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Low frequency oscillations in the plasma in the induced magnetosphere of Mars were recently measured by the Mars Express spacecraft. Velocity shear was observed, and the plasma was shown to be Kelvin-Helmholtz unstable. In this paper we examine the effects on the frequency and growth rate of the instability when the parameters are varied around the observed values. It is found that if the velocity shear is increased by decreasing the velocity on the low velocity side, i.e. close to the planet, the growth rate increases, while a frequency consistent with the observations can be maintained. When finite Larmor radius effects are included, the growth rate depends on the direction of the vorticity and in the maximum case it is seen to increase substantially, up to a factor of 50, for narrow shear layers. This does also increase the frequency of the oscillations to a large extent. [ABSTRACT FROM AUTHOR]

Using data from the Mars Express Ion Mass Analyzer (IMA) we investigate the distribution of ion beams of planetary origin and search for an influence from Mars crustal magnetic anomalies. We have concentrated on ion beams observed inside the induced magnetosphere boundary (magnetic pile-up boundary). Some north-south asymmetry is seen in the data, but no longitudinal structure resembling that of the crustal anomalies. Comparing the occurrence rate of ion beams with magnetic field strength at 400 km altitude below the spacecraft (using statistical Mars Global Surveyor results) shows a decrease of the occurrence rate for modest (< 40 nT) magnetic fields. Higher magnetic field regions (above 40 nT at 400 km) are sampled so seldom that the statistics are poor but the data is consistent with some ion outflow events being closely associated with the stronger anomalies. This ion flow does not significantly affect the overall distribution of ion beams around Mars. [ABSTRACT FROM AUTHOR]

Abstract: We present and compare energetic neutral atom (ENA) images that are calculated from plasma parameters given by three different simulation models of the interaction between the solar wind and Mars. The images are calculated by combining a model for the ion flow with a model of the neutral atmosphere using the cross-sections for the charge exchange collisions. The three ion models are: an empirical model that is based on Phobos 2 measurements; a three-dimensional hybrid simulation; and a three-dimensional MHD simulation. For the empirical and MHD models the images are obtained by integration of the ENA emission along lines of sight to a virtual ENA instrument. In the case of the hybrid model images are obtained by summing the contributions from all ions, whose positions, velocities, and weights are saved in files at regular intervals. Differences between the models can be detected in the images, for example the hybrid model produces ENA emissions from a larger region than the MHD model does. An asymmetry in the oxygen ion density develops in the hybrid model and can be seen in the oxygen ENA images. The images are influenced by finite gyro radius effects, which are included in the hybrid model but not in the other two. The total production rates of hydrogen ENAs are , , and for the empirical, hybrid and MHD models respectively. This study shows the importance of considering both the type of simulation model used and the proper inclusion of relevant physical phenomena and boundary conditions, when modelling the interaction between planets and the solar wind. Although the different models agree fairly well in terms of macroscopic plasma parameters they produce ENA images that differ substantially. [Copyright &y& Elsevier]

Data from the Cluster spacecraft during their magnetopause crossing on 25 January 2002 are presented. The magnetopause was in a state of slow non-oscillatory motion during the observational period. Coherent structures of magnetosheath plasma, here typified as plasmoids, were seen on closed magnetic field lines on the inside of the magnetopause. Using simultaneous measurements on two spacecraft, the inward motion of the plasmoids is followed from one spacecraft to the next, and it is found to be in agreement with the measured ion velocity. The plasma characteristics and the direction of motion of the plasmoids show that they have penetrated the magnetopause, and the observations are consistent with the concept of impulsive penetration, as it is known from theory, simulations, and laboratory experiments. The mean flux across the magnetopause observed was 0.2%-0.5% of the solar wind flux at the time, and the peak values of the flux inside the plasmoids reached approximately 20% of the solar wind flux. [ABSTRACT FROM AUTHOR]

Dispersion relations for ion-acoustic-like waves in plasmas with and without superthermal particles are calculated using a simple pole expansion of the distribution function for the ions. It is found that slow weakly damped modes exist in two-temperature plasmas, and the conditions when these modes appear are investigated. For plasmas with equal ion and electron temperatures weakly damped modes are found when there is a depletion of the high-energy tails. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Abstract: The general scientific objective of the ASPERA-4 (Analyser of Space Plasmas and Energetic Atoms) experiment is to study the solar wind–atmosphere interaction and characterise the plasma and neutral gas environment in the near-Venus space through energetic neutral atom (ENA) imaging and local charged particle measurements. The studies to be performed address the fundamental question: How strongly do the interplanetary plasma and electromagnetic fields affect the Venusian atmosphere? The ASPERA-4 instrument comprises four sensors; two ENA sensors, electron and ion spectrometers. The neutral particle imager (NPI) provides measurements of the integral ENA flux (0.1–60keV) with no mass and energy resolution but relatively high angular resolution. The neutral particle detector (NPD) provides measurements of the ENA flux, resolving velocity (0.1–10keV) and mass (H and O) with a coarse angular resolution. The electron spectrometer (ELS) is a standard top-hat electrostatic analyser in a very compact design. These three sensors are located on a scanning platform providing a coverage. The instrument also contains an ion mass composition sensor, IMA (ion mass analyser). Mechanically, IMA is a separate unit electrically connected with the ASPERA-4 main unit. IMA provides ion measurements in the energy range 0.01–36keV/ for the main ion components , , , and the ion group with . [Copyright &y& Elsevier]

Mars Global Surveyor detected cold electrons above the Martian ionopause, which can be interpreted as detached ionospheric plasma clouds. Similar observations by the Pioneer Venus Orbiter electron temperature probe showed also extreme spatial irregularities of electrons in the form of plasma clouds on Venus, which were explained by the occurrence of the Kelvin–Helmholtz instability. Therefore, we suggest that the Kelvin–Helmholtz instability may also detach ionospheric plasma clouds on Mars. We investigate the instability growth rate at the Martian ionopause resulting from the flow of the solar wind for the case where the interplanetary magnetic field is oriented normal to the flow direction. Since the velocity shear near the subsolar point is very small, this area is stable with respect to the Kelvin–Helmholtz instability. We found that the highest flow velocities are reached at the equatorial flanks near the terminator plane, while the maximum plasma density in the terminator plane appears at the polar areas. By comparing the instability growth rate with the magnetic barrier formation time, we found that the instability can evolve into a non-linear stage at the whole terminator plane but preferably at the equatorial flanks. Escape rates of O ions due to detached plasma clouds in the order of about –s are found. Thus, atmospheric loss caused by the Kelvin–Helmholtz instability should be comparable with other non-thermal loss processes. Further, we discuss our results in view of the expected observations of heavy ion loss rates by ASPERA-3 on board of Mars Express. [Copyright &y& Elsevier]