Yuki Harada

The most intense radio feature in the Solar System are the solar Type‐III radio bursts, which are associated with electron beams that propagate along open magnetic field lines. Although solar radio bursts have been studied since the 1940s, observations remain mainly restricted within 1 AU. In this study, we explore 16.5 years of data (2005–2021) from the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) onboard Mars Express. While not the central aim of the instrument, MARSIS has been recording solar radio bursts since its deployment in mid‐2005. Given the potential of this new radio burst data set for heliophysics studies, we have created a catalog of Type‐III solar radio burst events between 1.38 and 1.66 AU (Mars' orbit) within the frequency range of 0.1 and 5.5 MHz and with an instrument measuring cadence of 7.5 s. We demonstrate that these emissions seen at Mars come from the Sun, follow the solar cycle pattern, and are not influenced by the planet's own properties such as crustal magnetic fields. We also compare the propagation of a few illustrative events from the Sun to Mars to demonstrate the utility of this new data set for heliophysics studies. Mars catalogs provide a fundamentally new perspective on the spatial extent, directivity, and evolution of solar radio emission in the inner heliosphere. It serves as an essential benchmark for multi‐point heliospheric studies and for constraining radio emissions mechanisms beyond Earth orbit, and demonstrating the usefulness of planetary missions in contributing to heliophysics science in diverse ways.

Reconnection observations involving large density gradients and multiple heavyion species are scarce, motivating the analysis of satellite observations at Mars. The analyzed event exhibits anti-parallel magnetic fields with a guide field, accompanied by a substantial density gradient of approximately one to two orders of magnitude involving multiple species: e−, H+, O+, O+2 , and CO+2. The event is examined in terms of the temporal and spatial scales of the heavy ions of an assumed reconnection event, with the expectation that the structure is maintained by electron currents. The distance between the shocked solar wind and the collision-dominated ionosphere–thermosphere is small compared with the characteristic lengths of the X-line and outflow region. The ionospheric ions are accelerated within the reconnection region but transition to predominantly outflow-directed motion outside the exhaust. Away from the X-line, a boundary layer forms populated by accelerated ionospheric ions. The observations suggest that multiple X-lines form within the same areographic region and collectively sustain the outflow region over an extended period. For each newly formed Xline, reconnection is proposed to advance to lower altitudes, locally eroding the Martian atmosphere. This event demonstrates that magnetic reconnection can efficiently exchange plasma between the solar wind and the Martian ionosphere.

The lunar surface charging is controlled mainly by the balance of electron currents near the surface. One of the main electron currents is the photocurrent on the dayside, comprised of photoelectrons produced by solar photons ionizing the lunar surface. Yet, there lacks a systematic characterization of the statistical properties of lunar photoelectrons. Photoelectrons can be uniquely identified by the oxygen Auger electron flux peak in the photoelectron energy spectrum. In this study, we develop a methodology to select lunar photoelectron measurements from the Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS) mission. With such a photoelectron database, we statistically analyze several properties of the lunar photoelectrons when the Moon is in the magnetotail lobe environment and in the solar wind environment, including how the photoelectron fluxes and the photoelectron detection probability vary and depend on driving parameters.

As an airless body, the lunar surface is charged as a result of balancing electric currents into and away from the surface. Lunar surface charging is important for both scientific and human‐exploration purposes. The nightside surface charging has been well characterized by previous studies, but less so for the dayside. With a combination of observations from the Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS) mission and modeling efforts, we can infer the lunar dayside surface potential. This study provides a statistical analysis of the lunar dayside surface potential when the Moon is located in the Earth's magnetotail lobes and how it varies with surface solar zenith angles, local plasma density, and electron temperatures, as well as lunar crustal fields.

Tailward ion escape in the nightside magnetotail is one of the dominant paths of ion escape from Mars. The Martian nightside ionosphere can serve as a major reservoir for this escape channel, but its global distributions and dependence on the upstream solar wind have not been fully investigated due to observational limitations. In this study, we analyzed 8 years of Mars Express's Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) observations and Mars Atmosphere and Volatile EvolutioN (MAVEN) upstream solar wind data from October 2014 to November 2022, thereby ensuring global coverage. From MARSIS data, we derived the peak electron density and detection rate of the nightside ionosphere and investigated their dependence on the MAVEN upstream solar wind conditions. We find that the observed dependence is different between weak and strong crustal magnetic field regions. In the weak crustal magnetic field regions, relatively low density () ionospheres were observed, and high solar wind dynamic pressure and the outward direction of the solar wind electric field are particularly favored for the detection of nightside ionospheric echoes. As for the strong crustal magnetic field regions, very high‐density () nightside ionospheres were observed. They tend to be observed under near anti‐parallel conditions between draped interplanetary magnetic fields (IMF) and crustal magnetic fields, suggesting a contribution of magnetic reconnection between them. These results align with Martian electron auroral observations and highlight the importance of electron impact ionization.

Particles are energized—heated and accelerated to nonthermal energies—in laboratory, space, solar, and astrophysical plasmas. In collisionless plasmas, ion and electron temperatures are often unequal and cannot be fully understood within the framework of magnetohydrodynamics (MHD). In this context, a relation, Δ ε _s = q _s VBL _s , for each species can be useful, where Δ ε _s is the energy gain for species s, measured in the plasma rest frame, relative to the upstream region of shocks and magnetic reconnection; q _s is the charge; V is the plasma bulk flow speed; B is the magnetic field strength; and L _s is a characteristic length scale of energization. From this relation, we recently derived semiempirical scalings for ion and electron temperature increases across shocks and magnetic reconnection in Earth’s plasma environment. However, it remains unclear how broadly these scalings apply. Here we show that the same scalings explain temperature increases in other plasma environments such as laboratory experiments, planetary magnetospheres, solar flares, and supernova remnant shocks. Combined with another recent report that the maximum energy of particles in various plasma environments follows the same relation when L _s is taken as the system size, our results indicate that Δ ε _s = q _s VBL _s provides a novel framework that universally captures particle energization—both heating and acceleration to nonthermal energies. Additionally, the scaling captures the essential MHD trends while revealing systematic deviations that point to kinetic effects beyond fluid models, highlighting promising directions for theoretical and simulation studies.

On the dayside of the Moon, the surface potential is primarily determined by (a) photoelectron emission by solar radiation and (b) the ambient plasma environment. Both of these drivers have large variability, but the resulting variability of the lunar surface potential has not been fully characterized yet. To investigate the solar activity dependence of the dayside lunar surface potential in the terrestrial magnetotail, we compared long‐term observations of lunar surface photoelectrons with numerical calculations of photo‐emitted electron energy spectra. The comparison demonstrates that the observed solar activity dependence of lunar photoelectrons is well reproduced by the model. The current balance surface potential based on this model predicts more positive potentials for higher solar activities, possibly exceeding in the magnetotail lobes during solar flares. These results highlight the importance of evaluating the lunar electrostatic environment under various conditions, including extreme solar events.

Due to the lack of low‐altitude (50 km), high‐time‐resolution measurements, expected electron‐only magnetic reconnection near lunar magnetic anomalies (LMAs) has not been directly observed by lunar orbiters even though it is expected from numerical simulations and laboratory experiments. In this study, we analyze Kaguya's low‐altitude (29 km) electron data with a time resolution of 1 s. We first present an observation of a current sheet crossing near the South‐Pole Aitken magnetic anomaly, accompanied by an electron jet in the absence of any ion jet and by a magnetic flux rope. These signatures suggest potential in situ detection of electron‐only magnetic reconnection near the LMA. Our observational results offer further support for the occurrence of electron‐only magnetic reconnection near LMAs and imply that LMAs can provide plasma‐physics laboratories to investigate the nature of electron‐only magnetic reconnection.

Interplanetary coronal mass ejections (ICMEs) cause “Forbush decreases” (FDs), which are local decreases in background galactic cosmic rays (GCRs). Even though FDs can be observed with simple particle instruments, their amplitude and shape provide physical profiles of passing ICMEs. However, in some cases, previous statistical studies of the heliocentric distance dependence of FD changes associated with ICME propagation have found no strong correlation. We need the criteria for evaluating the relationship between ICME structure and FDs, necessary for the FD’s statistical analysis. This study investigates the effect of the evolution and interactions of ICMEs on FD profiles in the inner solar system using multipoint comparisons. We focus on multipoint ICME observations by Solar Orbiter, BepiColombo, and near-Earth spacecraft from 2022 March 10 to 16, when these spacecraft were ideally located for studying the radial and longitudinal evolution of ICMEs and accompanying FDs. We compared GCR variations with the multiple in situ data and ICME model, clarifying the correspondence between the evolution of each ICME structure in the radial and azimuthal directions and the depth and gradients of the FD. The radial comparison revealed decreases in FD intensities and gradients associated with the expansion of the ICME. The longitudinal difference found in FD intensity indicates longitudinal variations of the ICME’s shielding effect. These results suggest that accurate multipoint FD comparisons require determining the relationship between the observer’s position and the inner structure of the passing ICMEs.

Magnetic fields around Mars shape the pathways of energy input into, and ion escape from, the Martian atmosphere, thereby potentially affecting the atmospheric evolution of Mars. The Martian magnetic field environment is primarily determined by the global electric current system resulting from the solar wind interaction with the upper atmosphere and crustal magnetic fields of Mars. Recently, it has been recognized that neutral wind-driven ionospheric dynamo currents generate magnetic field perturbations observable in orbits and on the surface of Mars. However, the entire system of near-Mars currents is not fully understood. Here we present a newly identified current system in the Martian magnetosphere directly coupled to the wind-driven ionospheric currents, based on the spherical harmonic analysis of magnetic field data obtained by the Mars Atmosphere and Volatile Evolution spacecraft. This magnetospheric current system can be explained by diamagnetic currents associated with magnetic fields generated by the wind-driven ionospheric dynamo currents. Our results demonstrate a close coupling between the neutral atmospheric dynamics and the high-altitude magnetospheric current system at Mars mediated by wind-driven ionospheric dynamo currents. This reveals a previously unrecognized role of the neutral atmosphere in controlling magnetic field environments of unmagnetized planets.