Rosanna Rispoli

Investigation of the possible effects of comet Encke's meteoroid stream on the Ca exosphere of Mercury

The MESSENGER observations of the seasonal variability of Mercurys Ca-exosphere are consistent with the general idea that the Ca-atoms originate from the bombardment of the surface by particles from comet 2P/Encke. The generating mechanism is believed to be a combination of different processes including the release of atomic and molecular surface particles and the photo-dissociation of exospheric molecules. Considering different generation and loss mechanisms, we perform simulations with a 3D Monte Carlo model based on the exosphere generation model by Mura et al. [2009]. We present for the first time the 3D spatial distribution of the CaO and Ca exospheres generated through the process of micrometeoroid impact vaporization and we show that the morphology of the latter is consistent with the available MESSENGER/MASCS observations. The results presented in this paper can be useful in the exosphere observations planning for BepiColombo, the upcoming ESA-JAXA mission to Mercury.

ELENA microchannel plate detector: absolute detection efficiency for low energy neutral atoms

Microchannel plate (MCP) detectors are frequently used in space instrumentation for detecting a wide range of radiation and particles. The capability to detect non-thermal, low energy, neutral species is crucial for the Emitted Low-Energy Neutral Atoms (ELENA) sensor, which is part of the Search for Exospheric Refilling and Emitted Natural Abundances (SERENA) package on board the Mercury Planetary Orbiter (MPO) space- craft of the BepiColombo mission of European Space Agency to Mercury, which is scheduled for launch in August 2015. ELENA is a time-of-flight sensor based on a novel concept using an ultrasonic oscillating shutter (start section) and MCP detector (stop detector). The ELENA scientific objective is to monitor the emission of neutral atoms from the surface of Mercury by detecting energetic neutral atoms in the range 10 eV to 5 keV, within 76 deg FOV, perpendicular to the S/C orbital plane. The sur- face is scanned due to the spacecraft motion. In particular, processes of particle release from the surface will be investigated by identifying particles released via solar wind-induced ion sputtering (with energies >1 eV to <100 eV) as well as energetic hydrogen atoms, which are back-scattered solar wind protons, at energies of hundreds of eV. MCP absolute detection efficiency, for very low energy neutral atoms (E < 30 eV), is a crucial point for this investigation. At Messkammer fur Flugzeitinstrumente und time-of- flight facility of the University of Bern, measurements on three MCP, with different coatings, have been performed providing the first data of MCP detection efficiencies in the energy range 10 eV to 1 keV.

ELENA MCP detector: absolute detection efficiency for low-energy neutral atoms

Microchannel Plates (MCP) detectors are frequently used in space instrumentation for detecting a wide range of radiation and particles. In particular, the capability to detect non-thermal low energy neutral species is crucial for the sensor ELENA (Emitted Low-Energy Neutral Atoms), part of the package SERENA (Search for Exospheric Refilling and Emitted Natural Abundances) on board the BepiColombo mission of ESA to Mercury to be launched in 2015. ELENA is a Time of Flight (TOF) sensor, based on a novel concept using an ultra-sonic oscillating shutter (Start section), which is operated at frequencies up to 50 kHz; a MCP detector is used as a Stop detector. The scientific objective of ELENA is to detect energetic neutral atoms in the range 10 eV – 5 keV, within 76° FOV, perpendicular to the S/C orbital plane. ELENA will monitor the emission of neutral atoms from the whole surface of Mercury thanks to the spacecraft motion. The major scientific objectives are the interaction between the plasma environment and the planet’s surface, the global particle loss-rate and the remote sensing of the surface properties. In particular, surface release processes are investigated by identifying particles released from the surface, via solar wind-induced ion sputtering (< 1eV – < 100 eV) as well as Hydrogen back-scattered at hundreds eV. MCP absolute detection efficiency for very low energy neutral atoms (E < 30 eV) is a crucial point for this investigation. At the MEFISTO facility of the Physical Institute of the University of Bern (CH), measurements on three different types of MCP (with and without coating) have been performed providing the detection efficiencies in the energy range 10eV – 1keV. Outcomes from such measurements are discussed here.

Mercury sodium exospheric emission as a proxy for solar perturbations transit

The first evidence at Mercury of direct relation between ICME transit and Na exosphere dynamics is presented, suggesting that Na emission, observed from ground, could be a proxy of planetary space weather at Mercury. The link existing between the dayside exosphere Na patterns and the solar wind-magnetosphere-surface interactions is investigated. This goal is pursued by analyzing the Na intensity hourly images, as observed by the ground-based THEMIS solar telescope during 10 selected periods between 2012 and 2013 (with seeing, σ < = 2″), when also MESSENGER data were available. Frequently, two-peak patterns of variable intensity are observed, located at high latitudes in both hemispheres. Occasionally, Na signal is instead diffused above the sub-solar region. We compare these different patterns with the in-situ time profiles of proton fluxes and magnetic field data from MESSENGER. Among these 10 cases, only in one occasion the Na signal is diffused above the subsolar region, when the MESSENGER data detect the transit of two ICMEs. The selected cases suggest that the Na emission patterns are well related to the solar wind conditions at Mercury. Hence, the exospheric Na emission patterns, observed from ground, could be considered as a ‘natural monitor’ of solar disturbances when transiting near Mercury.