Stefano Massetti

Low energy high angular resolution neutral atom detection by means of micro‐shuttering techniques: the BepiColombo SERENA/ELENA sensor

The neutral sensor ELENA (Emitted Low‐Energy Neutral Atoms) for the ESA cornerstone BepiColombo mission to Mercury (in the SERENA instrument package) is a new kind of low energetic neutral atoms instrument, mostly devoted to sputtering emission from planetary surfaces, from E∼20 eV up to E∼5 keV, within 1‐D (2°×76°). ELENA is a Time‐of‐Flight (TOF) system, based on oscillating shutter (operated at frequencies up to a 100 kHz) and mechanical gratings: the incoming neutral particles directly impinge upon the entrance with a definite timing (START) and arrive to a STOP detector after a flight path. After a brief dissertation on the achievable scientific objectives, this paper describes the instrument, with the new design techniques approached for the neutral particles identification and the nano‐techniques used for designing and manufacturing the nano‐structure shuttering core of the ELENA sensor. The expected count‐rates, based on the Hermean environment features, are shortly presented and discussed. Such d...

Occurrence and average behavior of pulsating aurora

Motivated by recent event studies and modelling efforts on pulsating aurora, which conclude that the precipitation energy during these events is high enough to cause significant chemical changes in the mesosphere, this study looks for the bulk behaviour of auroral pulsations. Based on about 400 pulsating aurora events we outline the typical duration, geomagnetic conditions and change in the peak emission height for the events. We show that the auroral peak emission height for both green and blue emission decreases by about 8 km at the start of the pulsating aurora interval. This brings the hardest 10% of the electrons down to about 90 km altitude. The median duration of pulsating aurora is about 1.4 hours. This value is a conservative estimate since in many cases the end of event is limited by the end of auroral imaging for the night or the aurora drifting out of the camera field-of-view. The longest durations of auroral pulsations are observed during events which start within the substorm recovery phases. As a result, the geomagnetic indices are not able to describe pulsating aurora. Simultaneous Antarctic auroral images were found for ten pulsating aurora events. In eight cases auroral pulsations were seen in the southern hemispheric data as well, suggesting an equatorial precipitation source and a frequent inter-hemispheric occurrence. The long lifetimes of pulsating aurora, their inter-hemispheric occurrence and the relatively high precipitation energies make this type of aurora an effective energy deposition process which is easy to identify from the ground-based image data.

Short‐term observations of double‐peaked Na emission from Mercury's exosphere

We report the analysis of short-term ground-based observations of the exospheric Na emission (D1 and D2 lines) from Mercury, which was characterized by two high-latitude peaks confined near the magnetospheric cusp footprints. During a series of scheduled observations from the THEMIS telescope, achieved by scanning the whole planet, we implemented a series of extra measurements by recording the Na emission from a narrow North-South strip only, centered above the two emission peaks. Our aim was to inspect the existence of short-term variations, which were never analyzed before from ground-based observations, and their possible correlation with IMF variations. Though Mercury possesses a miniature magnetosphere, characterized by fast reconnection events that develops on a timescale of few minutes, ground-based observations show that the exospheric Na emission pattern can be globally stable for a prolonged period (some days), but also exhibits fluctuations in the time range of tens of minutes.

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.