S. Barabash

Charge exchange near Mars: The solar wind absorption and energetic neutral atom production

Charge exchange between solar wind protons and neutral atmospheric atoms is expected to affect the solar wind interaction with Mars, but its influences and significance have only been touched upon in previous work. Here several features associated with the charge exchange process between the solar wind protons and Martian neutral upper atmospheres are described. The analysis is based on an empirical proton model derived from Phobos 2 observations interacting with the Martian atomic (H) and molecular (H2) hydrogen, and oxygen (O) upper atmospheres representing solar minimum and solar maximum conditions. The region where the largest fraction of solar wind protons is lost by the charge exchange process is found to be a thin layer above the surface of Mars on the dayside resulting from charge exchange with the thermal oxygen. In general, the magnetosheath and “magnetosphere” (where the observed plasma takes on a different character in the Phobos 2 data) produce two distinguishable regions where the loss rate of solar wind protons is highest. Increasing solar activity increases the loss rate in the magnetosheath but decreases it in the magnetosphere. No significant increase of the absorption of the solar wind was found near the “magnetopause” suggesting that the decrease of the solar wind protons observed by Phobos 2 are not due to the charge exchange process. In addition to a reduction in the solar wind density, the charge exchange reaction results in energetic neutral atom (ENA) production. This paper considers some of the detailed properties expected for the ENA population at Mars. The ENA differential fluxes were found to be typically 106–107 cm−2 s−1 keV−1 in the energy range 0.01–1 keV. During solar minimum, the ENA production rate and ENA integral fluxes were found to be highest in the magnetosheath. At solar maximum the ENA production rate is highest in the magnetosphere, and ENA integral fluxes in the dayside magnetosphere appear to become comparable to the fluxes in the magnetosheath if the proton temperature in the magnetosphere is low. It is found that 1–3% of the original solar wind proton flux converts into ENAs before the bow shock. The ENAs produced upstream are undeflected and so may precipitate into the Martian upper atmosphere, depositing an energy flux of up to 3 × 109 eV cm−2 s−1 derived from the solar wind. These results both suggest the possible benefits of observing ENA fluxes around Mars and suggest the necessary parameters for detector design.

Little or no solar wind enters Venus' atmosphere at solar minimum.

Venus has no significant internal magnetic field, which allows the solar wind to interact directly with its atmosphere2,3. A field is induced in this interaction, which partially shields the atmosphere, but we have no knowledge of how effective that shield is at solar minimum. (Our current knowledge of the solar wind interaction with Venus is derived from measurements at solar maximum.) The bow shock is close to the planet, meaning that it is possible that some solar wind could be absorbed by the atmosphere and contribute to the evolution of the atmosphere. Here we report magnetic field measurements from the Venus Express spacecraft in the plasma environment surrounding Venus. The bow shock under low solar activity conditions seems to be in the position that would be expected from a complete deflection by a magnetized ionosphere. Therefore little solar wind enters the Venus ionosphere even at solar minimum.

Magnetic Reconnection in the Near Venusian Magnetotail

Magnetic Reconnection Magnetic reconnection (MR) has been observed in the magnetospheres of planets with an intrinsic magnetic field, such as Earth, Mercury, Jupiter, and Saturn. MR is a universal plasma process that occurs in regions of strong magnetic shear and converts magnetic energy into kinetic energy. On Earth, MR is responsible for magnetic storms and auroral events. Using data from the European Space Agency Venus Express spacecraft, Zhang et al. (p. 567, published online 5 April; see the Perspective by Slavin) present surprising evidence for MR in the magnetosphere of Venus, which is a nonmagnetized body. Venus Express observations show that magnetic reconnection occurs in the magnetotail of an unmagnetized planet. Observations with the Venus Express magnetometer and low-energy particle detector revealed magnetic field and plasma behavior in the near-Venus wake that is symptomatic of magnetic reconnection, a process that occurs in Earth’s magnetotail but is not expected in the magnetotail of a nonmagnetized planet such as Venus. On 15 May 2006, the plasma flow in this region was toward the planet, and the magnetic field component transverse to the flow was reversed. Magnetic reconnection is a plasma process that changes the topology of the magnetic field and results in energy exchange between the magnetic field and the plasma. Thus, the energetics of the Venus magnetotail resembles that of the terrestrial tail, where energy is stored and later released from the magnetic field to the plasma.

Ionospheric storms on Mars: Impact of the corotating interaction region

Measurements made by the ASPERA-3 and MARSIS experiments on Mars Express have shown, for the first time, that space weather effects related to the impact of a dense and high pressure solar wind (corotating interaction region) on Mars cause strong perturbations in the martian induced magnetosphere and ionosphere. The magnetic barrier formed by pile-up of the draped interplanetary magnetic field ceases to be a shield for the incoming solar wind. Large blobs of solar wind plasma penetrate to the magnetosphere and sweep out dense plasma from the ionosphere. The topside martian ionosphere becomes very fragmented consisting of intermittent cold/low energy and energized plasmas. The scavenging effect caused by the intrusions of solar wind plasma clouds enhances significantly (by a factor of ≥10) the losses of volatile material from Mars.

Evolution of the ion environment of comet 67P/Churyumov-Gerasimenko - Observations between 3.6 and 2.0 AU

Context. The Rosetta spacecraft is escorting comet 67P/Churyumov-Gerasimenko from a heliocentric distance of >3.6 AU, where the comet activity was low, until perihelion at 1.24 AU. Initially, the solar wind permeates the thin comet atmosphere formed from sublimation. Aims. Using the Rosetta Plasma Consortium Ion Composition Analyzer (RPC-ICA), we study the gradual evolution of the comet ion environment, from the first detectable traces of water ions to the stage where cometary water ions accelerated to about 1 keV energy are abundant. We compare ion fluxes of solar wind and cometary origin. Methods. RPC-ICA is an ion mass spectrometer measuring ions of solar wind and cometary origins in the 10 eV–40 keV energy range. Results. We show how the flux of accelerated water ions with energies above 120 eV increases between 3.6 and 2.0 AU. The 24 h average increases by 4 orders of magnitude, mainly because high-flux periods become more common. The water ion energy spectra also become broader with time. This may indicate a larger and more uniform source region. At 2.0 AU the accelerated water ion flux is frequently of the same order as the solar wind proton flux. Water ions of 120 eV–few keV energy may thus constitute a significant part of the ions sputtering the nucleus surface. The ion density and mass in the comet vicinity is dominated by ions of cometary origin. The solar wind is deflected and the energy spectra broadened compared to an undisturbed solar wind. Conclusions. The flux of accelerated water ions moving from the upstream direction back toward the nucleus is a strongly nonlinear function of the heliocentric distance.

Martian planetopause as seen by the plasma wave system onboard Phobos 2

The existence of a plasma boundary between the bow shock and the atmosphere of Mars has been confirmed by the Phobos 2 observations. This boundary is called planetopause, magnetopause, ion-composition boundary or protonopause, depending upon the authors. A careful examination of plasma wave system (PWS) data has revealed that planetopause signatures are generally clear and not questionable, enabling the calculation of the mean planetopause position up to 16 Martian radii in the distant tail. The planetopause model derived from PWS measurements is compared with models calculated from data sets obtained with other instruments on Phobos 2. With the exception of the transition plasma layer, called mass-loading boundary by the Automatic Space Plasma Experiment with a rotating analyzer (ASPERA) investigators, the published planetopause, magnetopause/areomagnetopause and ion-composition boundary locations are similar to the planetopause locations deduced from PWS data. This would suggest that, despite their different names, these boundaries correspond to the same plasma transition region. The PWS observations show that the planetopause position varies in the same way as does the bow shock position and is not significantly affected by the solar wind ram pressure. The absence of correlation between the planetopause position and solar wind ram pressure raises serious questions about the role that possible intrinsic magnetic field may play at the Martian planetopause. Finally, the planetopause is believed to be an ion-composition discontinuity rather than the “obstacle” predicted by gasdynamic models.

The solar wind interaction with Mars: Consideration of Phobos 2 mission observations of an ion composition boundary on the dayside

This paper describes the features of the boundary in the plasma ion composition near Mars which separates the region dominated by the solar wind protons from the plasma of planetary origin. This boundary was detected by the ASPERA experiment on Phobos 2. It is argued that the features of this boundary seem to be similar to those of other composition boundaries detected elsewhere: the cometopause near comet Halley, and a boundary in the ion composition which appears near Venus during periods of high solar wind dynamic pressure. Numerical modeling of the solar wind interaction with Mars supports the idea that during solar maximum the interaction of the Martian neutral atmosphere with the solar wind can result in a composition transition from solar wind to planetary ions in the low-altitude magnetosheath. This transition occurs because of charge exchange of solar wind protons with the neutral atmosphere and photoionization.

Plasma environment of Mars as observed by simultaneous MEX-ASPERA-3 and MEX-MARSIS observations

[1] Simultaneous in situ measurements carried out by the Analyzer of Space Plasma and Energetic Atoms (ASPERA-3) and Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) instruments on board the Mars Express (MEX) spacecraft for the first time provide us with the local parameters of cold ionospheric and hot solar wind plasma components in the different regions of the Martian magnetosphere and ionosphere. On the dayside, plasma of ionospheric and exospheric origin expands to large altitudes and gets in touch with the solar wind plasma. Formation of the magnetic field barrier which terminates the solar wind flow is governed by solar wind. The magnetic field rises up to the value which is just sufficient to balance the solar wind pressure while the position of the magnetospheric boundary varies insignificantly. Although, within the magnetic barrier, solar wind plasma is depleted, the total electron density increases owing to the enhanced contribution of planetary plasma. In some cases, a load caused by a planetaiy plasma becomes so strong that a pileup of the magnetic field occurs in a manner which forms a discontinuity (the magnetic pileup boundary). Generally, the structure of the magnetospheric boundary on the dayside varies considerably, and this variability is probably controlled by the magnetic field orientation. Inside the magnetospheric boundaiy, the electron density continues to increase and forms the photoelectron boundary which sometimes almost coincides with the magnetospheric boundary. The magnetic field strength also increases in this region, implying that the planetary plasma driven into the bulk motion transports the magnetic field inward. A cold and denser ionospheric plasma at lower altitudes reveals a tailward cometary-like expansion. Large-amplitude oscillations in the number density of the ionospheric plasma are another typical feature. Crossings of plasma sheet at low altitudes in the terminator region are characterized by depletions in the density of the ionospheric component. In some cases, density depletions correlate with large vertical components of the crustal magnetic field. Such anticorrelation in the variations of the densities of the cold ionospheric and hot magnetosheath/plasma sheet plasmas is also rather typical for localized aurora-type events on the nightside.

Remote energetic neutral atom imaging of electric potential over a lunar magnetic anomaly

The formation of electric potential over lunar magnetized regions is essential for understanding fundamental lunar science, for understanding the lunar environment, and for planning human explorati ...

Ion distributions in the vicinity of Mars: Signatures of heating and acceleration processes

More than three years of data from the ASPERA-3 instrument on-board Mars Express has been used to compile average distribution functions of ions in and around the Mars induced magnetosphere. We present samples of average distribution functions, as well as average flux patterns based on the average distribution functions, all suitable for detailed comparison with models of the near-Mars space environment. The average heavy ion distributions close to the planet form thermal populations with a temperature of 3 to 10 eV. The distribution functions in the tail consist of two populations, one cold which is an extension of the low altitude population, and one accelerated population of ionospheric origin ions. All significant fluxes of heavy ions in the tail are tailward. The heavy ions in the magnetosheath form a plume with the flow aligned with the bow shock, and a more radial flow direction than the solar wind origin flow. Summarizing the escape processes, ionospheric ions are heated close to the planet, presumably through wave-particle interaction. These heated populations are accelerated in the tailward direction in a restricted region. Another significant escape path is through the magnetosheath. A part of the ionospheric population is likely accelerated in the radial direction, out into the magnetosheath, although pick up of an oxygen exosphere may also be a viable source for this escape. Increased energy input from the solar wind during CIR events appear to mainly increase the number flux of escaping particles, the average energy of the escaping particles is not strongly affected. Heavy ions on the dayside may precipitate and cause sputtering of the atmosphere, though fluxes are likely lower than 0.4 × 1023 s−1.