Moa Persson

Mercury's magnetosphere is an extremely dynamic environment, where substorm‐like activity, including dipolarization fronts and particle acceleration, periodically disrupts the stability and radial extent of the closed field line region. The common narrowband ∼1 Hz waves at Mercury, near the local proton gyrofrequency are preferentially observed on closed field lines during quieter intervals, but their relationship to magnetospheric dynamics remains poorly understood. Here, we combine MESSENGER data sets of ∼1 Hz wave activity and dipolarization fronts to investigate whether newly closed field lines can sustain wave growth. We find that ∼1 Hz waves on the nightside are systematically absent during orbits with dipolarization intervals and typically confined within 1.3 R _M , compared to greater downtail extents during quiet intervals. The results indicate that ∼1 Hz waves require long‐lived, stable closed field lines to grow; therefore, when they are observed, they can serve as diagnostics of Mercury's magnetospheric state, establishing a clear connection between field‐line history, magnetospheric dynamics, and wave occurrence in Mercury's magnetotail.

The Venusian interaction with the solar wind leads to the formation of an induced magnetosphere structured by plasma boundaries. Their dynamics is complex, due to the combined influence of external (solar photons, solar wind plasma, and interplanetary magnetic field, IMF) and internal (ionized atmosphere) drivers. Studying these drivers helps understanding of the transfer of energy and momentum throughout the Venusian system, and has thus implications for the erosion of the atmosphere through its coupling with the solar wind. We here analyze and rank the influence of the main drivers of the Venusian bow shock and ion composition boundary locations. We revisit the results of C. Signoles et al. based on Venus Express measurements by combining several methods such as the Akaike Information Criterion, Least Absolute Shrinkage Selection Operator regression, and partial correlations. These methods allow us to investigate cross correlations that appear and can bias the interpretation, and allow us to rank drivers with robust approaches. The bow shock appears primarily driven by the IMF intensity or Mach number, the IMF θ _bn angle separating quasi-perpendicular versus quasi-parallel shocks, and then the solar extreme-ultraviolet fluxes and solar wind dynamic pressure (with little influence of the convective electric field induced asymmetries). The ion composition boundary is primarily driven by extreme-ultraviolet fluxes, with a more reduced influence of several solar wind parameters and IMF induced magnetic pileup asymmetries. We also compare the behaviors of both boundaries and then compare the bow shock driver rankings at Mars and Venus. Finally we propose an analysis of the drivers of the extreme bow shock and ion composition boundary excursions.

Mercury’s magnetosphere is highly dynamic, with processes occurring on much shorter spatial and temporal scales than at Earth due to its weak magnetic field and proximity to the Sun. Wave-driven transport of energy and momentum is a key manifestation of these dynamics. We present a comprehensive statistical survey of ~ 1 Hz waves using the full MESSENGER magnetometer dataset. These waves occur on average 10–20% of the time within Mercury’s magnetosphere, predominantly on closed field lines. Their occurrence is enhanced under conditions that favour an expanded closed-field region, such as a northward interplanetary magnetic field or low internal magnetospheric disturbance. Wave power peaks in the post-midnight sector, mirroring asymmetries seen in other plasma parameters and suggesting a generation mechanism linked to particle injections along closed field lines near the magnetic equator. While the precise wave mode is not determined here, their frequency range, polarization, and occurrence context are broadly consistent with electromagnetic ion cyclotron-like waves. These results provide the first global characterization of ~ 1 Hz waves and their dependence on upstream and internal conditions, establishing a foundation for interpreting future BepiColombo observations and for understanding wave-driven dynamics in Mercury’s magnetosphere.

Solar Orbiter entered the topside ionosphere of Venus for the first time when performing its 4th flyby of the planet, reaching an altitude of 378 km on 18 Feb 2025. High‐cadence electron density measurements showed previously unresolved fine‐structuring within plasma regions and boundaries, particularly at the ionopause. During the rapid flyby, a rare snapshot of the entire induced magnetosphere was captured during calm solar wind conditions around solar maximum. A well‐structured and a relatively steady plasma environment was observed. Assuming an electron temperature of 0.5 eV, pressure balance was found across the ionopause, while at the same time quasi‐periodic density and magnetic field variations suggest boundary oscillations. Near closest approach, non‐force free magnetic flux ropes were observed. Small‐scale perturbations in both magnetic field strength and density across them indicate them being dynamically evolving, rather than in a stationary state.

During its fourth Venus flyby on 18 February 2025, Solar Orbiter reached an altitude of 378 km, significantly deeper than during previous encounters, allowing the spacecraft to enter the Venusian ionosphere for the first time. The Magnetometer (MAG) and Radio and Plasma Wave (RPW) instruments operated in burst mode during most of the flyby, providing high-time-resolution measurements of the entire induced magnetosphere. The peak electron density reached approximately 2x104 cm-3, derived from a spacecraft potential of about –45 V and calibrated using the plasma frequency line.Solar Orbiter approached Venus from the tail region and entered the plasma environment without detecting a clear inbound bow shock. The upstream solar wind was steady and calm, as observed a few hours before and after the flyby and inferred from stable magnetosheath conditions, resulting in a structured and relatively steady plasma environment. High-cadence electron density measurements resolved fine-scale structures within plasma regions and boundaries, particularly at the ionopause, on spatial scales of 1–10 km, comparable to the local H+ and O+ ion length scales (2 and 8 km, respectively). Assuming an electron temperature of 0.5 eV, pressure balance was found across the ionopause, while quasi-periodic density and magnetic field variations suggest boundary oscillations on ion length scales during the pass. Near closest approach, magnetic flux ropes were observed. These features were generally not in full pressure balance with the surrounding plasma and exhibited small-scale perturbations in both magnetic field strength and density, consistent with a dynamically evolving rather than stationary state.

The magnetosphere of Mercury is highly dynamic, a consequence of its small size, weak intrinsic magnetic field, and proximity to the Sun. One intriguing phenomenon is the presence of magnetic field fluctuations around 1 Hz. Here, we present a comprehensive statistical survey of these waves using the full span of the MESSENGER magnetometer measurements from 2011 to 2015. We find that ~1 Hz waves are observed during 10-20 % of the time that the spacecraft spent on closed field lines in Mercury’s magnetosphere, as determined from the KT17 magnetic field model. Wave occurrence is increased under magnetospheric conditions that favour an expanded closed field line region. We present the first global characterisation of the ~1 Hz waves at Mercury and demonstrate their dependence on both external drivers, such as upstream IMF conditions, and internal magnetospheric activity, such as the occurrence of identified dipolarization events. These results are discussed in the context of the BepiColombo mission, which will provide new opportunities to identify the nature of these waves and to assess their role in Mercury’s highly dynamic plasma environment.

The Martian dayside ionosphere has been widely modeled using photochemical equilibrium calculations. These efforts have mostly focused on dominant ion species in order to make comparisons with orbital observations and on displaying non-negligible model-observation discrepancies. In this study, we investigate Ar⁺ions in the Martian dayside ionosphere, an ion species with a relatively simple chemistry, and perform both case-by-case orbital comparisons and a statistical comparison over five years of observations by the Neutral Gas and Ion Mass Spectrometer (NGIMS) on the Mars Atmosphere and Volatile Evolution (MAVEN) mission. Statistically, the ratio of modeled to observed Ar⁺densities increases from ∼1 near 130 km to ∼4 at 220 km, with notable variations as a function of the solar zenith angle. Pressure-dependent discrepancies show a weaker correlation with the solar zenith angle. Model performance improves when incorporating (i) a higher reaction rate coefficient for the charge transfer between Ar⁺and CO₂ and/or (ii) reduced solar irradiance. At altitudes above 200 km, Ar⁺loss via reactions with H₂ becomes increasingly important. However, we find that model-observation agreement varies between orbits: Some show strong consistency, particularly during Deep Dip campaigns, while others exhibit systematic deviations or significant discrepancies. We suggest that while systematic adjustments to reaction rate coefficients, ionization cross sections, solar irradiance, or background neutral densities may improve model fidelity for certain orbits, capturing the dynamic and time-varying nature of the Martian ionosphere requires further comprehensive investigations.

BepiColombo, the joint ESA/JAXA mission to Mercury, was launched in October 2018 and is scheduled to arrive at Mercury in November 2026 after an 8-year cruise. Like other planetary missions, its scientific objectives focus mostly on the nominal, orbiting phase of the mission. However, due to the long duration of the cruise phase covering distances between 1.2 and 0.3 AU, the BepiColombo mission has been able to outstandingly contribute to characterise the solar wind and transient events encountered by the spacecraft, as well as planetary environments during the flybys of Earth, Venus, and Mercury, and contribute to the characterisation of the space radiation environment in the inner Solar System and its evolution with solar activity. In this paper, we provide an overview of the cruise observations of BepiColombo, highlighting the most relevant science cases, with the aim of demonstrating the importance of planetary missions to perform cruise observations, to contribute to a broader understanding of Space Weather in the Solar System, and in turn, increase the scientific return of the mission. Graphical Abstract

The nightside ionosphere of Mars is formed by plasma transport from the dayside and electron precipitation. Significant progress has been made in our understanding of its composition and structure at low altitudes. However, what happens at higher altitudes remains unclear. Plasma structures escaping from the nightside of Mars could reveal the plasma transport paths from the dayside and from the nightside to space. Moreover, the response of escaping plasma structures to changing solar wind conditions will shed light on the dynamic evolution of the system. Mapping the paths of escaping plasma structures will result in a better understanding of the evolution of atmospheric escape at Mars and the contribution of escaping plasma structures to the total atmospheric loss. In this study we probe escaping plasma structures utilising two special campaigns of ESA's Mars Express mission as well as observations from NASA's MAVEN mission, in the high-altitude nightside ionosphere of Mars. Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) is the radar on board Mars Express and samples the ionosphere at altitudes no higher than ~1500 km. However, in our study we look at observations from consecutive orbits during two special MARSIS campaigns, each consisting of 5 orbits, that took place in September 2023 and April 2024, for which MARSIS was operated at altitudes up to ~5000 km. We see a variable nightside ionosphere at high altitudes that changes between consecutive MEX orbits. MARSIS detects plasma structures appearing at different altitudes or disappearing between orbits. We compare with MAVEN measurements to better evaluate both the escaping plasma structures and the solar wind conditions. MAVEN too sees plasma structures at high altitudes on the nightside, changing between orbits, confirming the variability of the high-altitude nightside ionosphere and revealing a dusk-dawn asymmetry in the observed electron densities.