The spatial structure of Martian magnetic flux ropes recovered by the Grad-Shafranov reconstruction technique

摘要

We applied the Grad-Shafranov (GS) reconstruction technique to Martian magnetic flux ropes observed by Mars Global Surveyor in order to estimate their spatial structures. This technique can provide a magnetic field map of their cross section from single spacecraft data, under the assumption that the structure is two-dimensional, magnetohydrostatic, and time independent. We succeeded in recovering the spatial structure for 70 events observed between April 1999 and November 2006. The reconstruction results indicate that the flux rope axes were mostly oriented horizontal to the Martian surface and were randomly distributed with respect to the typical plasma streamline. A subset of events with duration longer than 240 s was observed at solar zenith angles larger than 75?. These events all occur downstream from strong crustal magnetic field in the southern hemisphere, indicating an association between the crustal fields and the detected flux ropes. Using the shape and size of the flux ropes obtained from the GS reconstruction, we estimate lower limits on their volume that span 2–3 orders of magnitude, with larger flux ropes observed downstream from strong crustal magnetic fields. Estimated ion escape rates associated with flux ropes are of the order of 10~(22)–10~(23) ion/s, being approximately 10% of previously estimated escape rates during solar minimum.