J. S. Halekas

Initial mapping and interpretation of lunar crustal magnetic anomalies using Lunar Prospector magnetometer data

Maps of relatively strong crustal magnetic field anomalies detected at low altitudes with the magnetometer instrument on Lunar Prospector are presented. On the lunar nearside, relatively strong anomalies are mapped over the Reiner Gamma Formation on western Oceanus Procellarum and over the Rima Sirsalis rille on the southwestern border of Oceanus Procellarum. The main Rima Sirsalis anomaly does not correlate well with the rille itself but is centered over an Imbrian-aged smooth plains unit interpreted as primary or secondary basin ejecta. The stronger Reiner Gamma anomalies correlate with the locations of both the main Reiner Gamma albedo marking and its northeastward extension. Both the Rima Sirsalis and the Reiner Gamma anomalies are extended in directions approximately radial to the center of the Imbrium basin. This alignment suggests that Imbrium basin ejecta materials (lying in many cases beneath the visible mare surface) are the sources of the nearside anomalies. If so, then the albedo markings associated with the stronger Refiner Gamma anomalies may be consistent with a model involving magnetic shielding of freshly exposed mare materials from the solar wind ion bombardment. Two regions of extensive magnetic anomalies are mapped in regions centered on the Ingenii basin on the south central farside and near the crater Gerasimovic on the southeastern farside. These regions are approximately antipodal to the Imbrium and Crisium basins, respectively. The Imbrium antipode anomaly group is the most areally extensive on the Moon, while the largest anomaly in the Crisium antipode group is the strongest detected by the Lunar Prospector magnetometer. A consideration of the expected antipodal effects of basin-forming impacts as well as a combination of sample data and orbital measurements on the nearside leads to the conclusion that the most probable sources of magnetic anomalies in these two regions are ejecta materials from the respective impacts. In both regions the strongest individual anomalies correlate with swirl-like albedo markings of the Reiner Gamma class visible on available orbital photography.

Variability of the altitude of the Martian sheath

Received 31 March 2005; revised 18 August 2005; accepted 25 August 2005; published 24 September 2005. [1] Using electron energy spectra, we identify time periods when the Mars Global Surveyor (MGS) spacecraft is in or above the Martian magnetic pileup boundary (MPB). We use more than five years of data to develop a statistical picture of the location of the MPB relative to the MGS mapping altitude near 400 km. We show for the first time that the MPB location is sensitive to interplanetary magnetic field (IMF) orientation and to Martian season, and confirm a dependence upon solar wind pressure. We confirm that crustal magnetic sources raise the altitude of the MPB, and demonstrate that sheath electrons populate magnetic cusp regions in the southern hemisphere. During southern summer strong crustal fields near the subsolar point raise the altitude of the MPB over the entire dayside, implying that Martian crustal fields modify the solar wind interaction globally. Citation: Brain, D. A., J. S. Halekas, R. Lillis, D. L. Mitchell, R. P. Lin, and D. H. Crider (2005), Variability of the altitude of the Martian sheath, Geophys. Res. Lett., 32, L18203,

Mapping of crustal magnetic anomalies on the lunar near side by the Lunar Prospector electron reflectometer

Lunar Prospector (LP) electron reflectometer measurements show that surface fields are generally weak in the large mare basalt filled impact basins on the near side but are stronger over highland terranes, especially those lying antipodal to young large impact basins. Between the Imbrium and Nectaris basins, many anomalies correlate with the Cayley and Descartes Formations. Statistical analyses show that the most strongly magnetic nearside terranes are Cayley-type light plains, terra materials, and pre-Imbrian craters. Light plains and terrae include basin impact ejecta as a major component, suggesting that magnetization effects from basin-forming impacts were involved in their formation. The magnetization of pre-Imbrian craters, however, may be evidence of early thermal remanence. Relatively strong, small-scale magnetic anomalies are present over the Reiner Gamma feature on western Oceanus Procellarum and over the Rima Sirsalis rille on the southwestern border of Procellarum. Both Apollo subsatellite and LP data show that the latter anomaly is nearly aligned with the rille, though LP magnetometer and reflectometer data show that the anomaly peak is actually centered over a light plains unit. This anomaly and the Reiner Gamma anomaly are approximately radially aligned with the center of Imbrium, suggesting an association with ejecta from this basin.

On the origin of aurorae on Mars

] We report observations by Mars Global Surveyor(MGS) of thousands of peaked electron energy spectrasimilar to terrestrial auroral electrons. They are observed onthe Martian nightside, near strong crustal magnetic sources.The spectra have peak energies ranging from 100 eV –2.5 keV, and fluxes near the peak are 10–10000 timeshigher than typical nightside spectra. They occur onmagnetic field lines that connect the shocked solar windto crustal magnetic fields, and on adjacent closed field lines.Their detection is directly controlled by the solar wind,suggesting that magnetic reconnection is required for theirobservation. We calculate that the most energeticdistributions could produce atmospheric emission withintensity comparable to that recently reported from theMars Express (MEX) spacecraft. Half of the most energeticexamples occur during the passage of space weather eventspast Mars, suggesting that a disturbed plasma environmentis favorable for electron acceleration along magnetic fieldlines.

MAVEN observations of the response of Mars to an interplanetary coronal mass ejection

Coupling between the lower and upper atmosphere, combined with loss of gas from the upper atmosphere to space, likely contributed to the thin, cold, dry atmosphere of modern Mars. To help understand ongoing ion loss to space, the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft made comprehensive measurements of the Mars upper atmosphere, ionosphere, and interactions with the Sun and solar wind during an interplanetary coronal mass ejection impact in March 2015. Responses include changes in the bow shock and magnetosheath, formation of widespread diffuse aurora, and enhancement of pick-up ions. Observations and models both show an enhancement in escape rate of ions to space during the event. Ion loss during solar events early in Mars history may have been a major contributor to the long-term evolution of the Mars atmosphere.

Evidence for negative charging of the lunar surface in shadow

Research at the University of California, Berkeley, was supported by NASA through subcontract LRI-99-101 from the Lunar Research Institute.

The spatial distribution of planetary ion fluxes near Mars observed by MAVEN

We present the results of an initial effort to statistically map the fluxes of planetary ions on a closed surface around Mars. Choosing a spherical shell ~1000 km above the planet, we map both outgoing and incoming ion fluxes (with energies >25 eV) over a 4 month period. The results show net escape of planetary ions behind Mars and strong fluxes of escaping ions from the northern hemisphere with respect to the solar wind convection electric field. Planetary ions also travel toward the planet, and return fluxes are particularly strong in the southern electric field hemisphere. We obtain a lower bound estimate for planetary ion escape of ~3 × 1024 s−1, accounting for the ~10% of ions that return toward the planet and assuming that the ~70% of the surface covered so far is representative of the regions not yet visited by Mars Atmosphere and Volatile EvolutioN (MAVEN).

Strong plume fluxes at Mars observed by MAVEN: An important planetary ion escape channel†

The accepted version of the above article was posted prematurely on September 11, 2015. The final version of record will be made fully available at later date along with a special collection of related papers.

Episodic detachment of Martian crustal magnetic fields leading to bulk atmospheric plasma escape

[1] We present an analysis of magnetic field and supra-thermal electron measurements from the Mars Global Surveyor (MGS) spacecraft that reveals isolated magnetic structures filled with Martian atmospheric plasma located downstream from strong crustal magnetic fields with respect to the flowing solar wind. The structures are characterized by magnetic field enhancements and rotations characteristic of magnetic flux ropes, and characteristic ionospheric electron energy distributions with angular distributions distinct from surrounding regions. These observations indicate that significant amounts of atmosphere are intermittently being carried away from Mars by a bulk removal process: the top portions of crustal field loops are stretched through interaction with the solar wind and detach via magnetic reconnection. This process occurs frequently and may account for as much as 10% of the total present-day ion escape from Mars.

Observations of aurorae by SPICAM ultraviolet spectrograph on board Mars Express: Simultaneous ASPERA‐3 and MARSIS measurements

We present a new set of observations of Martian aurorae obtained by Spectroscopy for the Investigation of the Characteristics of the Atmosphere of Mars (SPICAM) on board Mars Express (MEX). Using nadir viewing, several auroral events have been identified on the Martian nightside, all near regions of crustal magnetic fields. For most of these events, two to three consecutive events with variable intensities and separated by a few seconds to several tens of seconds have been observed, whereas simultaneous observations with Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) and Analyzer of Space Plasma and Energetic Atoms (ASPERA-3) have been possible. In this paper, we present the data set for these events and discuss the possible correlation between the measured UV emission by SPICAM, the measured downward and/or upward flux of electrons by ASPERA-3 and the total electron content recorded by MARSIS. Despite the limited coverage of SPICAM ultraviolet spectrograph (UVS) on the Martian nightside (essentially in regions of high crustal magnetic fields), there is however a very good correlation between the regions with the locally smallest probability to be on closed crustal magnetic field lines, as derived from Mars Global Surveyor/Electron Reflectometer (MGS/MAG-ER), and the position of an aurora event. This suggests that the crustal magnetic fields, when organized into cusp-like structure, can trigger the few aurorae identified by SPICAM UVS. It confirms also the good probability, in the cases where SPICAM UVS measured UV emissions, that the increase in the measured total electron content by MARSIS and the simultaneous measured precipitating electron flux by the ASPERA-3/Electron Spectrometer may be related to each other.