K. Sauer

The plasma environment of Mars

When the supersonic solar wind reaches the neighborhood of a planetary obstacle it decelerates. The nature of this interaction can be very different, depending upon whether this obstacle has a large-scale planetary magnetic field and/or a well-developed atmosphere/ionosphere. For a number of years significant uncertainties have existed concerning the nature of the solar wind interaction at Mars, because of the lack of relevant plasma and field observations. However, measurements by the Phobos-2 and Mars Global Surveyor (MGS) spacecraft, with different instrument complements and orbital parameters, led to a significant improvement of our knowledge about the regions and boundaries surrounding Mars.

Physics of Mass Loaded Plasmas

In space plasmas the phenomenon of mass loading is common. Comets are one of the most evident objects where mass loading controls to a large extent the structure and dynamics of its plasma environment. New charged material is implanted to the fast streaming solar wind by planets, moons, other solar system objects, and even by the interstellar neutral gas flowing through our solar system. In this review we summarize both the current observations and the relevant theoretical approaches. First we survey the MHD methods, starting with a discussion how mass loading affects subsonic and supersonic gasdynamics flows, continuing this with single and multi-fluid MHD approaches to describe the flow when mass, momentum and energy is added, and we finish this section by the description of mass loaded shocks. Next we consider the kinetic approach to the same problem, discussing wave excitations, pitch angle and energy scattering in linear and quasi-linear approximations. The different descriptions differ in assumptions and conclusions; we point out the differences, but it is beyond the scope of the paper to resolve all the conflicts. Applications of these techniques to comets, planets, artificial ion releases, and to the interplanetary neutrals are reviewed in the last section, where observations are also compared with models, including hybrid simulations as well. We conclude the paper with a summary of the most important open, yet unsolved questions.

Mirror modes and fast magnetoacoustic waves near the magnetic pileup boundary of comet P/Halley

Large-amplitude ultralow-frequency wave structures observed on both sides of the magnetic pileup boundary of comet P/Halley during the flyby of the Giotto spacecraft have been analyzed using suprathermal electron density and magnetic field observations. Upstream of the boundary, electron density and magnetic field magnitude variations are anticorrelated, while in the pileup region these quantities are clearly correlated. Both in front of and behind the pileup boundary the observed waves are quasi-perpendicular wave structures as a minimum variance analysis shows. A detailed comparison of our observations in the prepileup region with theoretical and numerical results shows that the mirror mode mode waves may have been generated by a mirror instability driven by the pressure anisotropy of the ring-type distributions of the heavy (water group) pickup cometary ions. A mean wavelength λ1 = 2071 km is found, which is about 4 times the water group ion thermal gyroradius. For the fast mode wave structures in the pileup region, analysis of wave propagation on both sides of the pileup boundary shows that mode conversion from the mirror mode to fast mode type waves at the boundary is not a possible source mechanism. However, we show that the fast mode waves may be interpreted as stationary wave structures in a multifluid plasma. The wavelength λ2 = 1141 km derived from the observations, when using the stationary wave hypothesis, is in good agreement with the theoretical expected wave length λtheo = (774 ± 506) km computed from available ion observations.

Plasma characteristics of the boundary layer in the Martian magnetosphere

Plasma and magnetic field data from circular orbits of the Phobos 2 spacecraft near Mars are examined to provide a description of the plasma properties of inner regions of the Mars magnetosheath and the boundary layer/plasma mantle. The data are analyzed in the VB coordinate system, which is reasonable for draping magnetospheres of nonmagnetized planets and comets. It is shown that a boundary almost impermeable for protons is formed. The ion composition changes at this boundary, and a transition layer dominated by planetary ions is observed. The characteristics of the magnetosheath plasma is drastically changed near this ion composition boundary. A strong drop of the proton bulk velocity is accompanied by an increase of the proton temperature and intense fluxes of planetary ions. In dependence of the solar wind dynamic pressure and other factors, radius of the “magnetospheric cavity”, virtually void of solar wind plasma varies from 4500 to 9500 km. During time intervals of very high solar wind dynamic pressure, the cavity, divided up in lobe cells, is almost degenerated. The comparison of positions of different magnetospheric boundaries identified earlier from single-instrument measurements shows their collocation.

Dispersion analysis of ULF waves in the foreshock using cluster data and the wave telescope technique

Cluster provides us with a unique possibility to study ULF waves. We analyze ULF wave activity in the near-Earth upstream solar wind. Using Cluster as a wave telescope we investigate in detail wave propagation directions and wave numbers for various frequencies, obtaining, for the first time, three dimensional dispersion relations experimentally. After Doppler shift correction, we find that the dispersion relations are not linear and the waves are propagating in the sunward direction in the plasma rest frame. Comparison of the experimentally derived dispersion relation with that one for a beam plasma system shows good agreement. We suggest that the ULF waves in the foreshock are generated by a proton population backstreaming from the shock.

Observations of plasma boundaries and phenomena around Mars with Phobos 2

Magnetic field and plasma measurements on board the Soviet spacecraft Phobos 2 have been analyzed during five elliptical orbits around Mars. The existence of at least one separate plasma boundary and an adjacent plasma layer, called the planetopause and the transition region, between the bow shock and the ionopause seems to be a characteristic feature of the solar wind interaction with an almost nonmagnetized planetary ionosphere. It is suggested that the planetopause is a multiple-ion discontinuity, where a large number of solar wind protons are deflected at an exospheric density ramp. Strong changes in magnetic field and plasma flow direction within the transition region are interpreted as signatures of current sheets or internal shocks. The detected eclipse boundary in the tail is perhaps an elongation of the ionopause found at Venus. New ideas concerning the formation of multiple-ion flow boundaries by electrostatic plasma-plasma interaction are discussed. Finally, a remarkable Deimos event has been detected during the fifth orbit. This is explained as an interaction of the subsonic solar wind with a thin cloud of charged dust particles.

Subcritical multiple-ion shocks

Low Mach number collisionless shocks in a multiple-ion plasma are investigated theoretically using multiple-ion Rankine-Hugoniot relations (MIRHR) and hybrid simulations. The derived MIRHR permit tractable solutions for perpendicular shocks. It is found that starting from a synchronous flow of a mixture of different ion species, light ions and heavy ions jump to different downstream velocities. Although the resulting differential velocities between the species are unstable, they produce coherent gyration of the ion species around each other. This gyration excites standing ion hybrid waves in the downstream region. Both of these effects (the jump of different ion species ot different downstream velocities and the excitation of the ion hybrid waves) are confirmed by hybrid simulations.

Nonlinear stationary whistler waves and whistler solitons (oscillitons): Exact solutions

A fully nonlinear theory for stationary whistler waves propagating parallel to the ambient magnetic field in a cold plasma has been developed. It is shown that in the wave frame proton dynamics must be included in a self-consistent manner. The complete system of nonlinear equations can be reduced to two coupled differential equations for the transverse electron or proton speed and its phase, and these possess a phase-portrait integral which provides the main features of the dynamics of the system. Exact analytical solutions are found in the approximation of ‘small’ (but nonlinear) amplitudes. A soliton-type solution with a core filled by smaller-scale oscillations (called ‘oscillitons’) is found. The dependence of the soliton amplitude on the Alfven Mach number, and the critical soliton strength above which smooth soliton solutions cannot be constructed is also found. Another interesting class of solutions consisting of a sequence of wave packets exists and is invoked to explain observations of coherent wave emissions (e.g. ‘lion roars’) in space plasmas. Oscillitons and periodic wave packets propagating obliquely to the magnetic field also exist although in this case the system becomes much more complicated, being described by four coupled differential equations for the amplitudes and phases of the transverse motion of the electrons and protons.

The question of an internal martian magnetic field

Abstract The data of the near-Mars measurements of the two magnetometers FGMM and MAGMA onboard the Phobos-2 spacecraft are interpreted in favour of the existence of a weak Martian magnetic field. This conclusion is based mainly on a period analysis, leading to evidence for 12 hours and 24 hours periods in the magnetic data. Compatibility of this result with other data and interpretations is discussed.

The nature of the martian 'obstacle boundary'

Abstract The measurements made by the Mars Global Surveyor have determined the Marss magnetic moment M≤10 11 Tm 3 . This value is, at least, one order of magnitude below the value originally estimated from the pressure balance under the assumption that the observed solar wind obstacle boundary is identical with the earth-like magnetopause. This obvious discrepancy offers the limits of classical MHD for description of the solar wind/Mars interaction and calls for new theoretical models. To describe the main physical processes during the ‘collision’ of the solar wind with the continuously produced cold heavy-ion plasmas of planetary origin, a bi-ion fluid model is used. It is shown that a sharp electrostatic barrier is formed between the two plasmas with following properties: stoppage of the proton flow and change of the ion composition, pile-up of the magnetic field, a decrease of the electron temperature. These characteristic features are seen in single-instrument analysis of the Phobos-2 and MGS data.