Magnetic flux ropes in the Martian atmosphere:: Global characteristics

We report observations of magnetic fields amplitude, which consist of a series of individual spikes in the Martian atmosphere. A minimum variance analysis shows that these spikes form twisted cylindrical filaments. These small diameter magnetic filaments are commonly called magnetic flux ropes. We examine the global characteristics of magnetic flux ropes, which are observed on 5% of the elliptical orbits of Mars Global Surveyor. Flux ropes are more often observed in Venus' atmosphere (70% of the orbits). In this paper we report some of the global characteristics of the flux ropes identified in the Martian atmosphere. No flux ropes are observed in the southern hemisphere of Mars. Most of them occur at high solar zenith angles, close to the terminator plane, and at high latitude with altitudes below 400 km. The orientation of the flux ropes appears random while in the case of Venus the orientation is more horizontal near the terminator for altitudes greater than 200 km. We have identified fewer flux ropes for SZA between 40 to 60 deg and for SZA lower than 20 deg, like in the case of Venus (Elphic and Russell, 1983b). Statistically, Mars' ionosphere with SZA range between 40(circ) to 60(circ) is less magnetized than near the subsolar point. As the Martian ionosphere is quite often magnetized by the magnetic components of the crustal field, this crustal magnetic field seems to inhibit the flux ropes formation in the southern hemisphere. However, some orbits without crustal magnetic field, called magnetic cavities, were observed without flux ropes. So the flux ropes formation process seems to be suppressed by another factor, like the solar wind dynamic pressure for Venus (Krymskii and Breus, 1988).