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Context. The Mercury electron analyzer (MEA) obtained new electron observations during the first three Mercury flybys by BepiColombo on October 1, 2021 (MFB1), June 23 , 2022 (MFB2), and June 19, 2023 (MFB3). BepiColombo entered the dusk side magnetotail from the flank magnetosheath in the northern hemisphere, crossed the Mercury solar orbital equator around midnight in the magnetotail, traveled from midnight to dawn in the southern hemisphere near the closest approach, and exited from the post-dawn magnetosphere into the dayside magnetosheath. Aims. We aim to identify the magnetospheric boundaries and describe the structure and dynamics of the electron populations observed in the various regions explored along the flyby trajectories. Methods. We derive 4s time resolution electron densities and temperatures from MEA observations. We compare and contrast our new BepiColombo electron observations with those obtained from the Mariner 10 scanning electron spectrometer (SES) 49 yr ago. Results. A comparison to the averaged magnetospheric boundary crossings of MESSENGER indicates that the magnetosphere of Mercury was compressed during MFB1, close to its average state during MFB2, and highly compressed during MFB3. Our new MEA observations reveal the presence of a wake effect very close behind Mercury when BepiColombo entered the shadow region, a significant dusk-dawn asymmetry in electron fluxes in the nightside magnetosphere, and strongly fluctuating electrons with energies above 100s eV in the dawnside magnetosphere. Magnetospheric electron densities and temperatures are in the range of 10–30 cm−3 and above a few 100s eV in the pre-midnight-sector, and in the range of 1–100 cm−3 and well below 100 eV in the post-midnight sector, respectively. Conclusions. The MEA electron observations of different solar wind properties encountered during the first three Mercury flybys reveal the highly dynamic response and variability of the solar wind-magnetosphere interactions at Mercury. A good match is found between the electron plasma parameters derived by MEA in the various regions of the Hermean environment and similar ones derived in a few cases from other instruments on board BepiColombo. [ABSTRACT FROM AUTHOR]

Pulsating Aurora (PsA) is one of the major classes of diffuse aurora associated with precipitation of a few to a few tens of keV electrons from the magnetosphere. Recent studies suggested that, during PsA, more energetic (i.e., sub‐relativistic/relativistic) electrons precipitate into the ionosphere at the same time. Those electrons are considered to be scattered at the higher latitude part of the magnetosphere by whistler‐mode chorus waves propagating away from the magnetic equator. However, there have been no actual cases of simultaneous observations of precipitating electrons causing PsA (PsA electrons) and chorus waves propagating toward higher latitudes; thus, we still do not quite well understand under what conditions PsA electrons become harder and precipitate to lower altitudes. To address this question, we have investigated an extended interval of PsA on 12 January 2021, during which simultaneous observations with the Arase satellite, ground‐based all‐sky imagers and the European Incoherent SCATter (EISCAT) radar were conducted. We found that, when the PsA shape became patchy, the PsA electron energy increased and Arase detected intense chorus waves at magnetic latitudes above 20°, indicating the propagation of chorus waves up to higher latitudes along the field line. A direct comparison between the irregularities of the magnetospheric electron density and the emission intensity of PsA patches at the footprint of the satellite suggests that the PsA morphology and the energy of PsA electrons are determined by the presence of "magnetospheric density ducts," which allow chorus waves to travel to higher latitudes and thereby precipitate more energetic electrons. Plain Language Summary: Pulsating Aurora (PsA) is a kind of diffuse aurora associated with periodic precipitation of energetic electrons from the near‐Earth space into the atmosphere. Recent research has shown that, during PsA events, energetic particles at the sub‐relativistic energy range precipitate into the atmosphere. We speculate that such particles are scattered by wave‐particle resonance with natural electromagnetic waves, called chorus waves, at higher magnetic latitude regions. However, there has been no experimental case of PsA during which propagation of the chorus waves to higher magnetic latitudes was confirmed; thus, we still do not fully understand when and why PsA electrons become more energetic. Here, we investigate a PsA event on 12 January 2021, simultaneously observed by the Arase satellite, ground‐based all‐sky imagers and the European Incoherent SCATter (EISCAT) radar. We found that, when the PsA shape was patchy, the energy of precipitating electrons increased and chorus waves were observed at high latitudes in the magnetosphere. Comparing the magnetospheric electron density with the PsA brightness seen from the ground, we suggest that both the PsA shape and the energy of precipitating electrons were influenced by the so‐called magnetospheric ducts, which guide chorus waves to high‐latitudes regions where they interact with more energetic electrons. Key Points: Examined simultaneous observations of Pulsating Aurora (PsA) with the Arase satellite, ground‐based all‐sky imagers, and the EISCAT radarFound a relationship among the patchy PsA, the enhanced energy of PsA electrons, and the chorus wave propagation to high‐latitudes (>20°)Arase observations suggest that the observed relationship can be explained by the ducted propagation of chorus waves [ABSTRACT FROM AUTHOR]

This is the first report of significant energization (up to 7,000 eV) of low‐energy He+ ions, which occurred simultaneously with H‐band electromagnetic ion cyclotron (EMIC) wave activity, in a direction mostly perpendicular to the ambient magnetic field. The event was detected by the Arase satellite in the dayside plasmatrough region off the magnetic equator on 15 May 2019. The peak energy of the He+ flux enhancements is mostly above 1,000 eV. At some interval, the He+ ions are energized up to ∼7,000 eV. The H‐band waves are excited in a frequency band between the local crossover and helium gyrofrequencies and are close to a linear polarization state with weakly left‐handed or right‐handed polarization. The normal angle of the waves exhibits significant variation between 0° and 80°, indicating a non‐parallel propagation. We run a hybrid code with parameters estimated from the Arase observations to examine the He+ energization. The simulations show that cold He+ ions are energized up to more than 1,000 eV, similar to the spacecraft observations. From the analysis of the simulated wave fields and cold plasma motions, we found that the ratio of the wave frequency to He+ gyrofrequency is a primary factor for transverse energization of cold He+ ions. As a consequence of the numerical analysis, we suggest that the significant transverse energization of He+ ions observed by Arase is attributed to H‐band EMIC waves excited near the local helium gyrofrequency. Key Points: Strong H‐band EMIC waves were detected in the dayside plasmatrough region off the magnetic equator by the Arase spacecraftThe H‐band waves energize cold He+ ions to levels above ∼1,000 eV in the direction perpendicular to the background magnetic fieldSimulation indicates that the ratio of the wave frequency to He+ gyrofrequency is a primary factor in the strong energization of He+ ions [ABSTRACT FROM AUTHOR]

Author(s): S. Kasahara (corresponding author) [1]; Y. Miyoshi [2]; S. Yokota [3]; T. Mitani [4]; Y. Kasahara [5]; S. Matsuda [2]; A. Kumamoto [6]; A. Matsuoka [4]; Y. Kazama [7]; [...]