Search

In Huybrighs et al., 2020 we investigated energetic proton depletions along Galileo's Europa flyby E26. Based on a particle tracing analysis we proposed that depletions are caused by perturbed electrogmagnetic fields combined with atmospheric charge exchange and possible plumes. One depletion feature identified as a plume signature was shown to be an artefact Jia et al., 2021. Despite that, here we emphasize that Huybrighs et al., 2020 demonstrates that plumes can cause proton depletions and that these features should be sought after. Furthermore, the conclusions on the importance of perturbed electromagnetic fields and atmospheric charge exchange on the depletions are unaffected. We suggest that the artefact's cause is a mistagging of protons as heavier ions by EPD. The artefact prevents us from confirming or excluding that there is a plume associated depletion. We also address comments on the MHD simulations and demonstrate that 540-1040 keV losses are not necessarily inconsistent with 115-244 keV losses by plume associated charge exchange., Comment: accepted to journal of geophysical research letters

We report a new population of suprathermal H+(∼1.5–3 times the solar wind energy) around the Moon observed by SWIM/SARA on Chandrayaan-1. These ions have large initial velocity (>100 km s−1), and are observed on the dayside, near the terminator, and in the near wake region (100–200 km above the surface), when the Moon is located outside Earth's bowshock. Backtracing suggests that the source is located >500 km above the dayside lunar surface. Possible sources considered for these ions are ionization of backscattered lunar energetic neutral hydrogen atoms, ionization of lunar exospheric hydrogen, inner source pickup ions, interstellar pickup ions, ionization of neutral solar wind, and solar wind H+ reflected from Earth's bowshock. The comparison of the observed flux (density) with that expected from these sources and the velocity distribution suggests that an additional source is required to explain the population.

Understanding what processes govern atmospheric escape and the loss of planetary water is of paramount importance for understanding how life in the universe can exist. One mechanism thought to be important at all planets is an “ambipolar” electric field that helps ions overcome gravity. We report the discovery and first quantitative extraterrestrial measurements of such a field at the planet Venus. Unexpectedly, despite comparable gravity, we show the field to be five times stronger than in Earth's similar ionosphere. Contrary to our understanding, Venus would still lose heavy ions (including oxygen and all water-group species) to space, even if there were no stripping by the solar wind. We therefore find that it is possible for planets to lose heavy ions to space entirely through electric forces in their ionospheres and such an “electric wind” must be considered when studying the evolution and potential habitability of any planet in any star system.

An extremely strong Coronal Mass Ejection (CME) impacted Mars on 12 July 2011, while theMars Express spacecraft was present inside the nightside ionosphere. Estimated solar wind density andspeed du ...

Data from the neutral particle imager (NPI) on Venus Express reveals the presence of a persistent +Y (Venus Solar-Oriented) flow of Energetic Neutral Atoms (ENAs) in the Venus polar thermosphere. The overall flow morphology and altitude distribution of ENAs, solar wind H+ (SWH+), and ionospheric O+ imply that SWH+&O+ Charge Exchange (CE) with thermosphere neutrals causes the ENA flow. A dusk-dawn and noon-midnight ENA flux asymmetry is consistent with an asymmetric CE-interaction with higher ENA fluxes in the dayside and dawn (+Y). Model ENA fluxes derived from CE between SWH+&O+ and a model oxygen thermosphere is consistent with NPI data. Adding energy-corrected NPI fluxes and ion fluxes gives essentially constant fluxes of ≈1012 part/(m2s) in the altitude range of 200–1200 km. The energy and momentum of the 1–10 km/s ion and ENA flow, induced by solar wind forcing, should have implications for wind patterns in the polar thermosphere above 200 km.

Electron densities in the Martian nightside ionosphere are more than 90% of time too low to be detected by the Mars Advanced Radar for Subsurface and Ionosphere Sounding radar sounder on board the Mars Express spacecraft. However, the relative number of ionograms with peak electron density high enough to be detected represents a good statistical proxy of the ionospheric density. We focus on solar energetic particle (SEP) events, and we analyze their effects on ionospheric formation. SEP time intervals were identified in situ using the background counts recorded by the ion sensor of the ASPERA-3 instrument on board Mars Express. We show that peak electron densities during the SEP events are large enough to be detected in more than 30% of measurements, and, moreover, the reflections of the sounding signal from the ground almost entirely disappear. Nightside electron densities during SEP events are thus substantially increased as compared to normal nightside conditions.

[1] Solar cycle effects on the escape of planetary ions from Mars are investigated using Mars Express Analyzer of Space Plasmas and Energetic Atoms 3 data from June 2007 to January 2013. Average and median tail fluxes of low-energy (

[1] We investigate the dependence of the O+ escape rate on the upstream interplanetary magnetic field (IMF) direction by using data from the Analyser of Space Plasma and Energetic Atoms (ASPERA-4) instrument and the magnetometer (MAG) onboard Venus Express. We consider two cases, namely the perpendicular IMF case (167 events) and the parallel IMF case (82 events), where IMF is nearly perpendicular to the Venus-Sun line and nearly parallel to it. By integrating O+ fluxes observed on the nightside, total O+ escape rates of (5.8 ± 2.9) × 1024 s−1 (perpendicular IMF case) and (4.9 ± 2.2) × 1024 s−1 (parallel IMF case) are obtained. As these values are not significantly different, the upstream IMF direction does not affect the total amount of O+ outflow from Venus. The different acceleration mechanisms must balance each other in order to keep the escape rate the same.

1] Insights about the near-Mars space environment from Mars Express observations have motivated a revisit of the Phobos 2/ASPERA ion data from 1989. We have expanded the analysis to now include all ...

Eight years (2007-2015) of ion flux measurements from Mars Express are used to statistically investigate the influence of the Martian magnetic crustal fields on the atmospheric ion escape rate. We combine all ASPERA-3/IMA measurements taken during nominal upstream solar wind and solar Extreme Ultraviolet (EUV) conditions to compute global average ion distribution functions, individually for the north/south hemispheres and for varying solar zenith angles (SZA) of the strongest crustal magnetic field. Escape rates are subsequently calculated from each of the average distribution functions. The maximum escape rate (4.2 ± 1.2) × 1024s−1 is found for SZA = 60∘–80∘ while the minimum escape rate (1.7 ± 0.6) × 1024s−1 is found for SZA = 28∘–60∘, showing that the dayside orientation of the crustal fields significantly affects the global escape rate (p = 97%). However, averaged over time, independent of SZA, we find no statistically significant difference in the escape rates from the two hemispheres (escape from southern hemisphere 46% ± 18% of global rate).

We present first measurements of energetic neutral atoms that originate from solar wind plasma having interacted with the lunar nightside surface. We observe two distinct energetic neutral atom (ENA) distributions parallel to the terminator the spectral shape and the intensity of both of which indicate that the particles originate from the bulk solar wind flow. The first distribution modifies the dayside ENA flux to reach approximate to 6 degrees into the nightside and is well explained by the kinetic temperature of the solar wind protons. The second distribution which was not predicted reaches from the terminator to up to 30 degrees beyond the terminator with a maximum at approximate to 102 degrees in solar zenith angle. As most likely wake transport processes for this second distribution we identify acceleration by the ambipolar electric field and by the negatively charged lunar nightside surface. In addition our data provide the first observation indicative of a global solar zenith angle dependence of positive dayside surface potentials.

[1] The Sub-keV Atom Reflecting Analyzer instrument on board the lunar orbiter Chandrayaan-1 provided a large number of measurements of lunar energetic neutral atoms (ENAs). These ENAs were formerly solar wind ions, which were neutralized and backscattered from the lunar surface. The angles under which the ENAs are scattered strongly depend on the solar wind ions' incidence angle, which corresponds to the solar zenith angle (SZA). Our large dataset provides us with a complete coverage of the SZA and almost complete coverage of the scattering angles. When combining all available measurements, four distinct features are discernible with SZA increase: amplitude decrease, less azimuthal uniformity, bigger ratio of sunward versus anti-sunward flux and shallower scattering. We analyzed more than 290′000 measurements and derived a mathematical description of the features and their dependencies on the SZA.

[1] Solar wind alpha-particles are suggested to be the most important source of helium in the Martian atmosphere. Recent hybrid simulations show that as much as 30% of the alpha-particles passing through the planetary cross section can be lost from the solar wind due to charge-exchange processes associated with the Mars/solar wind interaction. We use ion data from the ASPERA-3 instrument on Mars Express to investigate the penetration of alpha-particles into the Martian atmosphere. We conclude that solar wind alpha-particles contribute to the helium observed in the atmosphere. In accordance with simulations we see an asymmetry in the capture efficiency related to the solar wind convection electric field.

We discuss the influence of lunar magnetic anomalies on the solar wind and on the lunar surface, based on maps of solar wind proton fluxes deflected by the magnetic anomalies. The maps are produced ...

The Sub-keV Atom Reflecting Analyzer (SARA) instrument on the Indian Chandrayaan-1 spacecraft has produced for the first time an image of a lunar magnetic anomaly in backscattered hydrogen atoms. The image shows that a partial void of the solar wind, a mini-magnetosphere, is formed above the strong magnetic anomaly near the Crisium antipode. The mini-magnetosphere is 360 km across at the surface and is surrounded by a 300-km-thick region of enhanced plasma flux that results from the solar wind flowing around the mini-magnetosphere. The mini-magnetosphere is visible only in hydrogen atoms with energy exceeding 150 eV. Fluxes with energies below 100 eV do not show corresponding spatial variations. While the high-energy atoms result from the backscattering process, the origin of the low-energy component is puzzling. These observations reveal a new class of objects, mini-magnetospheres, and demonstrate a new observational technique to study airless bodies, imaging in backscattered neutral atoms.

[1] We study atmospheric escape from Mars during solar wind pressure pulses. During the solar minimum of 2007–08 we have observed 41 high pressure events, which are predominantly identified as corotating interaction regions (CIR) while a few are coronal mass ejections (CME), in data from the Advanced Composition Explorer (ACE) upstream of the Earth. 36 of these events are also identified using Mars Express (MEX) data at Mars. We use MEX measurements at Mars to compare the antisunward fluxes of heavy planetary ions during the passage of these pulses to the fluxes during quiet solar wind conditions. The ion fluxes are observed to increase by a factor of ∼2.5, on average. Hence, a third of the total outflow from Mars takes place during ∼15% of the time, when a solar wind pressure pulse impacts on the planet. This can have important consequences for the total time-integrated outflow of plasma from Mars.