A. P. Remizov

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.

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.

First results of plasma and neutral gas measurements from Vega-12 near comet Halley

Based on the ion, electron and neutral gas observations, performed by five of the six sensors comprising the PLASMAG-1 experiment on board VEGA-1 and -2, the following results are discussed: (1) the existence of the bow shock and its location at ≀ 1.1×106 km for VEGA-1 inbound; (2) the existence of a cometopause and its location at ≀ 1.6×105 km for VEGA-2 inbound; (3) the plasma dynamical processes occurring inside the cometosheath; (4) the phenomena taking place within the cometary plasma region including mass-spectroscopy of cometary ions at distances ≀ 1.5×104 km; (5) the existence of keV electrons near closest approach to the nucleus; and (6) the radial dependence of the cometary neutral gas and the comparison with model calculations, yielding a mean ionization scale length of ≀ 2×106 km and an overall production rate of ≀ 1.3×1030 molecules s−1 for VEGA-1 inbound. The results are also discussed in the context of the other, both remote and in-situ, observations, performed on board the VEGA- and GIOTTO-spacecraft.

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.

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.

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.

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.

Interpretation of vega observations at comet Halley applying three-dimensional MHD simulations

Abstract In this work the velocity and density of the plasma flow were determined from Vega-1 and Vega-2 ion energy spectra at distances > 0.5 million km from the nucleus of comet Halley. Since the Solar Direction Analyser and the Cometary Ram Analyser of the PLASMAG instrument could observe the distribution of ions only in two limited sections of velocity space (and only the SDA worked aboard Vega-1), the predictions of a three-dimensional MHD simulation developed by Gombosi et al. (1996) were taken into account in order to find the best combination of the adjustable plasma parameters downstream of the bow shock. It was found that the density of the plasma flow was much higher when Vega-1 passed through the cometosheath compared to the Vega-2 fly by. The comparison of simulated and measured magnetic field profiles showed that there was a major disturbance in the interplanetary field after Vega-2 crossed the bow shock.

Quasi-periodic features and the radial distribution of cometary ions in the cometary plasma region of comet P/Halley

Based on the measurements of the ion electrostatic analyzer CRA of the Plasmag-1 instrument on board Vega-2, which was oriented along the spacecraft-comet relative velocity direction, we present observations (1) of quasi-periodic features in the intensity of cometry ions occurring inside the cometopause, (2) of the mass composition of the cometary ions in the mass range of 1 to 100 atomic mass units (amu), and (3) of the radial dependence of the density of certain, well-defined groups of cometary ions.