Anna Milillo

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.

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‐altitude energetic neutral atoms imaging of the inner magnetosphere: A geometrical method to identify the energetic neutral atoms contributions from different magnetospheric regions

Energetic neutral atoms (ENA) imaging is an innovative technique of global magnetospheric observations, useful for studying geomagnetic storm and substorm processes. An intrinsic limitation of this technique is the difficulty of spatial distinction of the various magnetospheric regions contributing to the generation of the detected ENA fluxes. Here we apply a geometrical method able to distinguish the contribution to the ENA fluxes given by the inner radiation belt ions and by the ring current ion distributions. The ENA energy spectra are simulated by using the Active Magnetospheric Particle Tracer Explorers CCE charge-energy-mass energetic ion distributions, averaged over 2 years. The calculations are made for a specific vantage point (altitude is 550 km, MLT is 1930 and magnetic latitude is 0°); however, the technique can be easily extended to all locations on the equatorial plane, at similar altitudes. The simulated ENA spectra, taken at +30° and at −30° with respect to the midnight direction are used to reconstruct the inner radiation belt/ring current ENA spectra integrated along a general radial direction within the considered angular interval. For estimating the two contributions, we assume that the ion distributions are isotropic (within 17%) over the 4-hour LT interval considered and that the functional form of the inner radiation belt ion distributions versus distance is not dependent on energy nor geomagnetic activity. The results show that there is a good agreement (within ∼5%) between the simulated ENA spectra along the radial direction (separately generated in the inner radiation belt and in the ring current) and those reconstructed through the proposed method. This technique can be applied to the real data for a zero-order analysis.

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.

Structure and dynamics of the proton energy density in the inner magnetosphere

Abstract The proton energy density distribution near the equatorial plane in the inner magnetosphere was statistically investigated by utilizing data from the Polar/MICS instrument. Results show that the proton energy density depends on the solar wind velocity and IMF Bz, and that during a storm main phase the energy density increases at midnight while the energy density decreases slightly or remains constant at noon. Three possible hypotheses are discussed to find a primary source of the energy density variation observed. After comparing with results of a numerical simulation, on the average, the convection hypothesis is thought to be reasonable to account for the storm-time enhancement of the energy density as well as the local time asymmetry of it. The local time asymmetry during the storm main phase is hardly produced during the storm that substorm-associated transport or diffusive transport dominates the convective transport. The diffusive or substorm-associated transport is likely needed to account for the “background” population which consists of high energy protons (more than hundreds of keV), but is expected to be minor in the storm-time enhancement of the energy density because on the average, the “background” population tends to remain rigidly throughout a storm.

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.