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In a two-week period between February and March of 2015, a series of interplanetary coronal mass ejections (ICMEs) and solar energetic particle (SEP) events encountered Mars. The interactions were observed by several spacecraft, including Mars Express (MEX), Mars Atmosphere and Volatile Evolution Mission (MAVEN), and Mars Odyssey (MO). The ICME disturbances were characterized by an increase in ion speed, plasma temperature, magnetic field magnitude, and energetic electron flux. Furthermore, increased solar wind density and speeds, as well as unusually high local electron densities and high flow velocities were detected on the nightside at high altitudes during the March 8 event. These effects are thought to be due to the transport of ionospheric plasma away from Mars. In the deep nightside, the peak ionospheric electron density at the periapsis of MEX shows a substantial increase, reaching number densities about 2.7 × 104 cm−3 during the second ICME in the deep nightside. This corresponds to an increase in the MO High-Energy Neutron Detector flux suggesting an increase in the ionization of the neutral atmosphere due to the high intensity of charged particles. Measurements of the SEP fluxs show a substantial enhancement before the shock of a fourth ICME causing impact ionization and absorption of the surface echo intensity which drops to the noise levels, below 10−15 V2m−2 Hz−1 from values of about 2 × 10−14 V2m−2 Hz−1. Moreover, the peak ionospheric density exhibits a discrete enhancement over a period of about 30 h around the same location, which may be due to impact ionization. Ion escape rates at this time are estimated to be in the order of 1025 to 1026 s−1.

The presence of photoelectrons in ionospheres, including that of unmagnetised Venus, can be inferred from their characteristic spectral peaks in the electron energy spectrum. The electrons within the peaks are created by the photoionisation of neutrals in the upper atmosphere by the solar HeII 30.4nm line. Here, we present some case studies of photoelectron spectra observed by the ASPERA-4 instrument aboard Venus Express with corresponding ion data. In the first case study, we observe photoelectron peaks in the sunlit ionosphere, indicating relatively local production. In the second case study, we observe broadened peaks in the sunlit ionosphere near the terminator, which indicate scattering processes between a more remote production region and the observation point. In the third case study, we present the first observation of ionospheric photoelectrons in the induced magnetotail of Venus, which we suggest is due to the spacecraft being located at that time on a magnetic field line connected to the dayside ionosphere at lower altitudes. Simultaneously, low energy ions are observed moving away from Venus. In common with observations at Mars and at Titan, these imply a possible role for the relatively energetic electrons in producing an ambipolar electric field which enhances ion escape.

The dayside of the Venus ionosphere at the top of the planet׳s thick atmosphere is sustained by photoionization. The consequent photoelectrons may be identified by specific peaks in the energy spectrum at 20–30eV which are mainly due to atomic oxygen photoionization. The ASPERA-4 electron spectrometer has an energy resolution designed to identify the photoelectron production features. Photoelectrons are seen not only in their production region, the sunlit ionosphere, but also at more distant locations on the nightside of the Venus environment. Here, we present a summary of the work to date on observations of photoelectrons at Venus, and their comparison with similar processes at Titan and Mars. We expand further by presenting new examples of the distant photoelectrons measured at Venus in the dark tail and further away from Venus than seen before. The photoelectron and simultaneous ion data are then used to determine the ion escape rate from Venus for one of these intervals. We compare the observed escape rates with other rates measured at Venus, and at other planets, moons and comets. We find that the escape rates are grouped by object type when plotted against body radius.

Magnetic field and plasma measurements carried out onboard Venus Express during solar minimum conditions suggest the existence of two kinds of magnetic field configuration in the Venusian ionosphere. We interpret these as the manifestation of two different types of generation mechanisms for the induced magnetosphere. A different magnetic field topology (toroidal and poloidal) arises if the induced currents are driven either by the solar wind motional electric field or by the Faraday electric field—a conducting ionosphere sees the magnetic field carried by solar wind as a time-varying field. At the dayside, both driving agents produce a similar draping pattern of the magnetic field. However, different magnetic field signatures inherent to both induction mechanisms appear at lower altitudes in the terminator region. The conditions at low solar EUV flux when the ionosphere of Venus becomes magnetized seem to be favorable to distinguish between two different types of the induced fields. We present cases of both types of the magnetic field topology. The cases when the effects of the Faraday induction become well noticeable are especially interesting since they provide us with an example of solar wind interaction with a tiny induced dipole field immersed into the ionosphere. Another interesting case when poloidal magnetic fields are evidently displayed is observed when the IMF vector is almost aligned with the solar wind velocity. In general case, both mechanisms of induction probably complement each other.

The different types of asymmetry observed in the energy distributions of electrons and heavy-ions (M/Q=16-44) during signatures of electron precipitation in the Martian ionosphere have been classified. This has been achieved using the space plasma instrumentation of MEX ASPERA-3 from peri-centre altitude to 2200 km. ASPERA-3 ELS observes signatures of electron precipitation on 43.0% of MEX orbits. Unaccelerated electrons in the form of sudden electron flux enhancements are the most common type of electron precipitation signature at Mars and account for ∼ 70 % of the events observed in this study. Electrons that form unaccelerated electron precipitation signatures are either local ionospheric electrons with enhanced density, or electrons transported from another region of ionosphere, solar wind or tail, or a combination of local and transported electrons. The heating of electrons has a strong influence on the shape of most electron energy spectra from accelerated precipitation signatures. On most occasions the general flow of heavy-ions away from Mars is unchanged during the precipitation of electrons, which is thought to be the result of the finite gyroradius effect of the heavy-ions on crustal magnetic field lines. Only ∼ 17 % of events show some form of heavy-ion acceleration that is either concurrent or at the periphery of an electron precipitation signature. The most common combination of electron and heavy-ion energy distributions for signatures of electron precipitation involves electrons that visually have very little asymmetry or are isotropic and heavy-ions that have a upward net flux, and suggest the upward current associated with aurora. Due to a lack of reliable measurements of electrons travelling towards Mars, it is likely we miss further evidence of upward currents. The second most common combination of electron and heavy-ion energy distributions for signatures of electron precipitation, are those distributions of electrons that are asymmetric and have an net upward flux, with distributions of heavy-ions that also have a net upward flux. Energy distributions of heavy-ions with a net flux towards Mars occur half as often as heavy-ions with an upward net flux. There is also evidence to suggest we observe downward currents during electron precipitation signatures when we find energy distributions of electrons that are asymmetric and have an upward net flux, combined with energy distributions of heavy-ions that have a downward net flux. Wave particle interactions and downward parallel electric fields may be responsible for electrons that display a large amount of asymmetry in the upward direction of the energy distribution and have a upward net flux.

A very tenuous solar wind regime, following a series of large coronal mass ejections, impacted Venus during early August, 2010. STEREO-B downstream from Venus observed that the solar wind density at Earth orbit dropped to similar to 0.1 #/cm(3) and persisted at this value over 1 day. A similar low value was observed at Earth in 1999 and has attracted comprehensive attention (Lazarus, A.J., 2000. Solar physics: the day the solar wind almost disappeared. Science 287, 2172-2173.), especially its consequences on Earth's ionosphere and magnetosphere (Lockwood, M., 2001. Astronomy: the day the solar wind nearly died. Nature 409, 677-679.). We now have an opportunity to examine the response of Venus' ionosphere to such a tenuous solar wind. After Venus Express spacecraft entered the ionosphere near the terminator, it continuously sampled O+ dominated planetary plasma on the nightside till it left the optical shadow region when Venus Express was located at 2 R-V (Venus' Radii) to the Venus center and 1.1 R-V to the Sun-Venus line. Moreover, the O+ speed was lower than the gravitational escape speed. We interpret this low-speed O+ as a constituent of the extended nightside ionosphere as a consequence of long-duration (18 h) tenuous solar wind, because the very low dynamic pressure enhances the source and reduces the sink of the nightside ionosphere. Though the full extent of the nightside ionosphere is not known due to the limitation of spacecraft's trajectory, our results suggest that the global configuration of Venus' ionosphere could resemble a teardrop-shaped cometary ionosphere. (C) 2012 Elsevier Ltd. All rights reserved.

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.

The sunlit portion of planetary ionospheres is sustained by photoionization. This was first confirmed using measurements and modelling at Earth, but recently the Mars Express, Venus Express and Cassini-Huygens missions have revealed the importance of this process at Mars, Venus and Titan, respectively. The primary neutral atmospheric constituents involved (O and CO2 in the case of Venus and Mars, O and N-2 in the case of Earth and N-2 in the case of Titan) are ionized at each object by EUV solar photons. This process produces photoelectrons with particular spectral characteristics. The electron spectrometers on Venus Express and Mars Express (part of ASPERA-3 and 4, respectively) were designed with excellent energy resolution (Delta E/E-8%) specifically in order to examine the photoelectron spectrum. In addition, the Cassini CAPS electron spectrometer at Saturn also has adequate resolution (Delta E/E=16.7%) to study this population at Titan. At Earth, photoelectrons are well established by in situ measurements, and are even seen in the magnetosphere at up to 7R(E). At Mars, photoelectrons are seen in situ in the ionosphere, but also in the tail at distances out to the Mars Express apoapsis (similar to 3R(M)). At both Venus and Titan, photoelectrons are seen in situ in the ionosphere and in the tail (at up to 1.45R(V) and 6.8R(T), respectively). Here, we compare photoelectron measurements at Earth, Venus, Mars and Titan, and in particular show examples of their observation at remote locations from their production point in the dayside ionosphere. This process is found to be common between magnetized and unmagnetized objects. We discuss the role of photoelectrons as tracers of the magnetic connection to the dayside ionosphere, and their possible role in enhancing ion escape. (C) 2010 Elsevier Ltd. All rights reserved.

The upper ionospheres of Mars and Venus are permeated by the magnetic fields induced by the solar wind. It is a long-standing question whether these fields can put the dense ionospheric plasma into motion. If so, the transterminator flow of the upper ionosphere could explain a significant part of the ion escape from the planets atmospheres. But it has been technically very challenging to measure the ion flow at energies below 20 eV. The only such measurements have been made by the ORPA instrument of the Pioneer Venus Orbiter reporting speeds of 1–5 km/s for O+ ions at Venus above 300 km altitude at the terminator ( Knudsen et al., 1980 , Knudsen et al., 1982 ). At Venus the transterminator flow is sufficient to sustain a permanent nightside ionosphere, at Mars a nightside ionosphere is observed only sporadically. We here report on new measurements of the transterminator ion flow at Mars by the ASPERA-3 experiment on board Mars Express with support from the MARSIS radar experiment for some orbits with fortunate observation geometry. We observe a transterminator flow of O+ and O2+ ions with a super-sonic velocity of around 5 km/s and fluxes of 0.8×109/cm2 s. If we assume a symmetric flux around the terminator this corresponds to an ion flow of 3.1±0.5×1025/s half of which is expected to escape from the planet. This escape flux is significantly higher than previously observed on the tailside of Mars. A possible mechanism to generate this flux can be the ionospheric pressure gradient between dayside and nightside or momentum transfer from the solar wind via the induced magnetic field since the flow velocity is in the Alfvenic regime. We discuss the implication of these new observations for ion escape and possible extensions of the analysis to dayside observations which may allow us to infer the flow structure imposed by the induced magnetic field.

We present the first two-spacecraft near-simultaneous observations of the Martian bow shock (BS), magnetic pileup boundary (MPB) and photo-electron boundary (PEB) obtained by the plasma instruments onboard Rosetta and Mars Express during the Rosetta Mars flyby on February 25, 2007. Our observations are compared with shape models for the BS and MPB derived from previous statistical studies. The MPB is found at its expected position but the BS for this event is found significantly closer to the planet than expected for the rather slow and moderately dense solar wind. Cross-calibration of the density measurements on the two spacecraft gives a density profile through the magnetosheath, indicating an increasing solar wind flux during the Rosetta passage which is consistent with the multiple BS crossings at the Rosetta exit.

The analyser of space plasma and energetic atoms (ASPERA-3) neutral particle imager (NPI) on board Mars Express (MEX) is devoted to energetic neutral atom (ENA) detection within the Martian environment. These ENAs originate from the interaction between the energetic ions flowing inside the Martian environment and the exospheric neutral gas, thus providing crucial information about the dynamics of this interaction. NPI records the instantaneous angular distribution of the energy-integrated ENA signal. In order to identify recurrent ENA signals in the Martian environment, we have performed a statistical analysis of the NPI data. Count rates have been averaged using different methods in order to be able to discriminate signals coming from the planet, from a selected direction, or from specific planetographic regions at the planetary surface. Possible recurrent ENA signals (about 5×10 6 (cm 2 sr s) −1 ) are found coming from the terminator direction and above the atmosphere toward nightside when the spacecraft was inside the planetary shadow, mainly close to the shadow edge. Some significant signal was found from the anti-Mars directions in 2005. No statistically significant signal related to pick-up ions from the atmosphere or related to magnetic anomalies above the sensor intrinsic error (estimated as 3×10 6 (cm 2 sr s) −1 ) was observed. Our analysis shows that particular attention should be given to the use of NPI data when performing statistical studies; in fact, the sensor has some intrinsic limitations due to inadequate UV suppression, difficulties in sector inter-calibrations, and variations in the sector response versus time.

In a previous paper, we showed a method for deriving the interplanetary magnetic field (IMF) orientation from the velocity distribution of ring-like distributed ions as measured by the Ion Mass Analyser (IMA) on board Mars Express (MEX). This method has been improved so that one can derive the IMF orientation from a very limited portion of the ring distributions, i.e., only the highest energy portion of the ring distribution. This method uses the maximum variance direction L instead of the minimum variance direction N , which are derived from manually selected ring data. Because IMA's count rate for a semi-persistent ring distribution is nearly proportional to energy squire, L is most likely aligned to the tangential direction of the ring distribution at its highest energy, and this tangential direction is parallel or anti-parallel to the electric field. A vector product of L and the solar wind direction ( X ) gives the IMF orientation projected to the Y – Z plane. The tilt angle of IMF toward the X direction from the Y – Z plane is the same as the angle between the X direction and the ring plane, and is obtained from two methods when the initial speed of the ring ions is estimated to be much smaller than the solar wind speed: (1) angle between the velocity of ring's maximum energy portion and the solar wind vector, and (2) energy ratio between the solar wind and the maximum energy of the ring. The present method is applied to the IMA data from 3 June 2005 (0605–0640 UT) when the Mars Global Surveyor (MGS) magnetometer data are available. Using these data, we also tried to determine the sign of the IMF direction by estimating the evolution direction of the ring ions.

Data from the Ion Mass Analyzer (IMA) sensor of the ASPERA-3 instrument suite onboard Mars Express and data from the Magnetometer/Electron Reflectometer (MAG/ER) on Mars Global Surveyor have been analyzed to determine whether ion beam events (IBEs) are correlated with the direction of the draped interplanetary magnetic field (IMF) or the proximity of strong crustal magnetic fields to the subsolar point. We examined 150 IBEs and found that they are organized by IMF draping direction. However, no clear dependence on the subsolar longitude of the strongest magnetic anomaly is evident, making it uncertain whether crustal magnetic fields have an effect on the formation of the beams. We also examined data from the IMA sensor of the ASPERA-4 instrument suite on Venus Express and found that IBEs are observed at Venus as well, which indicates the morphology of the Martian and Venusian magnetotails are similar.

We report the detection of electrons due to photo-ionization of atomic oxygen and carbon dioxide in the Venus atmosphere by solar helium 30.4 nm photons. The detection was by the Analyzer of Space Plasma and Energetic Atoms (ASPERA-4) Electron Spectrometer (ELS) on the Venus Express (VEx) European Space Agency (ESA) mission. Characteristic peaks in energy for such photoelectrons have been predicted by Venus atmosphere/ionosphere models. The ELS energy resolution (ΔE/E~7%) means that these are the first detailed measurements of such electrons. Considerations of ion production and transport in the atmosphere of Venus suggest that the observed photoelectron peaks are due primarily to ionization of atomic oxygen

Recently aurora-type UV emissions were discovered on the nightside of Mars [Bertaux, J.-L., Leblanc, F., Witasse, O., et al., 2005. Discovery of an aurora on Mars. Nature 439, doi:10.1038/nature03603]. It was suggested that these emissions are produced by suprathermal electrons with energies of tens of eV, rather than by the electrons with spectra peaked above 100 eV [Leblanc, F., Witasse, O., Winningham J., et al., 2006. Origin of the martian aurora observed by spectroscopy for investigation of characteristics of the atmosphere of Mars (SPICAM) onboard Mars Express. J. Geophys. Res. 111, A09313, doi:10.1029/2006JA011763]. In this paper we present observations of fluxes of suprathermal electrons ( E e ≈ 30 –100 eV) on the Martian nightside by the ASPERA-3 experiment onboard the Mars Express spacecraft. Narrow spikes of suprathermal electrons are often observed in energy-time spectrograms of electron fluxes at altitudes between 250 and 600 km. These spikes are spatially organized and form narrow strips in regions with strong upward or downward crustal magnetic field. The values of electron fluxes in such events generally could explain the observed auroral UV emissions although a question of their origin (transport from the dayside or local precipitation) remains open.

Mars Express (MEX) does not carry its own magnetometer which complicates interpretation of ASPERA-3/MEX ion measurements. The direction of the interplanetary magnetic field (IMF) is especially important because it, among other things, determines the direction of the convective electric field and orientation of the cross tail current sheet and tail lobes. In this paper we present a case study to show the properties of the magnetic field near Mars in a quasi-neutral hybrid (QNH) model at the orbits where the Mars Global Surveyor (MGS) has made measurements, present a method to derive the IMF clock angle by comparing fields in a hybrid model and the direction of the magnetic field measured by MGS by deriving the IMF clock angle. We also use H + ring velocity distribution observations upstream of the bow shock measured by the IMA/ASPERA-3 instrument on board MEX spacecraft. These observations are used to indirectly provide the orientation of the IMF. We use a QNH model (HYB-Mars) where ions are modeled as particles while electrons form a mass-less charge neutralizing fluid. We found that the direct MGS and non-direct IMA observations of the orientation magnetic field vectors in non-crustal magnetic field regions are consistent with the global magnetic field draping pattern predicted by the global model.

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.

In December 2006, a single active region produced a series of proton solar flares, with X-ray class up to the X9.0 level, starting on 5 December 2006 at 10:35 UT. A feature of this X9.0 flare is that associated MeV particles were observed at Venus and Mars by Venus Express (VEX) and Mars Express (MEX), which were ∼80° and ∼125° east of the flare site, respectively, in addition to the Earth, which was ∼79° west of the flare site. On December 5, 2006, the plasma instruments ASPERA-3 and ASPERA-4 on board MEX and VEX detected a large enhancement in their respective background count levels. This is a typical signature of solar energetic particle (SEP) events, i.e., intensive MeV particle fluxes. The timings of these enhancements were consistent with the estimated field-aligned travel time of particles associated with the X9.0 flare that followed the Parker spiral to reach Venus and Mars. Coronal mass ejection (CME) signatures that might be related to the proton flare were twice identified at Venus within

Energetic electron fluxes from more than two years of ASPERA-3 observations are organized in different coordinate systems for the investigation of asymmetries in the global dynamics of the Martian magnetosphere. A clear asymmetry is found in the distribution of high-flux events with respect to the solar wind convective electric field ( E sw ) direction. These events are frequently detected below the average magnetic pile-up boundary (MPB) location at the terminator region of the hemisphere to which the E sw points and extend toward the tail. A detailed investigation of the electron fluxes at the terminator region also reveals that the largest contribution to this E sw asymmetry comes from locations of moderate or strong crustal fields. These observations have implications about reconnection processes in the terminator and provide new insight on magnetic anomaly effects in the global dynamics of the Mars–solar wind interaction.