K. I. Gringauz

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.

Calculated ionization rates, ion densities, and airglow emission rates due to precipitating electrons in the nightside ionosphere of Mars

The calculations presented in this paper clearly establish that the electron fluxes measured by the HARP instrument, carried on board Phobos 2, could cause significant electron impact ionization and excitation in the nightside atmosphere of Mars, if these electrons actually do precipitate. The calculated peak electron densities were found to be about a factor of 2 larger than the mean observed nightside densities, indicating that if a significant fraction of the measured electrons actually precipitate, they could be the dominant mechanism responsible for maintaining the nightside ionosphere. The calculated zenith column emission rates of the O I 5577-A and 6300-A and CO Cameron band emissions, due to electron impact and dissociative recombination mechanisms, were found to be significant.

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.

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 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.

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.

Energetic particle composition measurements from Phobos 2: Results of the LET experiment

Abstract The Low Energy Telescope (LET) experiment carried on board the Phobos spacecraft measured the flux, spectra and elemental composition of nuclei from hydrogen up to iron, in the energy range ∼ 1–75 MeV nucleon −1 . Isotope separation for helium was also achieved. We present the results of a study of solar energetic particle (SEP) composition using LET data acquired during the period July 1988 to March 1989. The set of particle events selected for study comprises six large solar flare events, two 3 He-rich events and two energetic storm particle increases associated with interplanetary shocks. Three of the six large flare events occurred during a period of unusually high solar activity in March 1989. In two of these events, large Fe/O ratios were measured (0.44 ± 0.05 and 0.95 ± 0.24). The Fe/O ratios determined for the complete set of large flare events show an inverse correlation with the spectral index of oxygen, suggesting that the acceleration mechanism that produces events showing enhanced heavy-ion abundances is different from that responsible for lower Fe/O ratios. Our results in the case of the 3 He-rich events are in agreement with earlier work, showing an enrichment in heavy ions relative to the average SEP composition. The abundances measured for the shock-associated increases are consistent with the acceleration of the ambient population of solar flare particles.

Analysis of the electron measurements from the Plasmag-1 experiment on board Vega 2 in the vicinity of comet P/Halley

Measurements of electron spectra, as obtained by the Plasmag-1 experiment on board Vega 2 in the vicinity of comet Halley, are presented. It is shown that the temperature for thermal electrons gradually decreases when the comet is approached from about 4 105 K behind the cometary bow shock to about 2 105 K at the cometopause. In the region inside the cometopause a fast increase in the flux of energetic electrons of about 1 keV energy is observed. Various possibilities are discussed regarding the differences in the electron spectra measured by Vega 2 and Giotto, respectively.

Energy distribution of electrons with E <800 eV in the areomagnetosphere

Abstract The electron distribution functions measured in the neighborhood of Mars by means of the Hyperbolic Retarding Potential Analyzer (HARP) carried aboard the Phobos 2 spacecraft are presented. The measurements were carried out over an energy/charge ( E / q ) from ∼0.3 eV to ∼800 eV in eight independent angular sectors ∼ 20° × 10° covering the FOV ∼ 180° in the X - Z plane in the antisolar directions. The total intensity and energy distribution function of electrons downstream of the bow shock clearly differ from those in the undisturbed solar wind. The electron fluxes are significantly increased and the energy distribution of electrons in the magnetosheath was found to be characterized by the double-peaked structure. The high energy fluxes often exceed the flux values for the low energy peak.