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With the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) on the Mars Express (MEX) spacecraft, the electron density can be measured by two methods: from the excitation of local plasma oscillations and from remote sounding. A study of the local electron density versus time for 1664 orbits revealed that in 132 orbits very sharp gradients in the electron density occurred that are similar to the ionopause boundary commonly observed at Venus. In 40 of these cases, remote sounding data have also confirmed identical locations of steep ionopause‐like density gradients. Measurements from the Analyzer of Space Plasma and Energetic Atoms (ASPERA‐3) electron spectrometer and ion mass analyzer instruments (also on Mars Express) verify that these sharp decreases in the electron density occur somewhere between the end of the region where ionospheric photoelectrons are dominant and the magnetosheath. Combined studies of the two experiments reveal that the steep density gradients define a boundary where the magnetic fields change from open to closed. This study shows that, although the individual cases are from a wide range of altitudes, the average altitude of the boundary as a function of solar zenith angle is almost constant. The average altitude is approximately 500 km up to solar zenith angles of 60°, after which it shows a slight increase. The average thickness of the boundary is about 22 km according to remote sounding measurements. The altitude of the steep gradients shows an increase at locations with strong crustal magnetic fields.

The ion and electron sensor (IES) is part of the Rosetta Plasma Consortium (RPC). The IES consists of two electrostatic plasma analyzers, one each for ions and electrons, which share a common entrance aperture. Each analyzer covers an energy/charge range from 1 eV/e to 22 keV/e with a resolution of 4%. Electrostatic deflection is used at the entrance aperture to achieve a field of view of 90°× 360° (2.8π sr). Angular resolution is 5°× 22.5° for electrons and 5°× 45° for ions with the sector containing the solar wind being further segmented to 5°× 5°. The three-dimensional plasma distributions obtained by IES will be used to investigate the interaction of the solar wind with asteroids Steins and Lutetia and the coma and nucleus of comet 67P/Churyumov–Gerasimenko (CG). In addition, photoelectron spectra obtained at these bodies will help determine their composition.

A total of about of 400 orbits during the first year of the ASPERA-3 operation onboard the Mars Express spacecraft were analyzed to obtain a statistical pattern of the main plasma domains in the Martian space environment. The environment is controlled by the direct interaction between the solar wind and the planetary exosphere/ionosphere which results in the formation of the magnetospheric cavity. Ionospheric plasma was traced by the characteristic 'spectral lines' of photoelectrons that make it possible to detect an ionospheric component even far from the planet. Plasma of solar wind and planetary origin was distinguished by the ion mass spectrometry. Several different regions, namely, boundary layer/mantle, plasma sheet, region with ionospheric photoelectrons, ray-like structures near the wake boundary were identified. Upstream parameters like solar wind ram pressure and the direction of the interplanetary electric field were inferred as proxy from the Mars Global Surveyor magnetic field data at a reference point of the magnetic pile up region in the northern dayside hemisphere. It is shown that morphology and dynamics of the main plasma domains and their boundaries are governed by these factors as well as by local crustal magnetizations which add complexity and variability to the plasma and magnetic field environment.

The suite of SECCHI optical imaging instruments on the STEREO‐A spacecraft is used to track a solar storm, consisting of several coronal mass ejections (CMEs) and other coronal loops, as it propagates from the Sun into the heliosphere during May 2007. The 3‐D propagation path of the largest interplanetary CME (ICME) is determined from the observations made by the SECCHI Heliospheric Imager (HI) on STEREO‐A (HI‐1/2A). Two parts of the CME are tracked through the SECCHI images, a bright loop and a V‐shaped feature located at the rear of the event. We show that these two structures could be the result of line‐of‐sight integration of the light scattered by electrons located on a single flux rope. In addition to being imaged by HI, the CME is observed simultaneously by the plasma and magnetic field experiments on the Venus Express and MESSENGER spacecraft. The imaged loop and V‐shaped structure bound, as expected, the flux rope observed in situ. The SECCHI images reveal that the leading loop‐like structure propagated faster than the V‐shaped structure, and a decrease in in situ CME speed occurred during the passage of the flux rope. We interpret this as the result of the continuous radial expansion of the flux rope as it progressed outward through the interplanetary medium. An expansion speed in the radial direction of ∼30 km s−1 is obtained directly from the SECCHI‐HI images and is in agreement with the difference in speed of the two structures observed in situ. This paper shows that the flux rope location can be determined from white light images, which could have important space weather applications.

The general scientific objective of the ASPERA-3 experiment is to study the solar wind – atmosphere interaction and to characterize the plasma and neutral gas environment with within the space near Mars through the use of energetic neutral atom (ENA) imaging and measuring local ion and electron plasma. The ASPERA-3 instrument comprises four sensors: two ENA sensors, one electron spectrometer, and one ion spectrometer. The Neutral Particle Imager (NPI) provides measurements of the integral ENA flux (0.1–60 keV) with no mass and energy resolution, but high angular resolution. The measurement principle is based on registering products (secondary ions, sputtered neutrals, reflected neutrals) of the ENA interaction with a graphite-coated surface. The Neutral Particle Detector (NPD) provides measurements of the ENA flux, resolving velocity (the hydrogen energy range is 0.1–10 keV) and mass (H and O) with a coarse angular resolution. The measurement principle is based on the surface reflection technique. The Electron Spectrometer (ELS) is a standard top-hat electrostatic analyzer in a very compact design which covers the energy range 0.01–20 keV. These three sensors are located on a scanning platform which provides scanning through 180∘ of rotation. The instrument also contains an ion mass analyzer (IMA). Mechanically IMA is a separate unit connected by a cable to the ASPERA-3 main unit. IMA provides ion measurements in the energy range 0.01–36 keV/charge for the main ion components H+, He++, He+, O+, and the group of molecular ions 20–80 amu/q. ASPERA-3 also includes its own DC/DC converters and digital processing unit (DPU).

Still delivering ESA's Venus Express probe has been in orbit since April 2006. Eight research papers in this issue present new results from the mission, covering the atmosphere, polar features, interactions with the solar wind and the controversial matter of venusian lightning. Håkan Svedham et al . open the section with a review of the similarities and (mostly) differences between Venus and its 'twin', the Earth. Andrew Ingersoll considers the latest results, and also how the project teams plan to make the most of the probe's remaining six years of life. On Venus, the bulk of O and O 2 are gravitationally bound, but heavy ions have been observed to escape, though their composition could not be determined. Venus Express measurements report that the dominant escaping ions are O + , He + and H + . The escape of H + and O + , together with the estimated neutral hydrogen and oxygen escape, currently takes place near the stoichometric ratio corresponding to water. 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 1 . Present-day thermal escape is low for all atmospheric species. However, hydrogen can escape by means of collisions with hot atoms from ionospheric photochemistry 2 , and although the bulk of O and O 2 are gravitationally bound, heavy ions have been observed to escape 3 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.

Foreshock ions are compared between Venus and Mars at energies of 0.6~20 keV using the same ion instrument, the Ion Mass Analyser, on board both Venus Express and Mars Express. Venus Express often observes accelerated protons (2~6 times the solar wind energy) that travel away from the Venus bow shock when the spacecraft location is magnetically connected to the bow shock. The observed ions have a large field-aligned velocity compared to the perpendicular velocity in the solar wind frame, and are similar to the field-aligned beams and intermediate gyrating component of the foreshock ions in the terrestrial upstream region. Mars Express does not observe similar foreshock ions as does Venus Express, indicating that the Martian foreshock does not possess the intermediate gyrating component in the upstream region on the dayside of the planet. Instead, two types of gyrating protons in the solar wind frame are observed very close to the Martian quasi-perpendicular bow shock within a proton gyroradius distance. The first type is observed only within the region which is about 400 km from the bow shock and flows tailward nearly along the bow shock with a similar velocity as the solar wind. The second type is observed up to about 700 km from the bow shock and has a bundled structure in the energy domain. A traversal on 12 July 2005, in which the energy-bunching came from bundling in the magnetic field direction, is further examined. The observed velocities of the latter population are consistent with multiple specular reflections of the solar wind at the bow shock, and the ions after the second reflection have a field-aligned velocity larger than that of the de Hoffman-Teller velocity frame, i.e., their guiding center has moved toward interplanetary space out from the bow shock. To account for the observed peculiarity of the Martian upstream region, finite gyroradius effects of the solar wind protons compared to the radius of the bow shock curvature and effects of cold ion abundance in the bow shock are discussed.

We here report the first dual‐spacecraft detection of planetary flux ropes in the ionosphere of Mars. The Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS), on board Mars Express, can measure the magnetic field magnitude near the spacecraft. Typically, these measurements track the known crustal magnetic field strength very well; however, occasionally, MARSIS detects transient, intense magnetic fields that deviate significantly from the known crustal fields. Two such magnetic field enhancements occur in near‐coincidence with flux rope detections by the Mars Global Surveyor Magnetometer and Electron Reflectometer, which provides vector magnetic field measurements, allowing us to clearly identify the enhancements as flux ropes. The flux ropes detected are quasi‐stable for at least a half hour, have peak magnetic field strengths of ∼50 and 90 nT, and are ∼650–700 km in diameter. Both occur downstream of the region of strong crustal fields. In addition, MARSIS has detected 13 other magnetic enhancements over a 5 year period, which we infer to be flux ropes. These structures have peak field strengths up to 130 nT and measured horizontal dimensions of several hundred to over a thousand kilometers. They are clustered around the intense crustal fields in the southern hemisphere of Mars. The large spatial scale of these flux ropes distinguishes them from small‐scale flux ropes, with diameters of tens of kilometers, that have been seen in the ionospheres of Venus and Mars. These large‐scale flux ropes are believed to be caused by solar wind stretching and shearing of the Martian crustal fields.

We present the first electron and ion moment maps (density, velocity and temperature) of the martian plasma environment, using data from the ELS and IMA sensors of the ASPERA-3 experiment onboard Mars Express. Moments are calculated by integration and by Gaussian fits to the phase space distribution. The methods of calculation and the calibration parameters relevant for the calculation are described in detail in the first part of the paper. The estimation of ionospheric electron densities assumes that the thermal electron temperature can be determined by the instrument – despite a cut-off by a negative spacecraft potential. The spacecraft potential is estimated by the location of photoelectron peaks in the energy spectrum. For the magnetosheath we separate the low energy part of the electron spectrum – presumably spacecraft photo electrons and the high energy part. For ions, we present maps for solar wind protons and alpha particles. Protons with energies below 500 eV which may play an important role in the ionosphere are not measured by the instrument. As well the low speed solar wind protons are not sampled very well. The maps reveal all the boundaries of the Mars-solar wind interaction and give a good qualitative description of the plasma behavior at the different interaction regions.

By identifying peaks in the photoelectron spectrum produced by photoionization of CO2 in the Martian atmosphere, we have conducted a pilot study to determine the locations of these photoelectrons in the space around Mars. The significant result of this study is that these photoelectrons populate a region around Mars bounded externally by the magnetic pileup boundary, and internally by the lowest altitude of our measurements (∼250 km) on the dayside and by a cylinder of approximately the planetary radius on the nightside. It is particularly noteworthy that the photoelectrons on the nightside are observed from the terminator plane tailward to a distance of ∼3 RM, the Mars Express apoapsis. The presence of the atmospherically generated photoelectrons on the nightside of Mars may be explained by direct magnetic field line connection between the nightside observation locations and the Martian dayside ionosphere. Thus the characteristic photoelectron peaks may be used as tracers of magnetic field lines for the study of the magnetic field configuration and particle transport in the Martian environment.