E. Kallio

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.

The loss of ions from Venus through the plasma wake

Venus, unlike Earth, is an extremely dry planet although both began with similar masses, distances from the Sun, and presumably water inventories. The high deuterium-to-hydrogen ratio in the venusian atmosphere relative to Earth’s also indicates that the atmosphere has undergone significantly different evolution over the age of the Solar System. Present-day thermal escape is low for all atmospheric species. However, hydrogen can escape by means of collisions with hot atoms from ionospheric photochemistry, and although the bulk of O and O2 are gravitationally bound, heavy ions have been observed to escape through interaction with the solar wind. Nevertheless, their relative rates of escape, spatial distribution, and composition could not be determined from these previous measurements. Here we report Venus Express measurements showing that the dominant escaping ions are O+, He+ and H+. The escaping ions leave Venus through the plasma sheet (a central portion of the plasma wake) and in a boundary layer of the induced magnetosphere. The escape rate ratios are Q(H+)/Q(O+) = 1.9; Q(He+)/Q(O+) = 0.07. The first of these implies that the escape of H+ and O+, together with the estimated escape of neutral hydrogen and oxygen, currently takes place near the stoichometric ratio corresponding to water.

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 proton precipitation in the Martian atmosphere and its connection to the Mars-solar wind interaction

Atmospheric effects of precipitating solar wind protons in the Martian atmosphere are studied. The proton flux to the atmosphere is derived from a newly developed global quasineutral hybrid simulation which includes solar wind H + ions and planetary O + ions. The motion of the precipitating particles in the atmosphere is followed, and the effects of collisions to atmospheric neutrals are studied by a collision-to-collision Monte Carlo algorithm. Maximum atmospheric effects are estimated by using a fully absorbing boundary condition in the hybrid model where all solar wind protons are allowed to precipitate into the atmosphere without reflection. The developed mass-loaded hybrid code is found to reproduce many of the observed plasma and field features near Mars. When the vertical profiles of the energy deposition rates, CO 2 + ionization rates, and Lyman alpha emission rates are calculated at different solar zenith angles, the maximum atmospheric effects on the dayside under average solar wind conditions are found to be typically a few percent of the effects of EUV radiation. On the nightside the proton precipitation is estimated to be intensive enough to be able to produce the measured ionospheric electron densities. The analysis illustrates that the atmospheric effects are strongly coupled with the global plasma interaction process between Mars and the solar wind.

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.

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.

Hemispheric asymmetry of the magnetic field wrapping pattern in the Venusian magnetotail

We examine statistically the magnetic field in the Venusian magnetotail which is formed by the draping of interplanetary magnetic field lines. Although the near-planet and distant magnetotail regions have been sampled by the various missions to Venus and the general magnetic features of the distant magnetotail are well established, the near wake region from about 1.3 to 3 Venusian radii downstream of the planet remained unexplored until the Venus Express mission. Here we report the unanticipated finding of a draped field reversal in one hemisphere of the near Venus tail. When ordered by the interplanetary electric field orientation, the magnetic field lines in the hemisphere with inward motional electric field apparently are wrapped more tightly around Venus than in the other hemisphere, thus forming a field reversal region in the this portion of the near tail. A global hybrid simulation produces what we see and provides a three-dimensional view of the observed hemispherical asymmetry.

Venus-solar wind interaction: Asymmetries and the escape of O+ ions

Abstract We study the interaction between Venus and the solar wind using a global three-dimensional self-consistent quasi-neutral hybrid (QNH) model. The model treats ions ( H + , O + ) as particles and electrons as a massless charge neutralizing fluid. In the analysed Parker spiral interplanetary magnetic field (IMF) case ( IMF = [ 8.09 , 5.88 , 0 ] nT ) , a notable north–south asymmetry of the magnetic field and plasma exists, especially in the properties of escaping planetary O + ions. The asymmetry is associated with ion finite gyroradius effects. Furthermore, the IMF x-component results in a dawn–dusk asymmetry. Overall, the QNH model is found to reproduce the main observed plasma and magnetic field regions (the bow shock, the magnetosheath, the magnetic barrier and the magnetotail), implying the potential of the developed model to study the Venusian plasma environment and especially the non-thermal ion escape.

A test particle simulation of the motion of oxygen ions and solar wind protons near Mars

Outflowing oxygen ions (O+) have been observed to be a persistent feature in the Martian tail. However, not much is known of the spatial distribution of oxygen ions, of the correlation between oxygen ions and solar wind protons (H+), and, especially, of the acceleration of oxygen ions in the tail. We present a test particle simulation study of the motion of O+ and H+ ions and compare the results to Automatic Space Plasma Experiment with a Rotating Analyzer (ASPERA) Phobos 2 particle measurements. We have studied the spatial distribution, velocity, density, and flux of oxygen ions in the nightside. In the simulation the oxygen ions were ionized from the Martian hot oxygen corona and the H+ ions were solar wind protons. We used an empirical flow model to derive the magnetic and electric field everywhere around the planet. The work is the first test particle simulation for Mars where a fully three-dimensional magnetotail configuration has been used. The model reproduces many observed plasma features. The solar wind protons flow fluid-like near the terminator despite their finite Larmor radii. The oxygen ions produce a plasmasheet-like layer near the cross-tail current sheet and empty magnetic lobes, and have north-to-south asymmetry in the magnetosheath and in the tail much as observed. The energy of oxygen ions in the tail is close to, but slightly less than, observed. The particle density and the particle flux of oxygen ions also agree quite well with the observations, suggesting that the total O+ outflow rate is ∼ 2 × 1025 s−1. Overall, the study suggest that most of the observed O+ outflow features can be understood by assuming that the ions are accelerated by the convective electric field associated with the flow of the solar wind protons.

An empirical model of the solar wind flow around Mars

An empirical model that describes the proton velocity around Mars has been developed. The analytical axially symmetric model is based on Automatic Space Plasma Experiment with a Rotating Analyzer three-dimensional (3-D) proton velocity observations on the Phobos 2 spacecraft near Mars in early 1989. The model includes a bow shock, a magnetopause, and an impenetrable obstacle a few hundred kilometers above the surface of Mars. The flow model can be used to model both an open and a closed magnetosphere. The model velocity values are found to describe rather well the 3-D proton velocity observations in the Martian magnetosheath and magnetosphere both on the dayside and on the nightside. Characteristics of the velocity field are studied on the basis of an ideal MHD model, assuming that the magnetic field is frozen into the proton flow and that there are no proton sources or sinks. Under these assumptions the velocity field can be used to calculate the proton density and the magnetic field. When the proton density and the magnetic field are compared with the observations, quite good qualitative agreement between the model and the data is found. Our model shows that a relative simple flow model can predict several observed plasma and field features near Mars.