A. K. Richter

Ions of planetary origin in the Martian magnetosphere (Phobos 2/TAUS experiment)

Abstract The measurements onboard the Phobos 2 Martian orbiter revealed one more physical process of Martian neutral atmosphere dissipation—outflow of heavy ions of planetary origin through the magnetic tail of Mars. The distribution of heavy ions through the cross-section of the Martian magnetotail is studied based on TAUS spectrometer data. Average loss rate of heavy ions through the plasmasheet (separating magnetotail lobes) is evaluated as ∼ 5 × 10 24 s −1 . The revealed process of Martian atmosphere dissipation is important for cosmological time and constitutes ∼ 10% of non-thermal oxygen dissipation due to dissociative recombination of molecular oxygen ions near exobase.

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.

On the problem of the Martian atmosphere dissipation: Phobos: 2 TAUS Spectrometer results

The measurements of proton spectra obtained by the TAUS spectrometer on board the Phobos 2 spacecraft in elliptical orbits near Mars are presented. A strong deceleration of the solar wind upstream of the Martian bow shock was revealed. It can be caused by the mass loading of the plasma flow by ions originating from the hot oxygen/hydrogen corona of Mars and/or by protons specularly reflected from the bow shock. In the first case the deceleration of the solar wind by about 100 km/s implies that the hot oxygen corona of Mars could be several times denser than it was anticipated to be (at least during the observation period that was close to solar cycle maximum). Furthermore, the loss of planetary oxygen through the corona appears to be the main process of oxygen loss from Mars. The upper limit of loss rate for such a process is determined to be 1026 oxygen atoms or 2.5 kg of oxygen per second.

General features of comet P/Halley: solar wind interaction from plasma measurements

The Giotto RPA-COPERNIC plasma experiment identified several regions in which the solar wind interaction with the cometary plasma displayed characteristic features: Beginning ~ 4.6 106 km from the comet there is an upstream region with sporadic connection to the comet An electron foreshock is present up to 2.5 105 km away from the bow shock A bow shock is detected at 1.15 106 km Between the bow shock and the cometopause the outer regions can be divided into 3 parts A cometopause is found at ~ 1.35 105 km and a density decrease is detected at ~ 4.5 104 km from the comet. The detailed plasma features associated with these regions are successively described.

Quantitative model of the Martian magnetopause shape and its variation with the solar wind ram pressure based on Phobos 2 observations

A model of the Martian magnetopause is developed for the period of maximum solar activity which simultaneously describes (1) the observed relation between the solar wind ram pressure ρ V2 and the magnetopause position in the magnetotail, (2) the observed relation between ρ V2 and the flaring angle, and (3) a few magnetopause crossing observations above the day side of the planet. The shape of the magnetopause is determined from the equation of pressure balance across this boundary when both the magnetic pressure (with a planetary magnetic moment of (0.8–1.0)×l022 G cm3) and the ionospheric pressure are taken into account in the planetary magnetosphere. The specific feature of the model is the “stagnation” of the subsolar magnetopause when the ram pressure increases to higher values (≥ 6×10−9 dyn cm−2).

Stochastic Fermi acceleration of ions in the pre-shock region of comet P/Halley

Energetic cometary ion fluxes measured between 2.5106 km and 106 km from the cometary nucleus along the inbound trajectory of s/c VEGA-1 are used to derive the temperatures of the ion distributions in the solar wind frame. The increase of the temperature is modelled by the temperature change derived from a Fokker-Planck type equation with a source term and a stochastic acceleration term. The temperature increase predicted by theory is about 3 keV, higher than the observed one (≃ 1.4 keV). The difference may be due to the approximations applied. The second order Fermi mechanism is thus capable of producing the temperature increase observed.

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.

Description of the main boundaries seen by the Giotto electron experiment inside comet P/Halley-solar wind interaction region

The Giotto electron-plasma experiment identified several boundaries inside the comet Halley-solar wind region. Two of them are particularly interesting; they separate very different plasma regimes on quite sharp length scales and furthermore their existence was not foreseen in theoretical models. They are the limit between the transition region and the sheath detected at 550,000 km from the nucleus, and the cometopause detected at 135,000 km from the nucleus.

The low energy particle detector SLED (∼30 keV-3.2 MeV) and its performance on the PHOBOS Mission and its moons

Abstract A low energy particle detector system (SLED) is described which was designed to measure the flux densities of electrons and ions in the energy range from ≈30 keV to a few MeV in (a) the varying solar aspect angles and temperatures pertaining during the Cruise Phase of the Phobos Mission and (b) in the low temperature environment (reaching −25° C) pertaining during Mars Encounter. Representative data illustrating the excellent functioning of SLED during both phases of the mission are presented.

Energetic particle studies at Mars by SLED on Phobos 2

A preliminary overview of particle records obtained by the SLED instrument on Phobos 2, February–March, 1989 during Mars encounter, is presented. Data obtained while in close elliptical orbit around the planet (pericenter < 900 km), in both spin and three axis stabilised mode, display evidence of energy related particle shadowing by the body of Mars. This effect was also observed, under favourable conditions, in certain circular orbits (altitude 6330 km above the planet). Flux enhancements, inside the magnetopause, in the approximate range 30–350 keV, recorded in the same general location at < 900 km above Mars over an 8 day period during three consecutive elliptical orbits, are described. Possible explanations of these enhancements include the presence of quasi-trapped radiation at the planet and the detection of the propagation of accelerated particles along the boundary of the magnetopause from the day to the night side of Mars. Large anisotropic ion flux increases (1–1.5 orders of magnitude) in the approximate range 30–200 keV recorded in front of the bow shock (inbound and outbound) during certain circular orbits, provide evidence that the spacecraft traversed strongly anisotopic jets of energetic particles. These are suggested to have constituted O+ ions. The pickup process would have been sufficient to accelerate such ions to their observed energies in the prevailing solar wind conditions. Alternatively, they might have comprised particles that had leaked from inside the magnetopause, perhaps undergoing shock drift acceleration in the process. Significant flux enhancements were also sometimes identified in the magnetotail (approximate energy range 30–50 keV). These are suggested to represent the signatures of O+ beams, impelled by acceleration processes similar to those associated with terrestrial ion beams.