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The ESA-JAXA BepiColombo mission to Mercury will provide simultaneous measurements from two spacecraft, offering an unprecedented opportunity to investigate magnetospheric and exospheric particle dynamics at Mercury as well as their interactions with solar wind, solar radiation, and interplanetary dust. The particle instrument suite SERENA (Search for Exospheric Refilling and Emitted Natural Abundances) is flying in space on-board the BepiColombo Mercury Planetary Orbiter (MPO) and is the only instrument for ion and neutral particle detection aboard the MPO. It comprises four independent sensors: ELENA for neutral particle flow detection, Strofio for neutral gas detection, PICAM for planetary ions observations, and MIPA, mostly for solar wind ion measurements. SERENA is managed by a System Control Unit located inside the ELENA box. In the present paper the scientific goals of this suite are described, and then the four units are detailed, as well as their major features and calibration results. Finally, the SERENA operational activities are shown during the orbital path around Mercury, with also some reference to the activities planned during the long cruise phase.

The second Venus flyby of the BepiColombo mission offer a unique opportunity to make a complete tour of one of the few gas-dynamics dominated interaction regions between the supersonic solar wind and a Solar System object. The spacecraft pass through the full Venusian magnetosheath following the plasma streamlines, and cross the subsolar stagnation region during very stable solar wind conditions as observed upstream by the neighboring Solar Orbiter mission. These rare multipoint synergistic observations and stable conditions experimentally confirm what was previously predicted for the barely-explored stagnation region close to solar minimum. Here, we show that this region has a large extend, up to an altitude of 1900 km, and the estimated low energy transfer near the subsolar point confirm that the atmosphere of Venus, despite being non-magnetized and less conductive due to lower ultraviolet flux at solar minimum, is capable of withstanding the solar wind under low dynamic pressure. BepiColombo mission had two Venus flybys on its way to Mercury. Here, the authors show that during its second flyby of Venus BepiColombo has crossed the stagnation region, which was predicted by the models.

Mercury’s southern inner magnetosphere is an unexplored region as it was not observed by earlier space missions. In October 2021, BepiColombo mission has passed through this region during its first Mercury flyby. Here, we describe the observations of SERENA ion sensors nearby and inside Mercury’s magnetosphere. An intermittent high-energy signal, possibly due to an interplanetary magnetic flux rope, has been observed downstream Mercury, together with low energy solar wind. Low energy ions, possibly due to satellite outgassing, were detected outside the magnetosphere. The dayside magnetopause and bow-shock crossing were much closer to the planet than expected, signature of a highly eroded magnetosphere. Different ion populations have been observed inside the magnetosphere, like low latitude boundary layer at magnetopause inbound and partial ring current at dawn close to the planet. These observations are important for understanding the weak magnetosphere behavior so close to the Sun, revealing details never reached before. BepiColombo mission had already two flybys of Mercury, over the total of six, as planned before entering the planet’s orbit in 2025. Here, the authors show the first ion measurements of Mercury’s inner southern magnetosphere during BepiColombo mission’s first Mercury flyby.

After more than two years in orbit still no Venus Express observations were published concerning the hot oxygen corona of Venus which could verify the corresponding controversial observations of Venera 11 and PVO, three decades ago. Based on recent energy and mass dependent collision cross sections, the energy distributions of hot atomic oxygen created via dissociative recombination of O2+ are calculated in the daytime thermosphere by means of a 3D Monte Carlo approach. The exosphere density is obtained from the corresponding energy density and angular distribution at 240 km altitude by using a test particle model which traces the ballistic trajectories of hot O atoms in the exosphere. Our study indicates that upon taking into account proper input parameters, the hot oxygen corona appears to be substantially less dense than suggested by previous simulations.

We analyzed the measured thermospheric response of an extreme solar X17.2 flare that irradiated the Earth's upper atmosphere during the so-called Halloween events in late October/early November 2003. We suggest that such events can serve as proxies for the intense electromagnetic and corpuscular radiation environment of the Sun or other stars during their early phases of evolution. We applied and compared empirical thermosphere models with satellite drag measurements from the GRACE satellites and found that the Jacchia-Bowman 2008 model can reproduce the drag measurements very well during undisturbed solar conditions but gets worse during extreme solar events. By analyzing the peak of the X17.2 flare spectra and comparing it with spectra of young solar proxies, our results indicate that the peak flare radiation flux corresponds to a hypothetical Sun-like star or the Sun at the age of approximately 2.3 Gyr. This implies that the peak extreme ultraviolet (EUV) radiation is enhanced by a factor of about 2.5 times compared to today's Sun. On the assumption that the Sun emitted an EUV flux of that magnitude and by modifying the activity indices in the Jacchia-Bowman 2008 model, we obtain an average exobase temperature of 1950 K, which corresponds with previous theoretical studies related to thermospheric heating and expansion caused by the solar EUV flux.

We have evaluated the Lyman-α limb emission from the exospheric hydrogen of Mars measured by the neutral particle detector of the ASPERA-3 instrument on Mars Express in 2004 at low solar activity (solar activity index = 42, F10.7=100). We derive estimates for the hydrogen exobase density, nH = 1010 m−3, and for the apparent temperature, T > 600 K. We conclude that the limb emission measurement is dominated by a hydrogen component that is considerably hotter than the bulk temperature at the exobase. The derived values for the exosphere density and temperature are compared with similar measurements done by the Mariner space probes in the 1969. The values found with Mars Express and Mariner data are brought in a broader context of exosphere models including the possibility of having two hydrogen components in the Martian exosphere. The present observation of the Martian hydrogen exosphere is the first one at high altitudes during low solar activity, and shows that for low solar activity exospheric densities are not higher than for high solar activity.

We present a Monte Carlo study of the atomic hot oxygen corona of Venus. In this model we consider elastic, inelastic, and quenching collisions between the traced hot particle and the ambient neutral atmosphere as well as differential cross sections to determine the scattering angle in the collisions. We also include rotational and vibrational excitation energies for the calculation of the initial energy of the produced hot oxygen atoms. Our results indicate that the differential cross sections and the fraction between elastic, inelastic, and quenching collisions are the most sensitive parameters which effect the corona density. We found that the hot O densities inferred from PVO observations can only be reproduced during high solar activity based on a forward scattering model but without inelastic and quenching collisions. The corona densities for low solar activity (VEX solar conditions) are about a factor of 2–3 smaller than for high solar activity.

A Correction to this paper has been published: https://doi.org/10.1007/s11214-021-00809-8

The nightside oxygen exosphere of Venus is investigated for high and moderate solar activity by means of a Monte‐Carlo model. Hot O atoms are assumed to be produced by dissociative recombination of O2+ and NO+ molecular ions and by charge transfer processes between ionospheric O+ ions and neutral O and H atoms. The model considers rotational and vibrational excitation of the initial energy distribution of hot O atoms, includes elastic, inelastic, and quenching collisions between the suprathermal atoms and the ambient neutral atmosphere species, and uses differential cross sections for the determination of the scattering angle in the collisions. The results indicate that dissociative recombination of O2+ is, like at Venus' dayside, the most efficient source of hot O atoms at the planet's nightside. For high solar activity, the nightside exospheric density of hot O atoms is about one order of magnitude lower compared to the dayside, although between 2–10 times higher than in previous studies. Key Points Dissociative recombination of O2+ is the main source of hot O at Venus nightside For high solar activity, hot O density at nightside is ~10 lower than at dayside Our simulated nightside densities are 2‐10 times higher than previous studies

We use data of the ASPERA‐4 ion and electron spectrometers onboard Venus Express to determine the locations and shapes of the plasma boundaries (bow shock, ion composition boundary, and mantle) at Venus. We also investigate the variation of the terminator bow shock position as a function of the solar wind dynamic pressure and solar EUV flux. We compare the results with a 3‐D hybrid simulation. In the hybrid model, ions are treated as individual particles moving in self‐consistently generated electromagnetic fields and electrons are modeled as a massless charge neutralizing fluid. The planetary heavy ion plasma is generated by an oxygen ionosphere and exosphere adapted to a profile, which depends on the solar zenith angle (Chapman layer). A comparison between observations and simulations indicates that the hybrid model is able to produce an adequate picture of the global plasma environment at Venus. The positions of the plasma boundaries are well reproduced by the model but a significant disagreement appears in the absolute values of the considered parameters.