Takuya Hara

We studied a coronal mass ejection (CME) structure that propagated through the inner heliosphere in October 2021, which was observed by BepiColombo, Solar Orbiter, PSP, STEREO‐A, and DSCOVR spacecraft located at different radial distances and heliospheric longitudes. Notably, four of the five spacecraft detected a dip‐like signature in the magnetic field embedded within the CME. We investigated the cause of the dip‐like signature on the basis of both in situ solar wind data and solar surface data. We examined the flare‐triggering structure of the M1.6 flare, which probably caused the CME; however, there was no conclusive evidence found linking the flare to the dip‐like signature observed in the ejected CME. We applied the minimum variance analysis (MVA) to the in situ magnetic field data to determine whether the dip‐like signature could represent a planar magnetic structure (PMS) embedded in the CME‐driven shock sheath. Our MVA results confirmed that the magnetic field associated with the dip‐like structure lies along a plane perpendicular to the minimum variance direction, consistent with the characteristics of the PMS previously reported in the literature. Additionally, we examined the shock parameters and found that the CME‐driven shock developed more rapidly than usual, probably because of the passage of a stream interaction region (SIR) during the initial phase of its propagation. This fact suggests that the interaction between the CME and the SIR played a significant role in the formation of the PMS, resulting in the observation of dip‐like structure downstream at most of the spacecraft.

ESCAPADE is a twin-spacecraft low-cost Mars mission that will revolutionize our understanding of how space weather conditions drive magnetic structure and flows of energy and momentum throughout Mars’ unique hybrid magnetosphere, and how this interaction drives both ion escape and sputtering escape. ESCAPADE will measure magnetic field strength and topology, suprathermal ion distributions and electron flows, and thermal electron and ion densities, as well as possibly image visible aurora. Our 2-part scientific campaign of temporally and spatially-separated multipoint measurements in different regions of Mars’ diverse plasma environment, will allow us to untangle spatial from temporal variability, characterize short-term variability, and unravel the cause-and-effect of solar wind control of magnetospheric structure and ion and sputtering escape for the first time.ESCAPADE launched on November 13, 2025. Though it is a Mars mission, ESCAPADE’s journey begins with a 12 month “loiter” phase within ~2.5 million km of earth, primarily on the anti-sunward side, looping around the L2 Lagrange point and passing twice through the Earth’s magnetotail, at ~320 and again at ~80 earth radii. Instruments are due to be turned on in late February 2026, just prior to this first tail passage. ESCAPADE will provide the first two-point measurements of heliospheric conditions in these regions of space, addressing questions of solar wind and space weather structure on ~105 km scales and investigating distant magnetotail features, including spatial extent, dependence on solar wind conditions, and the existence of reconnection in the distant magnetotail. In November 2026, ESCAPADE will execute Oberth maneuvers at a ~500 km perigee to start their interplanetary journey, arriving at Mars in September 2027 and beginning their science mission in spring 2028. This presentation will focus on first results from the plasma instruments in the near-Earth heliospheric environment.

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

Billions of years ago, Mars’ ability to sustain liquid water waned as the solar wind and radiation began to erode the atmosphere. Sputtering is an atmospheric escape process that may have been dominant during earlier epochs of our Sun according to isotopic evidence, but is difficult to detect under current solar conditions. Using over 9 years of data from the Mars Atmosphere and Volatile Evolution mission, we present the first observations of present-day sputtering in the martian upper atmosphere. By correlating argon densities with solar electric fields, we find that sputtered rates of argon are over four times higher than model predictions. We also present evidence of enhanced sputtering during a solar storm, offering a glimpse at more intense conditions in the early solar system. Observationally establishing the role of sputtering in the loss of Mars’ atmosphere is critical to understanding the conditions that allowed liquid water to exist on the martian surface and the implications for habitability.

Ion pickup by the solar wind is ubiquitous in space plasma. Because pickup ions are originally produced by ionization of an exospheric neutral atmosphere, their measurements contain information on the exospheric neutral abundance. Here we established a method to retrieve exospheric number densities, by analyzing the ion velocity distribution functions of pickup ions measured by the Mars Atmosphere and Volatile EvolutioN spacecraft. We successfully retrieved exospheric oxygen density distributions at altitudes ranging from 1000 to 10,000 km around Mars except for the vicinity of the bow shock. This method can be applied to other space missions to study the upper atmosphere of planets, moons, and other small bodies in our solar system, where pickup ions exist.

Pickup ions around Mars are originally produced from an exospheric neutral atmosphere and their measurements thus contain information on the exospheric neutral abundance. Here we present a method to retrieve exospheric number densities by analyzing the ion velocity distribution functions of pickup ions measured by the SupraThermal And Thermal Ion Composition instrument on the Mars Atmosphere and Volatile EvolutioN spacecraft. We successfully retrieved exospheric oxygen density distributions at altitudes ranging from 1,000–10,000 km around Mars (Masunaga et al., submitted). In this study, we also apply this method to analyze temporal variations of O exospheric densities. We particularly examined the case of the dust storm event in September 2016 during which thermospheric O and ionospheric O+ densities varied with a period of ~7-8 days (Masunaga et al., 2022, Hara et al., 2024) and the comet approach event in September 2014 during which the cometary atmosphere was transferred to the upper atmosphere of Mars (Crismani et al., 2015), finding that the Martian O exosphere likely responds to these drivers.

Dust storms and the atmospheric waves can play a significant role in the dynamics in the upper neutral atmosphere of Mars. Recent observations found that a periodic variation of neutral H and O exists in the upper atmosphere, which is likely associated with atmospheric waves that occurred during the regional dust storm on 4 September 2016. However, such periodic variations accompanying the dust storm are far from understood in terms of the Martian ionized particles (i.e., ionosphere). Here we investigated the periodic variation of the ion density in the Martian ionosphere during the regional dust storm in September 2016 based on the MAVEN/STATIC observations. Assuming a simple Chapman layer model, we implemented numerical fitting for the ion density altitude profile to retrieve the peak ion densities in the Martian ionosphere. We then applied periodogram analysis to these peak ion densities in order to identify the peak periodicities together with their confidence levels. We identified several distinct peak periodicities with a scale from ∼1 day up to ∼20 days. The peak periodicities around 6–9 days are comparable to those seen in the Martian neutral atmosphere. In addition, the other peak periodicities likely correspond to the periodic crossings of the local crustal fields and periodic variation of the upstream solar wind, indicating a strong regional coupling between the lower atmosphere and the upstream solar wind.

In December 2022, an extremely low‐density solar wind stream encountered first the Earth and then Mars, shortly after the radial alignment of the two planets (i.e., Mars opposition). As the low‐density stream passed Mars, the properties of the Mars‐solar wind interaction changed dramatically in response to the low solar wind Alfvénic Mach number and dynamic pressure. The Martian magnetosphere and its boundaries expanded by thousands of kilometers, extending outside of the nominal average bow shock location. The low upstream Mach number resulted in a low ratio of plasma to magnetic field pressure in the magnetosheath, allowing the formation of a very high but stable ion temperature anisotropy, together with a very low level of electromagnetic instabilities. Meanwhile, the decrease in solar wind dynamic pressure caused the Martian upper ionosphere at the terminator to transition from a magnetized state to an unmagnetized state. This event provides an opportunity to study a unique end‐member state of the Mars‐solar wind interaction.

We analyzed Mars Atmosphere and Volatile Evolution (MAVEN) and Venus Express (VEX) data of magnetic flux ropes observed in both Mars and Venus nightside induced magnetosphere, in order to understand their statistical characteristics and possible formation processes. We statistically identified 382 (286) events at Mars (Venus) via the minimum variance analysis and investigated the flux rope properties including their geometrical configurations of axial core field direction and spatial distributions. Interestingly, there is no significant difference with respect to the variety of the flux rope axial orientation between Mars and Venus, indicating that about at least one fourth of events do not necessitate magnetotail reconnection for their formations. However, the spatial distribution shows that flux ropes at Venus tend to be observed near the central plasma sheet, whereas those at Mars are more spread out. Moreover, the events tend to be observed more frequently in the −E hemisphere, where the solar wind electric field (E_SW) is pointing toward the planet, rather than in +E hemisphere, where the E_SW direction is away from the planet. The geographical distribution shows that flux ropes at Mars tend to be observed more frequently in the southern hemisphere where the strong crustal magnetic fields are primarily distributed. Therefore, such a tendency indicates that the Martian crustal magnetic fields could play significant roles on generating flux ropes in the nightside magnetosphere of Mars.