Paul A. Cloutier

The solar wind interaction with Mars: Locations and shapes of the bow shock and the magnetic pile-up boundary from the observations of the MAG/ER Experiment onboard Mars Global Surveyor

The Mars Global Surveyor spacecraft was inserted into an elliptical orbit around Mars on September 12, 1997. It includes the MAG/ER instrument with two magnetometers providing in-situ sensing of the ambient magnetic field and an electron reflectometer measuring the local distribution function of the electrons in the energy range of 10 eV to 20 keV. This statistical study deals with the identification and the position of the Bow Shock (BS) and of another plasma boundary, the Magnetic Pile-up Boundary (MPB), proved as permanent by MAG/ER. During the first year of the MGS mission, a total of 290 orbits have been considered to fit the geometric characteristics of these boundaries. The position and shape of these boundaries are compared with previous studies. Good agreement is found with the Phobos 2 observations, suggesting than the mean bow shock and MPB locations are independent of solar cycle phase. The great number of crossings shows that the Bow Shock position and nightside MPB position are highly variable.

Venus-like interaction of the solar wind with Mars

The magnetometer and electron reflectometer experiment (MAG/ER) on the Mars Global Surveyor (MGS) spacecraft has obtained magnetic field and electron data which indicates that the solar wind interaction with Mars is primarily an ionospheric-atmospheric interaction similar to that at Venus. However, the global-scale electric currents and resulting magnetic fields due to the interaction at Mars are locally interrupted or perturbed over distance scales of several hundred kilometers by the effects of paleomagnetic fields due to crustal remanence. In this paper we compare the Mars-solar wind interaction with the Venus-solar wind interaction by selecting MGS orbits which do not show significant magnetic perturbations due to crustal magnetic anomalies, and demonstrate that a number of phenomena characteristic of the Venus-solar wind interaction are also observable at Mars.

Mars Observer magnetic fields investigation

The Mars Observer magnetic fields investigation will provide fast vector measurements of the Martian magnetic field over a wide dynamic range. The fundamental objectives of this investigation are (1) to establish the nature of the magnetic field of Mars, (2) to develop appropriate models for its representation, which take into account the internal sources of magnetism and the effects of the interaction with the solar wind, and (3) to map the Martian crustal remanent field to a resolution consistent with the Mars Observer orbit altitude and ground track separation. The basic instrumentation complement implemented for this mission is a synergistic combination of a dual, triaxial, flux gate magnetometer system and an electron reflectometer with sensors mounted on a spacecraft boom. The dual magnetometer system allows the real-time estimation and correction of spacecraft-generated fields, while the electron reflectometer provides remote magnetic field sensing capabilities. These instruments have an extensive spaceflight heritage, and similar versions of the same have been flown in numerous missions like Voyager, Magsat, International Solar Polar mission (ISPM), Giotto, Active Magnetospheric Particle Tracer Explorers, and Global Geospace Science (GGS). Depending on the telemetry rate supported, a minimum of 2–16 vector samples per second will be acquired. The instrument is microprocessor controlled, can be partially reprogrammed in flight, and supports the packet telemetry protocol implemented for Mars Observer.

Effects of magnetic anomalies discovered at Mars on the structure of the Martian ionosphere and solar wind interaction as follows from radio occultation experiments

The slopes of the electron density profiles obtained by radio occultation experiments at Mars revealed different variations with solar zenith angle in comparison with behavior of the electron density profiles in the magnetic field free ionosphere of Venus. The results obtained by the Mars-Global-Surveyor (MGS) spacecraft show the existence of highly variable and very localized magnetic fields of crustal origin at Mars. Addressing the difference between the ionosphere at Venus and Mars, the scale heights of electron density profiles obtained by radio occultation methods onboard Mariner 9 and Viking 1 are analyzed at altitudes higher than the topside boundary of the photoequlibrium region in the magnetic field-free ionosphere. The local increase of the mean scale height in the altitude region 180–250 km is assumed to be either an effect of a nonhorizontal magnetic field associated with the magnetic anomalies or diffusive equilibrium in the magnetic field free ionosphere. The areas where the scale height of electron density profile is increased in comparison with average one have been selected. The angle between the magnetic field measured by MGS MAG/ER at altitudes 120–250 km and local zenith direction is investigated throughout these selected areas.

Oxygen auger electrons observed in Mars' ionosphere

Over the course of 290 orbits, the Electron Reflectometer onboard Mars Global Surveyor consistently observed a plasma boundary at a median altitude of 380 km, where electron fluxes at energies greater than ∼100 eV change abruptly by about an order of magnitude. Above the boundary, electron energy spectra are consistent with solar wind electrons that have been shocked and then cooled by impact with exospheric neutrals. Below the boundary, electron energy spectra exhibit a broad feature from 20 to 50 eV, which likely results from a blend of unresolved photoionization peaks that have been predicted by published models of ionospheric photoelectrons at Mars. We attribute a second feature at ∼500 eV to oxygen Auger electrons. The 500-eV flux level measured below the boundary responds to variations in the solar soft x-ray flux and is consistent with a balance between photoionization and loss by impact with atmospheric neutral atoms.

Magnetic field draping enhancement at the Martian magnetic pileup boundary from Mars global surveyor observations

[1] The Magnetic Pileup Boundary (MPB) is a sharp and permanent plasma boundary located between the bow shock and the ionospheric boundary, reported so far at Mars and comets. We use Mars Global Surveyor Magnetometer data to do a quantitative analysis of the magnetic field geometry in the surroundings of the Martian MPB. As a result, we report for the first time a dramatic enhancement of the magnetic field draping at this boundary. This new feature, already reported at comets, is independent of the presence of the crustal magnetic sources. Comparisons with similar results across the Martian and cometary magnetotails reveal that the MPB and the magnetotail boundary are connected. Moreover, the study of this feature can help understand the physics of the Venusian magnetic barrier.

A cometary ionosphere model for Io

The ionosphere of Jupiters satellite Io, discovered by the Pioneer 10 radio-occultation experiment, cannot easily be understood in terms of a model of a gravitationally bound, Earth-like ionosphere. Ios gravitational field is so weak that a gravitationally bound ionosphere would probably be blown away by the ram force of the Jovian ‘magnetospheric wind’ — i.e., the plasma corotating in the Jovian magnetosphere. We propose here a model in which the material for Ios atmosphere and ionosphere is drawn from the ionosphere of Jupiter through a Birkeland current system that is driven by the potential induced across Io by the Jovian corotation electric field. We argue that the ionization near Io is caused by a comet-like interaction between the corotating plasma and Ios atmosphere. The initial interaction employs the critical velocity phenomenon proposed many years ago by Alfvén. Further ionization is produced by the impact of Jovian trapped energetic electrons, and the ionization thus created is swept out ahead of Io in its orbit. Thus, we suggest that what has been reported as a day-night ionospheric asymmetry is in fact an upstream-downstream asymmetry caused by the Jovian magnetospheric wind.

Atmospheric ion wakes of Venus and Mars in the solar wind

Abstract A model has been developed for the currents induced in the ionospheres of Venus and Mars by the flowing magnetized solar wind in a previous paper (Cloutier and Daniell, 1973). The altitudes of the ionopauses on both planets, determined from the electrodynamical models of the previous paper, are used here to calculate the total rates of atmospheric mass loss to the solar wind for Venus and Mars. These loss rates are compared to the rates calculated by Michel (1971) based upon the limit of mass loading of the solar wind flow determined from hydrodynamic constraints. The distributions of planetary ions in the downstream wakes of Venus and Mars are calculated, and the interpretation of ion spectrometer measurements from close planetary encounters is discussed.

Ionosphere of Venus - First observations of day-night variations of the ion composition

The Bennett radio-frequency ion mass spectrometer on the Pioneer Venus orbiter is returning the first direct composition evidence of the processes responsible for the formation and maintenance of the nightside ionosphere. Early results from predusk through the nightside in the solar zenith angle range 63� (dusk) to 120� (dawn) reveal that, as on the dayside, the lower nightside ionosphere consists of F1and F2 layers dominated by O2+ and O+, respectively. Also like the dayside, the nightside composition includes distributions of NO+, C+, N+, H+, He+, CO2+, and 28+ (a combination of CO+ and N2+). The surprising abundance of the nightside ionosphere appears to be maintained by the transport of O+ from the dayside, leading also to the formation of O2+ through charge exchange with CO2. Above the exobase, the upper nightside ionosphere exhibits dramatic variability in apparent response to variations in the solar wind and interplanetary magnetic field, with the ionopause extending to several thousand kilometers on one orbit, followed by the complete rertnoval of thermal ions to altitudes below 200 kilometers on the succeeding orbit, 24 hours later. In the upper ionosphere, considerable structure is evident in many of the nightside ion profiles. Also evident are horizontal ion drifts with velocities up to the order of 1 kilometer per second. Whereas the duskside ionopause is dominated by O+ H+ dominates the topside on the dawnside of the antisolar point, indicating two separate regions for ion depletion in the magnetic tail regions.

Evidence of electron impact ionization in the magnetic pileup boundary of Mars

A sharp decline in electron fluxes is observed in the Mars Global Surveyor Electron Reflectometer data in conjunction with the magnetic pileup boundary. We examine the characteristics of the evolution of the electron distribution function for one orbit. We determine that the spectra can best be explained by electron impact ionization of oxygen and hydrogen. To reproduce the observed spectral evolution, we construct a model of the effects of electron impact ionization on the electron distribution function as a flow element encounters the neutral atmosphere. Using the observed post-shock electron distribution function, we are able to reproduce the observed flux attenuation. We conclude that electron impact ionization is the physical mechanism responsible for the spectral feature.