Stas Barabash

Strong influence of lunar crustal fields on the solar wind flow

We discuss the influence of lunar magnetic anomalies on the solar wind and on the lunar surface, based on maps of solar wind proton fluxes deflected by the magnetic anomalies. The maps are produced ...

Extremely high reflection of solar wind protons as neutral hydrogen atoms from regolith in space

We report on measurements of extremely high reflection rates of solar wind particles from regolith-covered lunar surfaces. Measurements by the Sub-keV Atom Reflecting Analyzer (SARA) instrument on the Indian Chandrayaan-1 spacecraft in orbit around the Moon show that up to 20% of the impinging solar wind protons are reflected from the lunar surface back to space as neutral hydrogen atoms. This finding, generally applicable to regolith-covered atmosphereless bodies, invalidates the widely accepted assumption that regolith almost completely absorbs the impinging solar wind.

Birth of a comet magnetosphere: A spring of water ions

The Rosetta mission shall accompany comet 67P/Churyumov-Gerasimenko from a heliocentric distance of >3.6 astronomical units through perihelion passage at 1.25 astronomical units, spanning low and maximum activity levels. Initially, the solar wind permeates the thin comet atmosphere formed from sublimation, until the size and plasma pressure of the ionized atmosphere define its boundaries: A magnetosphere is born. Using the Rosetta Plasma Consortium ion composition analyzer, we trace the evolution from the first detection of water ions to when the atmosphere begins repelling the solar wind (~3.3 astronomical units), and we report the spatial structure of this early interaction. The near-comet water population comprises accelerated ions (<800 electron volts), produced upstream of Rosetta, and lower energy locally produced ions; we estimate the fluxes of both ion species and energetic neutral atoms.

Atmospheric effects of precipitating energetic hydrogen atoms on the Martian atmosphere

The Martian atmosphere is under the influence of an intense flux of precipitating energetic (≲1 keV) hydrogen atoms. In the solar wind and in the magnetosheath, fast hydrogen atoms are produced by charge exchange between solar wind protons and the hydrogen corona. Atmospheric effects of the precipitating hydrogen atoms are thus manifestations of the direct interaction between solar wind protons and the planetary neutrals. A three-dimensional Monte Carlo simulation model has been developed to study different atmospheric effects of the precipitating hydrogen atoms. The model is used to calculate the altitude profiles of the energy deposition rates, the ion production rates, and the photon emission rates at different solar zenith angles under low solar activity conditions. The peak loss and production rates under typical solar wind conditions caused by precipitating hydrogen atoms are estimated to be ∼1% of the corresponding peak values due to extreme ultraviolet radiation but comparable or larger than effects of H+ and O+ precipitation at low altitudes. The results indicate that a substantial part of the incoming particle and energy flux is scattered back from the Martian atmosphere.

Picked‐up protons near Mars: Phobos observations

The measurements carried out by the plasma spectrometer, ASPERA, onboard the PHOBOS-2 spacecraft show that protons, originating in the extended hydrogen corona of Mars, were observed at altitudes ≤7500 km. The cyclotron instability of these pickup ions appears to generate Alfven waves observed by the MAGMA magnetometer. Analysis of the plasma data shows that weak pitch-angle diffusion of a ring-distribution of pickup protons occurs. The altitude profiles of pickup proton fluxes and number densities of the parent hydrogen atoms are derived.

Low energy neutral atom imaging on the Moon with the SARA instrument aboard Chandrayaan-1 mission

This paper reports on the Sub-keV Atom Reflecting Analyzer (SARA) experiment that will be flown on the first Indian lunar mission Chandrayaan-1. The SARA is a low energy neutral atom (LENA) imaging mass spectrometer, which will perform remote sensing of the lunar surface via detection of neutral atoms in the energy range from 10 eV to 3 keV from a 100km polar orbit. In this report we present the basic design of the SARA experiment and discuss various scientific issues that will be addressed. The SARA instrument consists of three major subsystems: a LENA sensor (CENA), a solar wind monitor (SWIM), and a digital processing unit (DPU). SARA will be used to image the solar wind-surface interaction to study primarily the surface composition and surface magnetic anomalies and associated mini-magnetospheres. Studies of lunar exosphere sources and space weathering on the Moon will also be attempted. SARA is the first LENA imaging mass spectrometer of its kind to be flown on a space mission. A replica of SARA is planned to fly to Mercury onboard the BepiColombo mission.

Energetic neutral atom imaging at low altitudes from the Swedish microsatellite Astrid: Extraction of the equatorial ion distribution

Energetic neutral atom (ENA) images obtained by the imager on board the Astrid satellite in the polar cap at 1000 km during a moderate magnetic storm (Dst ≥ −80 nT) on February 8, 1995, are simulated using a parameterized model of the equatorial ion distribution and a six-component Chamberlain exosphere with parameters from the Mass Spectrometer Incoherent Scatter Extended (MSISE-90) model. By changing the ion parameters until a matching ENA image is obtained, one can extract the equatorial ion distribution. Four consecutive images from different view points several minutes apart are simulated assuming H+ and O+, respectively, as parent ions. The optimal set of parameters is extracted by minimizing the χ2 difference between simulated and observed ENA images using Powells minimization algorithm. The optimal equatorial model ion distribution consists of O+ peaked around dusk. The lower intensity of fluxes obtained from vantage points closer to the pole is an effect of the loss cone of the parent ion distribution being empty.

Protons in the near-lunar wake observed by the Sub-keV Atom Reflection Analyzer on board Chandrayaan-1

Significant proton fluxes were detected in the near wake region of the Moon by an ion mass spectrometer on board Chandrayaan-1. The energy of these nightside protons is slightly higher than the energy of the solar wind protons. The protons are detected close to the lunar equatorial plane at a

Energetic neutral atom imaging at low altitudes from the Swedish microsatellite Astrid: Images and spectral analysis

140^{\circ}

The lunar wake current systems

solar zenith angle, i.e., ~50