The Mercury Electron Analyzer (MEA) obtained new electron observations during the first threeMercury swingbys (MSBs) by BepiColombo on 01 October 2021 (MSB1), 23 June 2022 (MSB2),and 19 June 2023 (MSB3). We identify the magnetospheric boundaries and describe the structure and dynamics of the electron populations observed in the various regions explored along the swingby trajectories. We compare and contrast our new BepiColombo electron observations with those obtained from the Mariner 10 Scanning Electron Spectrometer (SES) 50 years ago.A comparison to the averaged magnetospheric boundary crossings by MESSENGER indicatesthat the magnetosphere of Mercury was compressed during MSB1, close to its average stateduring MSB2, and highly compressed during MSB3. Our new MEA observations revealed asignificant dusk-dawn asymmetry in electron fluxes on the nightside magnetosphere, and ofstrongly fluctuating electrons with energies above 100s eV on the dawnside magnetosphere.Magnetospheric electron densities and temperatures were 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.MEA electron observations of different solar wind properties encountered during the first threeMSBs revealed the highly dynamic response of the solar wind-magnetosphere interactions atMercury. A good match is found between the electron plasma parameters derived by MEA in the various regions of the Hermean environment with similar ones derived for a few cases from other instruments on board BepiColombo.

Although solar wind-driven convection is expected to dominate magnetospheric circulation at Mercury, its exact pattern remains poorly characterized by observations. Here we present BepiColombo Mio observations during the third Mercury flyby indicative of convection-driven transport of low-energy dense ions into the deep magnetosphere. During the flyby, Mio observed an energy-dispersed ion population from the duskside magnetopause to the deep region of the midnight magnetosphere. A comparison of the observations with backward test particle simulations suggests that the observed energy dispersion structure can be explained in terms of energy-selective transport by convection from the duskside tail magnetopause. We also discuss the properties and origins of more energetic ions observed in the more dipole-like field regions of the magnetosphere in comparison to previously reported populations of the plasma sheet horn and ring current ions. Additionally, forward test particle simulations predict that most of the observed ions on the nightside will precipitate onto relatively low-latitude regions of the nightside surface of Mercury for a typical convection case. The presented observations and simulation results reveal the critical role of magnetospheric convection in determining the structure of Mercury's magnetospheric plasma. The upstream driver dependence of magnetospheric convection and its effects on other magnetospheric processes and plasma-surface interactions should be further investigated by in-orbit BepiColombo observations. BepiColombo Mio observed low-energy dense ions deep within Mercury's magnetosphere during the third Mercury flyby Backward test particle simulations suggest that low-energy ions are transported by convection from the duskside tail magnetopause Forward test particle simulations imply convection-driven ion precipitation onto the nightside low-latitude surface of Mercury

An important technique of modern space plasma diagnostics is a detection and imaging of low energy (below 10 keV) energetic neutral atoms (ENA). Any space mission devoted to study of the planetary plasma environments, planetary magnetospheres and heliosphere boundaries, needs a low energy ENA imaging sensor in its payload list. A common approach to the ENA detection/imaging is to make energetic neutral atoms glance a high quality conductive surface and either produce a secondary electron, or produce a positive or negative reflection ion. In the first case we can collect and detect the yielded secondary electron and generate a start signal. The reflected neutral atom can be directed to another surface with a high secondary electron yield. Thus we can measure a time-of-flight of the reflected particle to get its velocity. In the second case we can analyze the reflected ion in an electrostatic analyzer to get the particle energy.Many types of conversion surfaces have been investigated over last decades in order to optimize an ENA sensor properties. We investigated properties of a thin layer of graphene applied to a silicon wafer surface. The experimental setup consisted of a secondary electron detector, neutral/ions separator and a high resolution particle imager. We used an incident He beam with energy of 200 eV - 3000 eV. We obtained a secondary electron emission, particle reflection efficiency, scattering properties, and a positive ion production rate as a function of the incident beam energy and the grazing angle. The experiment results show that 1) Graphene is a good source of secondary electrons even for low energy incident particles; 2) ENA scatter from the graphene surface similar to other surface types; 3) Graphene does not convert incident ENA to positive ions, especially for high grazing angles.