Ionospheric plasma acceleration at Mars: ASPERA-3 results

The Analyzer of Space Plasma and Energetic Atoms (ASPERA) on-board the Mars Express spacecraft (MEX) measured penetrating solar wind plasma and escaping/accelerated ionospheric plasma at very low altitudes (250 km) in the dayside subsolar region. This implies a direct exposure of the martian topside atmosphere to solar wind plasma forcing leading to energization of ionospheric plasma. The ion and electron energization and the ion outflow from Mars is surprisingly similar to that over the magnetized Earth. Narrow “monoenergetic” cold ion beams, ion beams with broad energy distributions, sharply peaked electron energy spectra, and bidirectional streaming electrons are particle features also observed near Mars. Energized martian ionospheric ions (O⁺, O⁺₂, CO⁺₂, etc.) flow in essentially the same direction as the external sheath flow. This suggests that the planetary ion energization couples directly to processes in the magnetosheath/solar wind. On the other hand, the beam-like distribution of the energized plasma implies more indirect energization processes like those near the Earth, i.e., energization in a magnetized environment by waves and/or parallel (to B) electric fields. The general conditions for martian plasma energization are, however, different from those in the Earth's magnetosphere. Mars has a weak intrinsic magnetic field and solar wind plasma may therefore penetrate deep into the dense ionospheric plasma. Local crustal magnetization, discovered by Acuña et al. [Acuña, M.J., Connerey, J., Ness, N., Lin, R., Mitchell, D., Carlsson, C., McFadden, J., Anderson, K., Rème, H., Mazelle, C., Vignes, D., Wasilewski, P., Cloutier, P., 1999. Science 284, 790-793], provide some dayside shielding against the solar wind. On the other hand, multiple magnetic anomalies may also lead to “hot spots” facilitating ionospheric plasma energization. We discuss the ASPERA-3 findings of martian ionospheric ion energization and present evidences for two types of plasma energization processes responsible for the low- and mid-altitude plasma energization near Mars: magnetic field-aligned acceleration by parallel electric fields and plasma energization by low frequency waves.     

Solar wind plasma protrusion into the martian magnetosphere: ASPERA-3 observations

The ASPERA-3 experiment onboard the Mars Express spacecraft revealed, near the wake boundary of Mars, a spatially narrow, strip-like plasma structure composed of magnetosheath-like electrons and planetary ions. The peak electron energy often exceeds the peak energy at the bow shock that indicates a significant heating (acceleration) during the structure formation. It is shown that this structure is formed during efficient plasma penetration into the martian magnetosphere in the region near the terminator. The penetration of sheath electrons and their gradual heating (acceleration) is accompanied by a change of the ion composition from a solar wind plasma to a planetary plasma dominated by oxygen ions. A possible mechanism of plasma inflow to the magnetosphere is discussed.     

Asymmetry of plasma fluxes at Mars. ASPERA-3 observations and hybrid simulations

The asymmetry of fluxes of solar wind and planetary ions is studied by using the ASPERA-3 observations onboard the Mars Express spacecraft in February 2004 to March 2006. Due to the small scale of the Martian magnetosphere and its induced origin, the flow pattern near Mars is sensitive to the directions of the interplanetary magnetic and electric (-V×B) fields. Asymmetry of the magnetic field draping produces an asymmetry in plasma flows in the plane containing the IMF. The crustal magnetic fields on Mars also influence the flow pattern. Scavenging of planetary ions is less efficient in the regions of strong crustal magnetization and therefore the escape fluxes of planetary ions in the southern hemisphere are smaller. The results of the observations are compared to simulations based on a 3D hybrid model with several ion species.     

GEOS-2 measurements of cold ions in the magnetosheath

The Suprathermal Plasma Analysers on GEOS-2 are able to make differential energy measurements of plasma particles down to sub-eV energies because the entire sensor package can be biased relative to the spacecraft. When the package is biased negatively with respect to space potential, low energy positive ions are sucked in and are more easily detected against the background. Large fluxes of ions with temperatures of the order of 1 eV or less were consistently detected at space potential when the spacecraft was in the magnetosheath though not when it was in the nearby magnetosphere. This apparent geophysical correlation, suggesting that the ions were part of the magnetosheath ion population, was contradicted by the fact that the ions showed no signs of the large drift velocity associated with the electric field in the magnetosheath. We conclude, after further investigation, that the observed ions were probably sputtered as neutrals from the spacecraft surface by the impact of solar wind ions and subsequently ionized by sunlight or electron impact. The effect of sputtering by solar wind ions has not been previously observed, although it could have consequences for the long-term stability of spacecraft surfaces.     

Measurements of hot plasmas in the magnetosphere of Jupiter

This paper presents an overview of a number of the principal findings regarding the hot plasmas (E 50 keV) in Jupiter's magnetosphere by the HISCALE instrument during the encounter of the Ulysses spacecraft with the planet in February 1992. The hot plasma ion fluxes measured by HI-SCALE in the dayside magnetosphere are similar to those measured in the same energy range in this region by the Voyager spacecraft in 1979. Within the dayside plasma sheet, the hot-ion energy densities are comparable with, or larger than, the magnetic field energy densities; these hot ions are found to corotate at about one-half the planetary corotational speed. For ions of energies 500 keV/nucleon, the protons contributed from 50–60% to as much as 80% of the energy content of these plasmas. Strong, magnetic-field-aligned streaming was found for both the ions and electrons in the high-latitude duskside magnetosphere. The ion and electron pitch-angle distributions could be characterized by cos²⁵ α throughout many of the high anisotropy intervals of the outbound pass. There is some evidence in the ion pitch-angle distributions for a corotational component in the hot plasmas at high Jovian latitudes. While there are limitations owing to the finite geometries of the detector telescope systems on the determination of the angular spreads of the ion and electron beams, the measurements show that there are intervals when the particle distributions are not bidirectional. At such times, locally the hot plasmas could be carrying currents of 10⁻⁴μAm⁻². The temporal variations in the streaming electron fluxes are substantially larger than the variations measured for the fluxes that are more locally mirroring. The temporal variations contain periodicities that may correspond to hydromagnetic wave frequencies in the magnetosphere as well as to larger scale motions of magnetospheric plasmas. On nearly half of the days for about a 130 day interval around the time of the Ulysses encounter with the planet, particles of Jovian origin were measured in the interplanetary medium. An event discussed herein shows evidence of an energy dependence of the particle release process from the planetary magnetosphere into the interplanetary medium.     

Energization of charged particles in planetary magnetospheres

A model is presented to describe the energization of charged particles in planetary magnetospheres. The model is based on the stochastic acceleration produced by a random electric field that is induced by the magnetic field fluctuations measured within the magnetospheres. The stochastic behavior of the electric field is simulated through a Monte Carlo method. We solve the equation of motion for a single charged particle—which comprises the stochastic acceleration due to the stochastic electric field, the Lorentz acceleration (containing the local magnetic field and the corotational electric field) and the gravitational planetary acceleration of the particle—under several initial conditions. The initial conditions include the ion species and the velocity distribution of the particles which depends on the sources they come from (solar wind, ionospheres, rings and satellites). We applied this model to Saturn’s inner magnetosphere using a sample of particles (H⁺, H₂O⁺, N⁺, O⁺ and OH⁺) initially located on Saturn’s north pole, above the C-Ring, on the south pole of Enceladus, in the north pole of Dione and above the E-Ring. The results show that the particles tend to increase the value of their energy with time reaching several eV in a few seconds and the large energization is observed far from the planet. We can distinguish three main energization regions within Saturn’s inner magnetosphere: minimum (Saturn’s ionosphere), intermediate (Dione) and high-energy (Enceladus and the E-ring). The resulting energy spectrum follows a power-law distribution (>1 keV), a logistic, an exponential decay or an asymmetric sigmoidal (<1 keV).     

Ion escape from Mars as a function of solar wind conditions: A statistical study

The influence of solar EUV and solar wind conditions on ion escape at Mars is investigated using ion data from the Aspera-3 instrument on Mars Express, combined with solar wind proxy data obtained from the Mars Global Surveyor (MGS) spacecraft. A solar EUV flux proxy based on data from the Earth position, scaled and shifted in time for Mars, is used to study relatively long time scale changes related to solar EUV variability. Data from May 2004 until November 2005 has been used. A clear dependence on the strength of the subsolar magnetic field as inferred from MGS measurements is seen in the ion data. The region of significant heavy ion flows is compressed and the heavy ion flux density is higher for high subsolar magnetic field strength. Because of the difference in outflow area, the difference in estimated total outflow is somewhat less than the difference in average flux density. We confirm previous findings that escaping planetary ions are mainly seen in the hemisphere into which the solar wind electric field is pointed. The effect is more pronounced for the high subsolar magnetic field case.The average ion motion has a consistent bias towards the direction of the solar wind electric field, but the main motion is in the antisunward direction. The antisunward flow velocity increases with tailward distance, reaching above at 2 to 3 martian radii downtail from Mars for O⁺ ions. Different ion species reach approximately the same bulk flow energy. We did not find any clear correlation between the solar EUV flux and the ion escape distribution or rate, probably because the variation of the solar EUV flux over our study interval was too small. The results indicate that the solar wind and its magnetic field directly interacts with the ionosphere of Mars, removing more ions for high subsolar magnetic field strength. The interaction region and the tail heavy ion flow region are not perfectly shielded from the solar wind electric field, which accelerates particles over relatively large tail distances.     

Comparative investigation of the terrestrial and Venusian magnetopause: Kinetic modeling and experimental observations by Cluster and Venus Express

In June 2006 Venus Express crossed several times the outer boundary of Venus induced magnetosphere, its magnetosheath and its bow shock. During the same interval the Cluster spacecraft surveyed the dawn flank of the terrestrial magnetosphere, intersected the Earth's magnetopause and spent long time intervals in the magnetosheath. This configuration offers the opportunity to perform a joint investigation of the interface between Venus and Earth's outer plasma layers and the shocked solar wind. We discuss the kinetic structure of the magnetopause of both planets, its global characteristics and the effects on the interaction between the planetary plasma and the solar wind. A Vlasov equilibrium model is constructed for both planetary magnetopauses and provides good estimates of the magnetic field profile across the interface. The model is also in agreement with plasma data and evidence the role of planetary and solar wind ions on the spatial scale of the equilibrium magnetopause of the two planets. The main characteristics of the two magnetopauses are discussed and compared.     

Jupiter's outer atmosphere

The current state of the theory of Jupiter's outer atmosphere is briefly reviewed. The similarities and dissimilarities between the terrestrial and Jovian upper atmospheres are discussed, including the interaction of the solar wind with the planetary magnetic fields. Estimates of Jovian parameters are given, including magnetosphere and auroral zone sizes, ionospheric conductivity, energy inputs, and solar wind parameters at Jupiter. The influence of the large centrifugal force on the cold plasma distribution is considered. The Jovian Van Alien belt is attributed to solar wind particles diffused in towards the planet by dynamo electric fields from ionospheric neutral winds and consequences of this theory are given.     

Electron transport and precipitation at Mercury during the MESSENGER flybys: Implications for electron-stimulated desorption

To examine electron transport, energization, and precipitation in Mercury's magnetosphere, a hybrid simulation study has been carried out that follows electron trajectories within the global magnetospheric electric and magnetic field configuration of Mercury. We report analysis for two solar-wind parameter conditions corresponding to the first two MESSENGER Mercury flybys on January 14, 2008, and October 6, 2008, which occurred for similar solar wind speed and density but contrasting interplanetary magnetic field (IMF) directions. During the first flyby the IMF had a northward component, while during the second flyby the IMF was southward. Electron trajectories are traced in the fields of global hybrid simulations for the two flybys. Some solar wind electrons follow complex trajectories at or near where dayside reconnection occurs and enter the magnetosphere at these locations. The entry locations depend on the IMF orientation (north or south). As the electrons move through the entry regions they can be energized as they execute non-adiabatic (demagnetized) motion. Some electrons become magnetically trapped and drift around the planet with energies on the order of 1–10 keV. The highest energy of electrons anywhere in the magnetosphere is about 25 keV, consistent with the absence of high-energy (>35 keV) electrons observed during either MESSENGER flyby. Once within the magnetosphere, a fraction of the electrons precipitates at the planetary surface with fluxes on the order of 10⁹ cm⁻² s⁻¹ and with energies of hundreds of eV. This finding has important implications for the viability of electron-stimulated desorption (ESD) as a mechanism for contributing to the formation of the exosphere and heavy ion cloud around Mercury. From laboratory estimates of ESD ion yields, a calculated ion production rate due to ESD at Mercury is found to be on par with ion sputtering yields.     

Plasma interactions of exoplanets with their parent star and associated radio emissions

The relatively high contrast between planetary and solar low-frequency radio emissions suggests that the low-frequency radio range may be well adapted to the direct detection of exoplanets. We review the most significant properties of planetary radio emissions (auroral as well as satellite induced) and show that their primary engine is the interaction of a plasma flow with an obstacle in the presence of a strong magnetic field (of the flow or of the obstacle). Scaling laws have been derived from solar system planetary radio emissions that relate the emitted radio power to the power dissipated in the various corresponding flow–obstacle interactions. We generalize these scaling laws into a “radio-magnetic” scaling law that seems to relate output radio power to the magnetic energy flux convected on the obstacle, this obstacle being magnetized or unmagnetized. Extrapolating this scaling law to the case of exoplanets, we find that hot Jupiters may produce very intense radio emissions due to either magnetospheric interaction with a strong stellar wind or to unipolar interaction between the planet and a magnetic star (or strongly magnetized regions of the stellar surface). In the former case, similar to the magnetosphere–solar wind interactions in our solar system or to the Ganymede–Jupiter interaction, a hecto-decameter emission is expected in the vicinity of the planet with an intensity possibly 10³–10⁵ times that of Jupiter's low frequency radio emissions. In the latter case, which is a giant analogy of the Io–Jupiter system, emission in the decameter-to-meter wavelength range near the footprints of the star's magnetic field lines interacting with the planet may reach 10⁶ times that of Jupiter (unless some “saturation” mechanism occurs). The system of HD179949, where a hot spot has been tentatively detected in visible light near the sub-planetary point, is discussed in some details. Radio detectability is addressed with present and future low-frequency radiotelescopes. Finally, we discuss the interests of direct radio detection, among which access to exoplanetary magnetic field measurements and comparative magnetospheric physics.     

Modeling of the magnetosphere of Mercury at the time of the first MESSENGER flyby

The MESSENGER spacecraft flyby of Mercury on 14 January 2008 provided a new opportunity to study the intrinsic magnetic field of the innermost planet and its interaction with the solar wind. The model presented in this paper is based on the solution of the three-dimensional, bi-fluid equations for solar wind protons and electrons in the absence of mass loading. In this study we provide new estimates of Mercury’s intrinsic magnetic field and the solar wind conditions that prevailed at the time of the flyby. We show that the location of the boundary layers and the strength of the magnetic field along the spacecraft trajectory can be reproduced with a solar wind ram pressure P_sw = 6.8 nPa and a planetary magnetic dipole having a magnitude of 210 R_M³ − nT and an offset of 0.18 R_M to the north of the equator, where R_M is Mercury’s radius. Analysis of the plasma flow reveals the existence of a stable drift belt around the planet; such a belt can account for the locations of diamagnetic decreases observed by the MESSENGER Magnetometer. Moreover, we determine that the ion impact rate at the northern cusp was four times higher than at the southern cusp, a result that provides a possible explanation for the observed north-south asymmetry in exospheric sodium in the neutral tail.     

Structure of the martian wake

We present the first results from the ion mass analyzer IMA of the ASPERA-3 instrument on-board of Mars Express. More than 200 orbits for May 2004-September 2004 time interval have been selected for the statistical study of the distribution of the atmospheric origin ions in the planetary wake. This study shows that the martian magnetotail consists of two different ion regimes. Planetary origin ions of the first regime form the layer adjacent to the magnetic pile-up boundary. These ions are accelerated to energy greater than 2000 eV and exhibit a gradual decreasing of energy down to the planetary tail. The second plasma regime is observed in the planetary shadow. The heavy ions (considered as planetary ones) are accelerated to the energy of the solar wind protons. Obviously the acceleration mechanism is different for the different plasma regimes. Study of two plasma regimes in the frame referred to the interplanetary magnetic field (IMF) direction (we used MGS magnetometer data to obtain the IMF clock angle) clearly shows their spatial anisotropy. The monoenergetic plasma in the planetary shadow is observed only in the narrow angular sector around the positive direction of the interplanetary electric field.     

Distribution of ionospheric currents induced by the solar wind interaction with Venus

The electric currents induced in the atmosphere of a non-magnetic planet such as Venus by the interaction of the solar wind satisfy a generalized Ohm's Law relationship with tensor conductivity. The distribution of these currents within the planetary ionosphere may be calculated by a variational technique which minimizes the Joule heating over the ionospheric volume. In this paper, we present the development of the variational technique, and apply it to a model of the solar wind interaction with Venus.Potential and current distributions are shown, and the use of these distributions in determining convective transport patterns of planetary ions is discussed.     

Centrifugal instability of the jovian magnetosphere and its interaction with the solar wind

The outer regions (r > 2.3 R_j; R_j = radius of Jupiter) of the magnetosphere of Jupiter will systematically accumulate plasma. If sufficient plasma accumulates, the field lines must open to allow the plasma to escape. Available energy sources appear able to supply plasma at a high enough rate to keep the field lines constantly open beyond about 60 R_J. We suggest that the solar wind interaction with Jupiter may be essentially different from that with the Earth, with the Jovian magnetosphere opening up to form a planetary wind.     

Dayside H circulation at Mercury and neutral particle emission

In this study we discuss the proton circulation and the neutral atom emission at Mercury. The H⁺ distribution in space, energy and pitch angle has been simulated by means of a single-particle Monte Carlo model. The applied electric and magnetic field model has been parameterized to take into account different boundary conditions such as interplanetary magnetic field and cross-tail potential drop. Particular attention has been paid to the estimation of the surface-sputtered neutral atoms and the energetic neutral atoms generated via charge-exchange process. The peculiar configuration of the hermean magnetosphere, as it is expected after the Mariner-10 observation of a weak magnetic field, allows a significant part of the incoming solar wind to enter Mercury's environment. For the Bz<−10 nT conditions, intense ion fluxes are expected in the cusp regions, which are extremely large when compared to the Earth's ones. The solar wind particles are likely to rapidly leave the hermean magnetosphere or precipitate onto the planetary surface, thus originating neutral particle emission via ion-sputtering. Two instruments, proposed to fly on board ESA mission BepiColombo (namely: the NPA-IS SERENA suite on the MPO segment and the ENA instrument on the MMO segment) will monitor the neutral signal as well as the precipitating ion particles. The modeled distribution presented here may be considered as a reference tool for the future observations.     

Comparative study of ion cyclotron waves at Mars, Venus and Earth

Ion cyclotron waves are generated in the solar wind when it picks up freshly ionized planetary exospheric ions. These waves grow from the free energy of the highly anisotropic distribution of fresh pickup ions, and are observed in the spacecraft frame with left-handed polarization and a wave frequency near the ion’s gyrofrequency. At Mars and Venus and in the Earth’s polar cusp, the solar wind directly interacts with the planetary exospheres. Ion cyclotron waves with many similar properties are observed in these diverse plasma environments. The ion cyclotron waves at Mars indicate its hydrogen exosphere to be extensive and asymmetric in the direction of the interplanetary electric field. The production of fast neutrals plays an important role in forming an extended exosphere in the shape and size observed. At Venus, the region of exospheric proton cyclotron wave production may be restricted to the magnetosheath. The waves observed in the solar wind at Venus appear to be largely produced by the solar-wind-Venus interaction, with some waves at higher frequencies formed near the Sun and carried outward by the solar wind to Venus. These waves have some similarity to the expected properties of exospherically produced proton pickup waves but are characterized by magnetic connection to the bow shock or by a lack of correlation with local solar wind properties respectively. Any confusion of solar derived waves with exospherically derived ion pickup waves is not an issue at Mars because the solar-produced waves are generally at much higher frequencies than the local pickup waves and the solar waves should be mostly absorbed when convected to Mars distance as the proton cyclotron frequency in the plasma frame approaches the frequency of the solar-produced waves. In the Earth’s polar cusp, the wave properties of ion cyclotron waves are quite variable. Spatial gradients in the magnetic field may cause this variation as the background field changes between the regions in which the fast neutrals are produced and where they are re-ionized and picked up. While these waves were discovered early in the magnetospheric exploration, their generation was not understood until after we had observed similar waves in the exospheres of Mars and Venus.     

First ENA observations at Mars: Solar-wind ENAs on the nightside

We present measurements with an Energetic Neutral Atom (ENA) imager on board Mars Express when the spacecraft moves into Mars eclipse. Solar wind ions charge exchange with the extended Mars exosphere to produce ENAs that can spread into the eclipse of Mars due to the ions' thermal spread. Our measurements show a lingering signal from the Sun direction for several minutes as the spacecraft moves into the eclipse. However, our ENA imager is also sensitive to UV photons and we compare the measurements to ENA simulations and a simplified model of UV scattering in the exosphere. Simulations and further comparisons with an electron spectrometer sensitive to photoelectrons generated when UV photons interact with the spacecraft suggest that what we are seeing in Mars' eclipse are ENAs from upstream of the bow shock produced in charge exchange with solar wind ions with a non-zero temperature. The measurements are a precursor to a new technique called ENA sounding to measure solar wind and planetary exosphere properties in the future.     

Electron reflectometry in the martian atmosphere

The technique of electron reflectometry, a method for remote estimation of planetary magnetic fields, is expanded from its original use of mapping crustal magnetic fields at the Moon to achieving the same purpose at Mars, where the presence of a substantial atmosphere complicates matters considerably. The motion of solar wind electrons, incident on the martian atmosphere, is considered in detail, taking account of the following effects: the electrons' helical paths around the magnetic field lines to which they are bound, the magnetic mirror force they experience due to converging field lines in the vicinity of crustal magnetic anomalies, their acceleration/deceleration by electrostatic potentials, their interactions with thermal plasma, their drifts due to magnetic field line curvature and perpendicular electric fields and their scattering off, and loss of energy through a number of different processes to, atmospheric neutrals. A theoretical framework is thus developed for modeling electron pitch angle distributions expected when a spacecraft is on a magnetic field line which is connected to both the martian crust and the interplanetary magnetic field. This framework, along with measured pitch angle distributions from the Mars Global Surveyor (MGS) Magnetometer/Electron Reflectometer (MAG/ER) experiment, can be used to remotely measure crustal magnetic field magnitudes and atmospheric neutral densities at ∼180 km above the martian datum, as well as estimate average parallel electric fields between 200 and 400 km altitude. Detailed analysis and full results, concerning the crustal magnetic field and upper thermospheric density of Mars, are left to two companion papers.     

A statistical study of flux ropes in the Martian magnetosphere

Using minimum variance analysis of the circular mapping data from the Mars Global Surveyor (MGS) spacecraft during four selected weeks of observation, we identify 360 magnetic field structures in the Martian topside ionosphere with characteristic signatures of flux ropes. Physical parameters including size, peak field strength, helicity, orientation, and external conditions at the time of each observation are compiled for the events in each population. We observe that Martian flux ropes typically have a peak field amplitude of ∼15 nT and a diameter of ∼80–100 km assuming they are stationary. Flux ropes tend to be aligned approximately parallel to the planetary surface, and perpendicular to the direction from which the solar wind flows. They are more frequently observed during times of low solar wind pressure, but do not show a clear preference for a particular Interplanetary Magnetic Field (IMF) draping direction. Flux rope characteristics of peak field amplitude, diameter, and helicity vary with solar zenith angle. Amplitudes tend to be higher during periods of high solar wind pressure. The events are sorted into three populations based on the location at which they were observed, possibly corresponding to distinct formation mechanisms. Flux ropes observed in eclipse tend to have smaller peak amplitudes and are larger than those observed in sunlight, and are less likely to be oriented parallel to the planetary surface. Proximity to crustal fields does not appear to influence the characteristics of flux ropes observed at the 400 km spacecraft altitude. The frequent observation of flux rope structures near Mars in a variety of locations suggests that the low-altitude plasma environment is quite dynamic, with magnetic shear playing a prominent role in determining magnetic field structure near the planet.