Heliosphere-Geosphere Interactions Using Low Energy Neutral Atom Imaging

Development of the low energy neutral atom (LENA) imager was originally motivated by a need to remotely sense plasma heating in the topside ionosphere, with the goal of greatly enhanced temporal resolution of an otherwise familiar phenomenon. During ground test and calibration, the LENA imager was found to respond to neutral atoms with energies well above its nominal energy range of 10–750 eV, up to at least 3–4 keV, owing to sputtering interactions with its conversion surface. On orbit, LENA has been found to respond to a ubiquitous neutral atom component of the solar wind, to the neutral atoms formed by magnetosheath interactions with the geocorona during periods of high solar wind pressure, and to the interstellar neutral atoms flowing through the heliosphere during the season of maximal relative wind velocity between spacecraft and interstellar medium. LENA imaging has thus emerged as a promising new tool for studying the interplanetary medium and its interaction with the magnetosphere, in addition to the ionospheric heating and outflow that result from this interaction. LENA emissions from the ionosphere consist of a fast component that can be observed at high altitudes, and slower components that evidently create a quasi-trapped extended superthermal exosphere. The more energetic emissions are responsive to solar wind energy inputs on time scales of a few minutes.     

Plasma transfer processes at the magnetopause

Space Science Reviews -     

Polar/TIMAS statistical results on the outflow of molecular ions from earth at solar minimum

Ion composition data from the first 22 months of operation of the Polar/TIMAS instrument, covering the 15-eV/e to 33-keV/e energy range, have been surveyed to determine the typical abundance, at solar minimum, of N₂⁺, NO⁺ and O₂⁺ ions in the auroral ion outflow, as compared to that of the better known O⁺ ions. The results indicate that molecular ions have roughly the same energy distribution as the O⁺ ions, with maximum differential flux occurring below 400 eV, but are far less abundant, by two orders of magnitude. The molecular ions also differ from the O⁺ ions in that they seem more specifically associated with enhanced geomagnetic activity.     

Source and loss processes in the magnetotail

Space Science Reviews -     

THE CLUSTER ION SPECTROMETRY (CIS) EXPERIMENT

The Cluster Ion Spectrometry (CIS) experiment is a comprehensive ionic plasma spectrometry package on-board the four Cluster spacecraft capable of obtaining full three-dimensional ion distributions with good time resolution (one spacecraft spin) with mass per charge composition determination. The requirements to cover the scientific objectives cannot be met with a single instrument. The CIS package therefore consists of two different instruments, a Hot Ion Analyser (HIA) and a time-of-flight ion COmposition and DIstribution Function analyser (CODIF), plus a sophisticated dual-processor-based instrument-control and Data-Processing System (DPS), which permits extensive on-board data-processing. Both analysers use symmetric optics resulting in continuous, uniform, and well-characterised phase space coverage. CODIF measures the distributions of the major ions (H⁺, He⁺, He⁺⁺, and O⁺) with energies from ~0 to 40 keV/e with medium (22.5°) angular resolution and two different sensitivities. HIA does not offer mass resolution but, also having two different sensitivities, increases the dynamic range, and has an angular resolution capability (5.6° × 5.6°) adequate for ion-beam and solar-wind measurements.     

The Toroidal Imaging Mass-Angle Spectrograph (TIMAS) for the polar mission

The science objectives of the Toroidal Imaging Mass-Angle Spectrograph (TIMAS) are to investigate the transfer of solar wind energy and momentum to the magnetosphere, the interaction between the magnetosphere and the ionosphere, the transport processes that distribute plasma and energy throughout the magnetosphere, and the interactions that occur as plasma of different origins and histories mix and interact. In order to meet these objectives the TIMAS instrument measures virtually the full three-dimensional velocity distribution functions of all major magnetospheric ion species with one-half spin period time resolution. The TIMAS is a first-order double focusing (angle and energy), imaging spectrograph that simultaneously measures all mass per charge components from 1 AMU e^–1 to greater than 32 AMU e^–1 over a nearly 360° by 10° instantaneous field-of-view. Mass per charge is dispersed radially on an annular microchannel plate detector and the azimuthal position on the detector is a map of the instantaneous 360° field of view. With the rotation of the spacecraft, the TIMAS sweeps out very nearly a 4 solid angle image in a half spin period. The energy per charge range from 15 eV e^–1 to 32 keV e^–1 is covered in 28 non-contiguous steps spaced approximately logarithmically with adjacent steps separated by about 30%. Each energy step is sampled for approximately 20 ms;14 step (odd or even) energy sweeps are completed 16 times per spin. In order to handle the large volume of data within the telemetry limitations the distributions are compressed to varying degrees in angle and energy, log-count compressed and then further compressed by a lossless technique. This data processing task is supported by two SA3300 microprocessors. The voltages (up to 5 kV) for the tandem toroidal electrostatic analyzers and preacceleration sections are supplied from fixed high voltage supplies using optically controlled series-shunt regulators.     

Relative contributions of terrestrial and solar wind ions in the plasma sheet

A major uncertainty concerning the origins of plasma sheet ions is due to the fact that terrestrial H⁺ can have similar fluxes and energies as H⁺ from the solar wind. The situation is especially ambiguous during magnetically quiet conditions (AE < 60γ) when H⁺ typically contributes more than 90% of the plasma sheet ion population. In this study we examine that problem using a large data set obtained by the ISEE-1 Plasma Composition Experiment. The data suggest that one component of the H⁺ increases in energy with increasing activity, roughly in proportion to $\text{1}\text{4}$ the energy of the He⁺⁺, whereas the other H⁺ component has about the same energy at all activity levels, as do the O⁺ and the He⁺. If we can assume that the H⁺ of solar wind origin on the average has about the same energy-per-nucleon as the He⁺⁺, which is presumably almost entirely from the solar wind, then the data imply that as much as 20–30% of the H⁺ can be of terrestrial origin even during quiet conditions.     

ISEE Ion Composition Data with Implications for Solar Wind Entry into Earth's Magnetotail

Energetic (0.1-16 keV/e) ion data from a plasma composition experiment on the ISEE-1 spacecraft show that Earth's plasma sheet (inside of 23 RE) always has a large population of H+ and He++ ions, the two principal ionic components of the solar wind. This population is the largest, in terms of both number density and spatial thickness, during extended periods of northward interplanetary magnetic field (IMF) and is then also the most "solar wind-like" in the sense that the He++/H+ density ratio is at its peak (about 3% on average in 1978 and 79) and the H+ and He++ have mean (thermal) energies that are in the ratio of about 1:4 and barely exceed the typical bulk flow energy in the solar wind. During geomagnetically active times, associated with southward turnings of the IMF, the H+ and He++ are heated in the central plasma sheet, and reduced in density. Even when the IMF is southward, these ions can be found with lower solar wind-like energies closer to the tail lobes, at least during plasma sheet thinning in the early phase of substorms, when they are often seen to flow tailward, approximately along the magnetic field, at a slow to moderate speed (of order 100 km s-1 or less). These tailward flows, combined with the large density and generally solar wind-like energies of plasma sheet H+ and He++ ions during times of northward IMF, are interpreted to mean that the solar wind enters along the tail flanks, in a region between the lobes and the central plasma sheet, propelled inward by ExB drift associated with the electric fringe field of the low latitude magnetopause boundary layer (LLBL). In order to complete this scenario, it is argued that the rapid (of order 1000 km s-1) earthward ion flows (mostly H+ ions), also along the magnetic field, that are more typically the precursors of plasma sheet "recovery" during substorm expansion, are not proof of solar wind entry in the distant tail, but may instead be a time-of-flight effect associated with plasma sheet redistribution in a dipolarizing magnetic field.