M. Tatrallyay

The dependence of the Martian magnetopause and bow shock on solar wind ram pressure according to Phobos 2 TAUS ion spectrometer measurements

The location of the Martian magnetopause and that of the bow shock are studied on the basis of three-dimensional solar wind proton spectra measured by the TAUS spectrometer on board Phobos 2 in its 56 circular orbits. The clear and strong dependence of the areomagnetopause position on solar wind ram pressure was revealed, while the position of the bow shock was practically independent of this parameter. In the power law expression telling the dependence of the Martian magnetotail thickness D on the solar wind ram pressure: D∼(ϱυ²)−1/k, the power index turned out to be k∼5.9±0.5. The close coincidence of this index with k = 6 for a dipole geomagnetic field, and the large areomagnetotail thickness compared with the planetary diameter, suggest that an intrinsic dipole magnetic field is likely to be an important factor in the solar wind interaction with Mars. On the other hand, the relatively stable position of the subsolar point of the Martian magnetopause and unambiguous induction effects observed by the Phobos 2 MAGMA magnetic experiment in the magnetotail indicate the essential role of an induced magnetic field, too. The weak dependence of the terminator bow shock position on the solar wind ram pressure may be related to the relatively stable position of the subsolar magnetopause.

Plasma properties from the upstream region to the cometopause of comet P/Halley: Vega observations

Based on the Plasmag-1 plasma measurements on board Vega-1 and -2, evidence is provided for the deceleration upstream, for the heating at and for the thermalization and deceleration behind the bow shock of comet Halley. In the cometosheath region two separate ion populations are observed: the first one consists of cometary ions being picked up in the vicinity of the point of observation; the energy of these ions coming from the solar direction decreases much faster than the energy of the solar wind ions. The second one consists of cometary ions being picked up by the solar wind far away from the point of observation. Considerable oscillations in the plasma flow direction occur in the cometosheath region.

The HARP plasma experiment on-board the Phobos 2 spacecraft: Preliminary results

Abstract The HARP differential electrostatic analyzer measured thermal and supra thermal electron and ion fluxes and distributions in the Mars environment. High bit-rate data obtained around the pericenter passage on elliptical orbits early in February 1989 will be highlighted, and correlation with results of other experiments will be discussed. A similarity between the HARP total count rate variations and the spacecraft potential measurements of the Plasma-Wave System experiment is established. The variation of electron energy spectra is discussed in various regions of the solar wind interaction with Mars. Tentative results on electron and ion anisotropy are presented, making use of regular variations of the ratio of count rates of two sensor heads looking in perpendicular directions. Magnetic field data of the MAGMA magnetometer are used for comparison.

The HARP electron and ion sensor on the phobos mission

Abstract The HARP instrument is a hyperbolic electrostatic analyzer working in the retarding potential mode. It is the lowest-energy member of the ESTER particle detector family. The energy range extends from 0.25 eV to 850 eV for both electrons and ions. The eight viewing sectors are arranged in a fan-shaped geometry in the antisolar hemisphere. They are simultaneously sampled while energy is stepped over a maximum number of 75 logarithmically spaced channels. The instrument is intended for thermal and superthermal solar-wind electron observations during the cruise phase and — more importantly — for electron and ion observations in the Mars environment.

Evidence of the influence of equatorial Martian crustal magnetization on the position of the planetary magnetotail boundary by Phobos 2 data

Abstract In addition to earlier found variations of the Martian magnetotail thickness as a function of the solar wind ram pressure, succeeding analysis of the magnetotail boundary crossings by Phobos 2 orbiter revealed modulation of magnetopause positions by planetary crustal magnetization. It was shown that magnetic anomalies mostly influence the magnetotail thickness when they are approaching the terminator region due to planetary rotation. The areographic position of the equatorial regions of Mars, which efficiently increase planetary magnetotail thickness, corresponds to strongly magnetized regions recently mapped by the Mars Global Surveyor. Local enhancement of the magnetotail thickness due to non-uniform crustal magnetization can be as high as 500–1000 km.

Planetary heavy ions in the magnetotail of Mars: Results of the TAUS and MAGMA experiments aboard PHOBOS

Abstract Both kinetic and hydrodynamic properties of specific ‘mushroom-cap’ distributions of heavy ions in the plasma sheet of the Martian magnetotail are analyzed to identify possible processes of planetary ion acceleration. A number of correlations of different plasma sheet and solar wind parameters are studied to find signatures of ion acceleration by magnetic field line shear stress, field aligned and cross tail electric field, as well as by direct interaction with the magnetosheath plasma in the ‘pole’ regions.

On the distribution of pickup ions as observed by the Vega spacecraft at comet Halley

Abstract Two electrostatic analysers of the PLASMAG-1 plasma instrument package detected energy/charge spectra of ions (without mass separation) aboard the Vega spacecraft when encountering comet Halley. They could observe only two relatively small sections of velocity space showing a mixed effect of decelerated solar wind particles and cometary pickup ions downstream of the bow shock. The separation of the different ion components was attempted by comparing the energy spectra with simple plasma distributions. The effect of cometary pickup ions observed by Vega-1 was different from that observed by Vega-2 at cometocentric distances >0.5 million km. These differences could be interpreted when using the bispherical shell distribution based on magnetic field vectors measured by the magnetometer.

Studies of the Martian bow shock response to the variation of the magnetosphere dimensions according to TAUS and MAGMA measurements aboard the Phobos 2 orbiter

Abstract A self-consistent quantitative model is presented which describes the planetary bow shock motion and its shape variation due to variations of the external plasma flow parameters and the magnetopause shape. This model is applied to the analysis of the Martian bow shock motions and for the explanation of its unusual properties.

Solar wind deceleration at Mars and Earth: A comparison

Abstract Phobos 2 plasma measurements have revealed the solar wind deceleration upstream of the Martian terminator bow shock. Earlier studies have shown that the solar wind is decelerated and deflected in the Earths foreshock region. At Mars, the solar wind decelerated more at duskside than at dawnside. At Earth the solar wind decelerates most upstream of the quasi-parallel bow shock in the foreshock region. In this paper, we investigate and compare the solar wind deceleration at Mars and Earth.

Plasma flow in the cometosheath of comet Halley

Plasma parameters are investigated in the regions upstream and downstream of the bow shock of comet Halley based on the ion spectra measured by the Plasmag-1 instrument aboard the Vega-2 spacecraft. Proton velocities observed by the Solar Direction Analyser and by the Cometary Ram Analyser are compared with the simulated bulk velocity profile provided by a three-dimensional multiscale MHD model (Gombosi et al., 1996). Disregarding the effects of presumably interplanetary disturbances, the simulated and measured values are in good agreement at cometocentric distances larger than about 0.5 million km.