S. Orsini

The loss of ions from Venus through the plasma wake

Venus, unlike Earth, is an extremely dry planet although both began with similar masses, distances from the Sun, and presumably water inventories. The high deuterium-to-hydrogen ratio in the venusian atmosphere relative to Earth’s also indicates that the atmosphere has undergone significantly different evolution over the age of the Solar System. Present-day thermal escape is low for all atmospheric species. However, hydrogen can escape by means of collisions with hot atoms from ionospheric photochemistry, and although the bulk of O and O2 are gravitationally bound, heavy ions have been observed to escape through interaction with the solar wind. Nevertheless, their relative rates of escape, spatial distribution, and composition could not be determined from these previous measurements. Here we report Venus Express measurements showing that the dominant escaping ions are O+, He+ and H+. The escaping ions leave Venus through the plasma sheet (a central portion of the plasma wake) and in a boundary layer of the induced magnetosphere. The escape rate ratios are Q(H+)/Q(O+) = 1.9; Q(He+)/Q(O+) = 0.07. The first of these implies that the escape of H+ and O+, together with the estimated escape of neutral hydrogen and oxygen, currently takes place near the stoichometric ratio corresponding to water.

Empirical model of proton fluxes in the equatorial inner magnetosphere: Development

We present an empirical model of the inner magnetosphere, based on proton measurements performed by the charge-energy-mass instrument in the energy range 1-300 keV and in the L range 3-9 during 3 years of the Active Magnetospheric Particle Tracer Explorers/CCE mission (1985 to 1987, i.e., near solar minimum). We have fitted the average proton fluxes at low geomagnetic activity (AE < 100 nT) and obtained a multi parametric function, which is the sum of five different parts and depends on energy, L shell, magnetic local time, and a few independent factors. The model is able to accurately reproduce the average perpendicular proton fluxes in the equatorial inner magnetosphere. The various segments of the model function reproduce the effects of physical processes, such as diffusion, on magnetospheric plasma. We describe the methodology followed for the development of the model. Empirical models like this one are critical in our effort to exploit global neutral atom imaging measurements of ongoing and future missions. Furthermore, they serve as a benchmark of theoretical models and numerical simulations of the inner magnetosphere.

Model calculation of energetic neutral atoms precipitation at low altitudes

It has become apparent during recent years that global magnetospheric imaging through the detection of energetic neutral atoms (ENA) will be a crucial tool for the advancement of magnetospheric research via remote sensing technique. This report presents model calculations of ENA precipitation as detectable from a satellite orbiting at low altitudes (<1500 km). The ENA generation via charge exchange between energetic ions and geocoronal neutrals is simulated in the inner magnetosphere, between 1.5RE and 10RE, where both neutral densities and ion fluxes are significantly high. AMPTE/CCE energetic ion measurements and the geocoronal neutral H altitude profile are used to simulate the charge exchange process leading to ENA production. We discuss the relative contribution of ring current and radiation belts, as well as the relative contribution of H+ and O+ to the ENA generation.

Energetic neutral atoms in the outer magnetosphere: An upper flux limit obtained with the HEP-LD spectrometer on board GEOTAIL

The energetic particle spectrometer HEP-LD on board the Japanese mission GEOTAIL is the first instrument in the energy range above 77 keV which can discriminate between energetic neutral atoms (ENA) and charged particles. The principle for particle identification is based on a time-of-flight (T) and energy (E) measurement which uniquely determines the particle species. A rather compact and highly anisotropic active collimation system in front of the (E,T)-detector head acts as an effective charged particle rejector (CPR) in the ENA mode. The resulting high-pass filter function for charged particles allows the detection of ENAs with energies below 200 keV. This report concentrates on ENA observations in the outer magnetosphere on the dayside (GSE x=−3, y=+12, z=+1 RE) and establishes an upper limit for the ENA flux in quiet conditions. The measurements agree reasonable well with predictions, however, the statistical uncertainties are considerable.

Remote sensing of Mercury's magnetospheric plasma environment via energetic neutral atoms imaging

Abstract The potentiality of exploring the plasma dynamical behavior of Mercury via energetic neutral atom (ENA) analysis is briefly discussed. The analysis of the Hermean ENA would give clues to the global magnetospheric configuration, clarifying crucial issues like the existence of radiation belts. In order to demonstrate the feasibility of ENA imaging in the Hermean environment, an empirical model of the equatorial proton distributions, originally developed for the Earths magnetosphere, is here modified in order to be applicable to the expected conditions, according to the present knowledge of the planet. Simulations of the ENA radiation are then presented and discussed. It is shown that the simulated radiation is significantly high, especially when pointing the field of view towards the Hermean surface. The way of determining the proton flux distributions and the cross-tail electric field characteristics by using the ENA signal is discussed. Finally, a preliminary design of an ENA detector suitable for the ESA mission to Mercury ‘BepiColombo’ is proposed and discussed.

Ring current ion flows and convection electric field as expected from observations by SAC‐B/ISENA

In the present paper, ENA energy spectra at the location at the SAC-B satellite are simulated by using AMPTE/CCE/CHEM ion distributions as originating source, with the purpose of analysing the SACB/ISENA response at different magnetic local times and latitudes. The shape of the ENA spectra simulated along different lines of sight on the equatorial plane is analysed to show that an instantaneous estimate of the global convection electric field will be possible with the ISENA data every few minutes. This unprecedented time resolution for the measurement of such a crucial parameter of the Earths magnetosphere will allow testing of many theories, like tail convection, pressure balance, global circuits, ring-current shielding, etc,

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...

An empirical model of the ion distributions in the equatorial inner magnetosphere

Abstract The ion flux spectra as observed by AMPTE-CCE/CHEM have been averaged over more than two years, and organized as a function of energy, L-shell, magnetic local time and pitch angle. The obtained data set has been best fit with an appropriate multi-parametric function. This preliminary study deals with 90° pitch angle proton distributions, during low geomagnetic activity. The resulting model distributions are presented and compared to the input distributions. The significance of the model parameters with respect to the ring current properties is discussed, with particular attention on the ion flow pattern characteristics.

Empirical Model of the Inner Magnetosphere H+ Pitch Angle Distributions

An empirical model is presented in order to describe the pitch angle distributions of H + particles in inner magnetosphere. The data analysis is based on three-year observations made by the AMPTE/CCE/CHEM instrument in the energy range 1-300 keV and in the L-shell range 3-9, using the average proton fluxes with AE < 100 nT. The model consists of a multi-parametric functional form, that depends on pitch angle, energy, L-shell and a few independent factors. The factors are determined for every magnetic local time. This is the first model, able to accurately reproduce the average proton pitch angle distributions in the whole inner magnetosphere, revealing interesting statistical features. Many of these features have been already evidenced by previous studies and can be explained by processes theoretically interpreted. Furthermore, the model outlines some new features never analyzed before.

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.