance versus clock angle
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Figure 3: Map of the crustal magnetic field of Mars [Acuña et al., 1999].
Figure 4 shows the extrapolated MGS terminator Bow Shock positions versus the longitude of the subsolar point. If the variability of the Bow Shock location were the caused by high field regions corotating with Mars, the figure should show a distinct maximum for the Terra Sirenum region. The mean shock position appears to be independent of the planetary longitude with no net variation when all of the crossings are considered.
Bow shock distance versus subsolar longitude
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Axisymmetric fits of all these Bow Shock crossings, using conic section, is displayed and compared with the earlier models of Slavin et al. [1991] and Trotignon et al. [1993]. MGS and Phobos 2 observations give nearly the same mean Bow Shock fits for sunspot numbers of ~30 - 90 and ~140 180, respectively, suggesting that the mean surface is independent of the phase of the solar cycle [Vignes et al., 2000]. Others factors that could explain this variability include crustal magnetic sources, the interplanetary magnetic field (IMF) orientation, and upstream solar wind parameters. Figure 3, on the right, shows the map of Mars with the position of the crustal magnetic sources, mainly confined in the southern hemisphere [Acuña et al., 1999].
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Figure 1: Magnetic field and electron data recorded on July 3rd, 1998.
Extrapolated terminator distance (RM)
Bow Shock crossings positions and fits
314 eV 191 eV 116 eV 79 eV 61 eV 47 eV 36 eV 27 eV 21 eV
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However, the shock appears farthest from Mars in the hemisphere of locally upward interplanetary electric field consistent with the idea that mass loading play a role in controlling the Bow Shock location, which confirms previous results. Further, the Bow Shock position appears slightly dependent on the solar wind dynamic pressure.
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Figure 1, on the right, shows an example of data measured during an elliptical orbit of MGS. The 3 upper panels show the orientation and intensity of the magnetic field B measured by magnetometers onboard MGS. The lower panel shows the flux of electron for energies between 20 to 300 eV and with a time resolution as high as 2 sec.
The shape of the shock has been determined by examining the MGS spacecraft Magnetometer / Electron Reflectometer (MAG/ER) data. The location of the shock was found highly variable.
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Figure 5 & 6:
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MGS Bow Shock positions versus clock angle
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