Mercury: Current and Future Science 2018 (LPI Contrib. No. 2047) 6117.pdf

ENERGETIC NEUTRAL ATOM (ENA) IMAGING OF MERCURY’S MAGNETOSPHERE ONBOARD BEPICOLOMBO. S. Barabash1, M. Wieser1, Y. Futaana1, M. Holmström1, K. Asamura2, Y. Saito2 and P. Wurz 3, 1 Swedish Institute of Space Physics, Box 812, S-98128, Kiruna, Sweden ([email protected]), 2 Institute of Space and Astronautical Science, 3-1-1 Yoshinodai, Sagamihara, Japan, 3University of Bern, Physikalisches Institut, CH- 3012 Bern, Switzerland

ENAs at Mercury: ENAs (Energetic Neutral At- tive ions on an ionization surface and then pass oms) at Mercury are produced by (1) charge-exchange through an electrostatic analyzer (ESA) of a specific of energetic ions in the near-planet environment with (“wave”) shape that effectively blocks photons. The the planetary exosphere, (2) sputtering, and (3) electrostatic analyzer also provides crude energy anal- backscattering of precipitating ions from the planetary ysis. After exiting the ESA, ions are post accelerated surface [1], [2], [3]. Mercury has a tenuous exosphere up to an energy of 1.5 keV and impact on a START but sufficiently dense to convert solar wind and plane- surface releasing kinetic secondary electrons and are tary ions into ENAs via charge - exchange. The ENA reflected towards the STOP MCPs where they are de- fluxes are close to the ones emerging from the Earth’s tected and produce a STOP pulse. The secondary elec- ring current. What is unique for Mercury is the high trons from the START surfaces are guided to the variability of Mercury’s magnetosphere giving rise to START MCPs and produce a START pulse. The pulsating ENA emissions (ENA “flashes”) with a peri- START and STOP timing gives the particle velocity. od of a minute or less. Due to the small size of the Combining the TOF measurements and ESA settings magnetosphere, the particles injected from the tail can one determines the neutral atoms energy and mass. fill up the entire dayside magnetosphere [4], making Measuring the azimuth of the ion hit on the START possible ENA imaging the shape of the magneto- surface by position sensitive START MCPs allows sphere. accurate determination of the arrival azimuth angles of Ions of the solar wind, accelerated in the tail, and the incoming neutral atoms. The instrument character- planetary ions, precipitate on Mercury’s surface, result istics are given up in Table 1. in sputtering. The energy spectrum of the sputtered Table 1. ENA instrument performance products falls off as E-2 (Thompson-Siegmund sputter- Parameter Value ing) and may result in high fluxes at energies larger Energy Range 10 eV – 3.3 keV than 10-100 eV, sufficient for detection. Energy Resolution, ΔE/E = 50% The third process resulting in ENA production is Mass Resolution H, O, Heavy backscattering of the precipitating solar wind and plan- Pure G-Factor/sector 10-2 cm2 sr eV/eV etary ions from Mercury’s surface. Recently it was Total Efficiency ~1 % shown that the lunar regolith backscatters up to 20% of Angular resolution the impinging protons as neutral hydrogen atoms. For < 50 eV (FWHM) 9° x 40° Since the solar wind flux impinging on the surface is For > 50 eV (FWHM) 9° x 25° high (up to 109 cm-2s-1sr-1 ) this process results in very Field of view 15° x 160° high hydrogen ENA fluxes (106 – 107 cm-2s-1sr-1), which will dominate all other sources [5]. The energy of the backscattering hydrogen is up to the energy of the impinging ions and thus well above few 10s eV. ENAs originated from backscattering (and also sputter- ing) can be used to visualize the precipitation regions in the same way as the terrestrial aurora shows magne- tospheric dynamics (ENA “aurora”). ENA instrument: The ENA instrument (ENA,

Energetic Neutrals Analyzer) onboard the Mercury Fig. 1 ENA instrument on BepiColombo / MMO Magnetospheric Orbiter (MMO) of BepiColombo (Fig. 1) is based on the surface conversion technique and References: [1] Barabash S. et al. (2001) PSS, 49, provides measurements of ENAs in the energy range 1685-1692. [2] Lukyanov, A., S. Barabash S. et al. 10 eV – 3.3 keV. ENAs enter the instrument through (2004) Adv. Space Res., 33, 1888-1896. [3] Mura, A. et an electrostatic charged particle deflector, which re- al. (2006) PSS 54, 144-152. [4] Kallio E. and Janhunen jects ambient charged particles by a static electric field. P. (2003) Geophys. Res. Lett. 30, 1877. [5] Lue C. et The incoming neutral atoms are then converted to posi- al. (2017) Icarus 296, 39–48.