The Advanced Small Analyzer for Neutrals (ASAN) is a compact mass resolving energetic neutral atom analyzer flown on the Yutu-2 rover of the Chinese Chang’E-4 mission to the Moon. ASAN measures energetic neutral atoms in the energy range from 10 eV to 10 keV with typically 30% energy resolution. The field-of-view is a single angular pixel of 37° × 30° pointing at lunar surface. Energetic neutral atoms enter the instrument through a charge particle deflection system and are then ionized on a diamond-like carbon conversion surface. Positive ions from the conversion surface are energy analyzed using an electrostatic analyzer and their velocity is subsequently determined in a time-of-flight section using ceramic channel electron multipliers as detectors. The obtained mass resolution m/$\Delta$m is about 2, comfortably separating neutral hydrogen, helium, and the oxygen-group (≥16 amu). A full energy spectrum for energetic neutral atoms is obtained in 3 seconds. Switching the charged particle deflection system off, ASAN also directly measures positive ions with similar performance as for energetic neutral atoms. Including electronics the instrument weighs 970 g and consumes 3.4 W.

Oxygen isotope ratios provide important constraints on the history of the Martian volatile system, revealing the impact of several processes that might fractionate them, such as atmospheric loss into space or interaction with the surface. We report infrared measurements of the Martian atmosphere obtained with the mid-infrared channel (MIR) of the Atmospheric Chemistry Suite (ACS), onboard the ExoMars Trace Gas Orbiter. Absorption lines of the three main oxygen isotopologues of water vapour (H216O, H218O, and H217O) observed in the transmission spectra allow, for the first time, the measurement of vertical profiles of the 18O/16O and 17O/16O ratios in atmospheric water vapour. The observed ratios are enriched with respect to Earth-like values (δ18O = 200 ± 80‰ and δ17O = 230 ± 110‰ corresponding to the Vienna Standard Mean Ocean Water). The vertical structure of these ratios does not appear to show significant evidence of altitudinal variations.

We analyze the data on hydrogen energetic neutral atoms (ENAs) emissions from the dayside of Mars, recorded by a Neutral Particle Detector of the Analyzer of Space Plasmas and Energetic Atoms aboard Mars Express from 14 March to 9 July 2004. We first identify and analyze events of the ENA flux enhancement coinciding with the presence of the crustal magnetic anomalies on the dayside of Mars. We then backtrace the ENA emissions to the lower altitudes (source region) and build up an average map of the flux intensities in the geographic coordinates with all the available data. The map shows a peak-to-valley ENA flux enhancement of 40%-90% close to the crustal magnetic anomaly regions. These results suggest the influence of the magnetic anomalies on the ENA emission from the dayside of Mars. The enhancement may result from the deviation of the highly directional plasma flow above anomalies toward the detectors such that more charge exchange ENAs would be recorded. Alternatively, higher exospheric densities above the anomalies would also result in an increase of the charge exchange ENA flux.

Using data collected by the neutral particle detector of the analyzer of space plasmas and energetic atoms on board Mars Express, we establish the global morphology of hydrogen energetic neutral atom (ENA) fluxes. The observations were conducted in March–July 2004 and reveal that low flux ENAs (<5×105cm−2s−1sr−1) are emitted roughly isotropically from the dayside of the planet. These ENA emissions originate from the backscattering of the hydrogen ENAs produced by the charge exchange of the upstream solar wind in the extended Martian exosphere. ENAs of higher fluxes are emitted preferentially in the directions perpendicular to the undisturbed solar wind velocity. This population is due to the charge exchange of the magnetosheath plasma. At a great distance, these ENA emissions resemble a thick layer or a wall. The emissions are enhanced in the direction opposite to the convective electric field (Ec). This enhancement reflects the asymmetric shape of the induced magnetic boundary that is close to the planet hemisphere where the Ec points toward the Sun‐Mars line (−Ec hemisphere). The ENA flux shows a positive correlation with the solar wind dynamic pressure. The asymmetry in the ENA emissions is modest during quiet solar wind conditions but becomes more pronounced when the solar wind dynamic pressure increases.

We re-analyze the signal of non-planetary energetic neutral atoms (ENAs) in the 0.4–5.0 keV range measured with the Neutral Particle Detector (NPD) of the ASPERA-3 and ASPERA-4 experiments on board the Mars and Venus Express satellites. Due to improved knowledge of sensor characteristics and exclusion of data sets affected by instrument effects, the typical intensity of the ENA signal obtained by ASPERA-3 is an order of magnitude lower than in earlier reports. The ENA intensities measured with ASPERA-3 and ASPERA-4 now agree with each other. In the present analysis, we also correct the ENA signal for Compton–Getting and for ionization loss processes under the assumption of a heliospheric origin. We find spectral shapes and intensities consistent with those measured by the Interstellar Boundary Explorer (IBEX). The principal advantage of ASPERA with respect to the IBEX sensors is the two times better spectral resolution. In this study, we discuss the physical significance of the spectral shapes and their potential variation across the sky. At present, these observations are the only independent test of the heliospheric ENA signal measured with IBEX in this energy range. The ASPERA measurements also allow us to check for a temporal variation of the heliospheric signal as they were obtained between 2003 and 2007, whereas IBEX has been operational since the end of 2008.

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I. Whittaker, G. Guymer, M. Grande, B. Pintér, S. Barabash, A. Federov, C. Mazelle, J. A. Sauvaud, R. Lundin, C. T. Russell, Y. Futaana, M. Fränz, T. L. Zhang, H. Andersson, A. Grigoriev, M. Holmström, M. Yamauchi, K. Asamura, W. Baumjohann, H. Lammer, A. J. Coates, D. O. Kataria, D. R. Linder, C. C. Curtis, K. C. Hsieh, H. E. J. Koskinen, E. Kallio, P. Riihelä, W. Schmidt, J. Kozyra, S. McKenna‐Lawlor, J. J. Thocaven, S. Orsini, R. Cerulli‐Irelli, A. Mura, M. Milillo, M. Maggi, E. Roelof, P. Brandt, R. A. Frahm, J. R. Sharber, P. Wurz, and P. Bochsler

The Analyzer of Space Plasmas and Energetic Atoms (ASPERA‐4) instrument on Venus Express is used to determine bow shock position at Venus using ion data alone, using data recorded during a solar minimum from the Ion Mass Analyzer (IMA) which is part of the ASPERA‐4 package. Previous models constructed from solar minimum data using Venus Express, Pioneer Venus Orbiter (PVO) and Venera 9 and 10 are also compared to the current fit. An important feature of this new fit is a statistical accuracy introduced in the form of a probability weighting function for the data points, based on the time spent in particular locations. The bow shock curve is then compared to two‐dimensional ion maps. These verify the accuracy of this and previous solar minimum fit curves based on PVO and Venus Express magnetic data. Comparing all bow shock models to the 2D ion maps shows that a combination of models produces the best fit. Since all the fitted curves show differences in position they are investigated relative to the solar conditions pertaining at the times when the individual data sets were measured. The sub solar point and terminator distance were thus found to vary linearly with sunspot number and hence with solar activity. This relationship, which was already known to exist between solar maximum and solar minimum, is now shown to exist between different solar minima and even within the same minimum. This indicates a need for the mechanisms for bow shock maintenance and variance to be more closely modeled.

We present the measurement and analysis of hydrogen energetic neutral atoms (ENAs) recorded with the NPD sensor of the ASPERA-3 instrument on board Mars Express during the cruise phase and the Mars orbit phase. We conclude that the origin of these ENAs is the inner heliosheath. The ENA energy spectra are all very similar and can be fitted well by a two-component power law. The ENA intensities, integrated from 0.3 to 10 keV, vary in the range of 5 × 103 to 3 × 104 cm−2 sr−1 s−1. This report is an update of our earlier paper using the final NPD calibration data and improved sensor knowledge from two years of NPD operation. The present ENA measurements fit together well with earlier ENA data that were obtained from other spacecraft at higher energies, and which also have their likely origin in the inner heliosheath. Comparison of the measured ENA energy spectra with results from several heliospheric models shows that some of these models predict significantly lower ENA intensities at Earth orbit.

The ASPERA-4 instrument on board the Venus Express spacecraft offers for the first time the possibility to directly measure the emission of energetic neutral atoms (ENAs) in the vicinity of Venus. When the spacecraft is inside the Venus shadow a distinct signal of hydrogen ENAs usually is detected. It is observed as a narrow tailward stream, coming from the dayside exosphere around the Sun direction. The intensity of the signal reaches several 10 5 cm - 2 sr - 1 s - 1 , which is consistent with present theories of the plasma and neutral particle distributions around Venus.

[ 1 ] The ASPERA-3 experiment on Mars Express provides the first measurements of energetic neutral atoms (ENAs) from Mars. These measurements are used to study the global structure of the interaction of the solar wind with the Martian atmosphere. In this study we describe the tailward ENA flow observed at the nightside of Mars. After characterizing energy spectra of hydrogen ENA signals, we present composite images of the ENA intensities and compare them to theoretical predictions (empirical and MHD models). We find that the tailward flow of hydrogen ENAs is mainly generated by shocked solar wind protons. Despite intensive search, no oxygen ENAs above the instrument threshold are detected. The results challenge existing plasma models and constrain the hydrogen exospheric densities and atmospheric hydrogen and oxygen loss rates at low solar activity.

We have an unique opportunity to compare the magnetospheres of two non-magnetic planets as Mars and Venus with identical instrument sets Aspera-3 and Aspera-4 on board of the Mars Express and Venus Express missions. We have performed both statistical and case studies of properties of the magnetosheath ion flows and the flows of planetary ions behind both planets. We have shown that the general morphology of both magnetotails is generally identical. In both cases the energy of the light (H+) and the heavy (O+, etc.) ions decreases from the tail periphery (several keV) down to few eV in the tail center. At the same time the wake center of both planets is occupied by plasma sheet coincident with the current sheet of the tail. Both plasma sheets are filled by accelerated (500–1000 eV) heavy planetary ions. We report also the discovery of a new feature never observed before in the tails of non-magnetic planets: the plasma sheet is enveloped by consecutive layers of He+ and H+ with decreasing energies.

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 <43 and <67 h after the flare. Although these CMEs did not necessarily originate from the X9.0 flare on December 5, 2006, they most likely originated from the same active region because these characteristics are very similar to flare-associated CMEs observed on the Earth. These observations indicate that CME and flare activities on the invisible side of the Sun may affect terrestrial space weather as a result of traveling more than 90° in both azimuthal directions in the heliosphere. We would also like to emphasize that during the SEP activity, MEX data indicate an approximately one-order of magnitude enhancement in the heavy ion outflow flux from the Martian atmosphere. This is the first observation of the increase of escaping ion flux from Martian atmosphere during an intensive SEP event. This suggests that the solar EUV flux levels significantly affect the atmospheric loss from unmagnetized planets.

[1] The Analyzer of Space Plasma and Energetic Atoms (ASPERA-4) experiment on Venus Express provides the first measurements of energetic neutral atoms (ENAs) from Venus. The results improve our knowledge on the interaction of the solar wind with a nonmagnetized planet and they present an observational constraint to existing plasma models. We characterize the tailward flow of hydrogen ENAs observed on the nightside by providing global images of the ENA intensity. The images show a highly concentrated tailward flow of hydrogen ENAs tangential to the Venus limb around the Sun’s direction. No oxygen ENAs above the instrument threshold are detected. The observed ENA intensities are reproduced with a simple ENA model within a factor of 2, indicating that the observed hydrogen ENAs originate from shocked solar wind protons that charge exchange with the neutral hydrogen exosphere.

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.

The Analyzer of Space Plasma and EneRgetic Atoms (ASPERA-3) on board Mars Express is designed to study the interaction between the solar wind and the atmosphere of Mars and to characterize the plasma and neutral gas environment in near-Mars space. Neutral Particle Detectors (NPD-1 and 2), which form part of the ASPERA-3 instrument suite, are Energetic Neutral Atom (ENA) detectors which use the time-of-flight (ToF) technique to resolve the energy of detected particles. In the present study, we perform a statistical analysis of NPD ToF data collected between 14 March 2004 and 17 June 2004 when Mars Express was located at the dayside of Mars looking toward the planet. After pre-processing and removal of UV contamination, the ToF spectra were fitted with simple analytical functions so as to derive a set of parameters. The behavior of these parameters, as a function of spacecraft position and attitude, is compared with a model, which describes ENA generation by charge exchange between shocked solar wind protons and extended Martian exosphere. The observations and the model agree well, indicating that the recorded signals are charge-exchanged shocked solar wind.

Plasma and magnetic field measurements made onboard the Venus Express on June 1, 2006, are analyzed and compared with predictions of a global model. It is shown that in the orbit studied, the plasma and magnetic field observations obtained near the North Pole under solar minimum conditions were qualitatively and, in many cases also, quantitatively in agreement with the general picture obtained using a global numerical quasi-neutral hybrid model of the solar wind interaction (HYB-Venus). In instances where the orbit of Venus Express crossed a boundary referred to as the magnetic pileup boundary (MPB), field line tracing supports the suggestion that the MPB separates the region that is magnetically connected to the fluctuating magnetosheath field from a region that is magnetically connected to the induced magnetotail lobes.

For the first time since 1992 when the Pioneer Venus Orbiter (PVO) ceased to operate, there is again a plasma instrument in orbit around Venus, namely the ASPERA-4 flown on Venus Express (inserted into an elliptical polar orbit about the planet on April 11, 2006). In this paper we report on measurements made by the ion and electron sensors of ASPERA-4 during their first five months of operation and, thereby, determine the locations of both the Venus bow shock (BS) and the ion composition boundary (ICB) under solar minimum conditions. In contrast to previous studies based on PVO data, we employ a 3-parameter fit to achieve a realistic shape for the BS. We use a different technique to fit the ICB because this latter boundary cannot be represented by a conic section. Additionally we investigate the dependence of the location of the BS on solar wind ram pressure (based on ASPERA-4 solar wind data) and solar EUV flux (using a proxy from Earth).