Magnetic field of Mars: Summary of results from the aerobraking and mapping orbits

The Mars Global Surveyor (MGS) Magnetic Field Investigation was designed to provide fast vector measurements of the ambient magnetic field in the near -Mars environment and over a wide dynamic range. The fundamental objectives of this investigation were to (1) establish the nature of the magnetic fie ld of Mars; (2) develop appropriate models for its representation; and (3) map the Martian crustal remanent field (if one existed) to a resolution con sistent with the spacecraft orbit altitude and ground track separation. Imp ortant and complementary objectives were the study of the interaction of Ma rs with the solar wind and of its ionosphere. The instrumentation is a syne rgistic combination of a twin-triaxial, fluxgate magnetometer system and an electron reflectometer. The twin-magnetometer system allows the real-time detection of spacecraft-generated fields, while the electron reflectometer adds remote magnetic field sensing capabilities as well as information abou t the local electron population. After Mars orbit injection in September 19 97 and through the aerobraking (AB) and science-phasing orbits (SPO) that f ollowed, observations were acquired from more than 1000 elliptical orbits w ith periapses ranging from 85 to 170 km above Mars' surface. Following inje ction into the final similar to 400 km altitude circular-mapping orbit, dat a have been acquired from more than 6000 orbits in the fixed 02100-1400 loc al time plane. Major results obtained so far by the Magnetometer/Electron R eflectometer (MAG/ER) investigation in the course of the mission include (1 ) the determination that Mars does not currently possess a magnetic field o f internal origin (dynamo), (2) the discovery of linear, strongly magnetize d regions in its crust, closely associated with the ancient, cratered terra in of the highlands in the southern hemisphere, and (3) multiple magnetic " cusps" that connect the crustal magnetic sources to the Martian tail and sh ocked solar wind plasma. The solar wind interaction with Mars is therefore similar in many ways tic) that at Venus and at an active comet, primarily a n ionospheric/atmospheric interaction. A comet-like "magnetic pileup" regio n and boundary develop that stand off the solar wind, and mass loading by p ickup ions of planetary origin plays an important role in defining interact ion regions and overall geometry. This paper focuses primarily on the resul ts obtained by the magnetometer (MAG) portion of the investigation during t he MGS aerobraking, science-phasing, and mapping orbits. A companion paper on this issue summarizes the results obtained from the Electron Reflectomet er (ER) sensor.