C. C. Curtis

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.

Imaging two geomagnetic storms in energetic neutral atoms

The IMAGE mission is the first of its kind. It is designed to comprehensively image a variety of emissions from the Earths magnetosphere with sufficient time resolution to follow the dynamics associated with the development of magnetospheric storms. This paper describes initial observations of two qualitatively different geomagnetic storms by the IMAGE High Energy Neutral Atom imager (HENA). HENA images formed at energies between 10 and 60 keV/nucleon reveal the distribution and the evolution of energetic ion distributions as they are injected into the ring current during geomagnetic storms, drift about the Earth on both open and closed drift paths, and decay through charge exchange to pre-storm levels. Substorm ion injections are also imaged, as are regions of low altitude, high latitude ion precipitation into the upper atmosphere. Two events are discussed: one a major magnetic storm (the “Bastille Day” storm of July 15 and 16, 2000, Dst=−300nT), and the other, a minor storm (June 10, 2000, Dst=−55nT). The larger storm is characterized by ion injection deep into the magnetosphere (L∼3 RE), while the images from the minor storm are consistent with injection to L∼7 RE.

Direct Measurements of Energetic Neutral Hydrogen in the Interplanetary Medium

We present an analysis of hydrogen energetic neutral atoms (ENAs) measured by the ASPERA-3 instrument on boardMars Express. We focus on ENAs that have no Martian origin. The energy spectra of these ENAs are all very similar and can be fitted well by a two-component power law. The fluxes, integrated from 0.2 to 10 keV, vary between 5 ; 10 3 and10 5 cm � 2 sr � 1 s � 1 .Wecheckedfor possiblesourcesfor theseENAs,but wecanruleoutaplanetaryorigin, a solar wind origin, contamination by UV from UV bright stars, and contamination by high-energy protons. With our present knowledge we conclude that the heliospheric termination shock is the most plausible source region. Subject headingg acceleration of particles — instrumentation: detectors — interplanetary medium — methods: data analysis — plasmas — solar wind

Techniques for the remote sensing of space plasma in the heliosphere via energetic neutral atoms - A review

ABSTRACT Remote sensing of space plasma via its neutral component was ushered in by ground observations in 1950. Direct detection of energetic neutral atoms (ENA) at rocket altitudes was first reported in 1969. It is now recognized by the community that charge-exchange between energetic ions of space plasma and ambient neutral atoms is an important process and remote sensing space plasma via ENA should be implemented. In the past decade a host of techniques for direct ENA detection has been proposed and developed, but none of which is presently operating in space. We review some of the techniques for direct detection of ENA between 100 eV and 300 keV for diverse regions ranging from the heliospheric boundary to auroral zones, where the solar wind plays a crucial role.

Imaging extreme ultraviolet photons and energetic neutral atoms: A common approach

Abstract New techniques for studying the dynamics of Earths plasmasphere and magnetosphere from space are at hand. Global images of these regions can be obtained by remote sensors which detect two types of emissions: extreme ultraviolet (EUV) photons and energetic neutral atoms (ENA). Efficient imagers for EUV and ENA share several important features, including a wide field of view, high throughput, the absence of transmitting optics, appropriate time and spatial resolution, and similar detectors. We have built and tested prototypes of imagers whose designs recognize these common requirements. Coded apertures replace lenses to provide the needed spatial resolution. A special class of aperture designs permits artifact‐free reconstruction (limited only by noise) of any source region within the field of view, even if the source overfills the field of view. These designs also provide signal to noise ratios superior to those of a single‐aperture imager in most applications contemplated here. We have evaluated...

Proposal of an Italian experiment for the mission SAC-B. ISENA: Imaging particle spectrometer for energetic neutral atoms

We propose to perform in situ measurements of precipitating and escaping energetic neutral atoms (ENA) of energies between approximately 5 and 200 keV. The interface characteristics of this new type of instrument, named ISENA (Imaging particle Spectrometer for Energetic Neutral Atoms), are consistent with the SAC-B spacecraft specifications, so that it could be included in its scientific payload. The main technical properties of this experiment are here briefly described.

Aperture codes for sensors viewing extended objects from space

In a companion paper, Curtis, et. al. discuss coded aperture sensors appropriate for viewing an extended object such as Earths magnetosphere seen at relatively close range. The aperture codes used may be different from those generally found in X or (gamma) ray telescopes, where the field of view (FOV) may encompass an entire region containing some small number of point sources. The aperture codes described here find application in imaging an extended object which may have relatively low contrast, and whose lateral limits extend beyond the FOV of the sensor. Those elements of an extended object lying near the FOV limits are only partially coded, that is, flux from those elements cannot cast a shadow of the entire aperture code onto the detector, as can elements near the center of the FOV. This has consequences for the algorithms used to reconstruct the image. The object field is divided into a number of elements which is smaller than the number of detector pixels, and a least squares fit to the data is performed. A discussion of the methods used for choosing the matrices representing the aperture codes is given, and computer simulations of the effects of noise are described.