Probing Mars' crustal magnetic field and ionosphere with the MGS Electron Reflectometer

Abstract

Abstract. The Electron Reflectometer (ER) on board Mars Global Surveyor measures the energy and angular distributions of solar wind electrons and ionospheric photoelectrons. These data can be used in conjunction with magnetometer data to probe Mars' crustal magnetic field and to study Mars' ionosphere and solar wind interaction. During aerobraking, ionospheric measurements were obtained in the northern hemisphere at high solar zenith angles (SZAs, typically -78ø). The ionopause was crossed at altitudes ranging from 180 km to over 800 km, with a median of 380 km. The 400-km-altitude polar mapping orbit allows observations at SZAs from 25 ø to 155 ø in both the northern and southern hemispheres. The near-planet ionosphere and magnetotail structure of the night hemisphere is dominated by the presence of intense crustal magnetic fields, which can exceed 200 nT at the spacecraft altitude. Closed field lines anchored to highly elongated crustal sources form "magnetic cylinders," which exclude solar wind plasma traveling up the magnetotail. When the spacecraft passes through one of these structures, the ER count rate falls to the instrumental background, representing an electron flux drop of at least two orders of magnitude. A map of these flux dropouts in longitude and latitude closely resembles a map of the crustal magnetic sources. When the crustal magnetic cylinders rotate into sunlight, they fill with ionospheric plasma. Since many of these crustal fields are locally strong enough to stand off the solar wind to altitudes well above 400 km, the ionosphere can extend much higher than would otherwise be possible in the absence of crustal fields. Even weak crustal fields may locally bias the median ionopause altitude, which provides an indirect method of detecting crustal fields using ER observations.

On the relation between plasma escape and the Martian crustal magnetic field

“…The Mars Global Surveyor (MGS) findings of strong crustal anomalies at Mars [Acuña et al, 1998] considerably changed this picture. We now expect to find diverging magnetic field "cusps" above Mars [Krymskii et al, 2002] and "minimagnetospheres" [Mitchell et al, 2001] with local magnetic conditions similar to those found on magnetized planets such as the Earth. A distinctive difference, though, is that the crustal magnetization is weak compared to the Earth's dipole field.…”

Solar Wind Electrons Alphas and Protons (SWEAP) Investigation: Design of the Solar Wind and Coronal Plasma Instrument Suite for Solar Probe Plus

“…Berkeley Space Sciences Laboratory, including sounding rockets, Mars Global Surveyor ER (Mitchell et al 2001), Wind 3D-Plasma (Lin et al 1995), Lunar Prospector, FAST Plasma Experiment ), THEMIS (McFadden et al 2008, and MAVEN (Halekas et al 2013;McFadden et al 2015). The UCB group also participated in the development of the STEREO IMPACT SWEA instrument (Sauvaud et al 2007).…”

Modeling of Venus, Mars, and Titan

“…Ma et al (2002) found that crustal magnetic fields did not effect the location of the bow shock but did effect the location of the MPB. The interaction between the piled-up IMF field in the MPB and the crustal magnetic fields also produced small-scale magnetic structures known as magnetocylinders, which had been observed by spacecraft (Mitchell et al 2001). Ma et al (2002) also described the general circulation of ionospheric plasma which, in agreement simulations of Venus' plasma interaction, was upward on the day side ionosphere, day to night at higher altitudes, downward on the night side below about 250 km altitude, and down Mars magnetotail at higher altitudes.…”