F. A. Herrero

THE LOW-ENERGY NEUTRAL ATOM IMAGER FOR IMAGE

The ‘Imager for Magnetosphere-to-Aurora Global Exploration’ (IMAGE) will be launched early in the year 2000. It will be the first mission dedicated to imaging, with the capability to determine how the magnetosphere changes globally in response to solar storm effects in the solar wind, on time scales as short as a few minutes. The low energy neutral atom (LENA) imager uses a new atom-to-negative ion surface conversion technology to image the neutral atom flux and measure its composition (H and O) and energy distribution (10 to 750 eV). LENA uses electrostatic optics techniques for energy (per charge) discrimination and carbon foil time-of-flight techniques for mass discrimination. It has a 90° x 8° field-of-view in 12 pixels, each nominally 8° x 8°. Spacecraft spin provides a total field-of-view of 90° x 360°, comprised of 12 x 45 pixels. LENA is designed to image fast neutral atom fluxes in its energy range, emitted by auroral ionospheres or the sun, or penetrating from the interstellar medium. It will thereby determine how superthermal plasma heating is distributed in space, how and why it varies on short time scales, and how this heating is driven by solar activity as reflected in solar wind conditions.

Thermospheric gravity waves: observations and interpretation using the transfer function model (TFM)

Gravity waves are prominent in the polar region of the terrestiral thermosphere, and can be excited by perturbations in Joule heating and Lorents force due to magnetospheric processes. We show observations from the Dynamics Explorer-2 satellite to illustrate the complexity of the phenomenon and review the transfer function model (TFM) which has guided our interpretation. On a statistical basis, the observed atmospheric perturbations decrease from the poles toward the equator and tend to correlate with the magnetic activity index, Ap, although individual measurements indicate that the magnetic index is often a poor measure of gravity wave excitation. The theoretical models devised to describe gravity waves are multifaceted. On one end are fully analytical, linear models which are based on the work of Hines. On the other end are fully numerical, thermospheric general circulation models (TGCMs) which incorporate non-linear processes and wave mean flow interactions. The transfer function model (TFM) discussed in this paper is between these two approaches. It is less restrictive than the analytical approach and relates the global propagation of gravity waves to their excitation. Compared with TGCMs, the TFM is simplified by its linear approximation; but it is not limited in spatial and temporal resolution, and the TFM describes the wave propagation through the lower atmosphere. Moreover, the TFM is semianalytical which helps in delineating the wave components. Using expansions in terms of spherical harmonics and Fourier components, the transfer function is obtained from numerical height integration. This is time consuming computationally but needs to be done only once. Once such a transfer function is computed, the wave response to arbitrary source distributions on the globe can then be constructed in very short order. In this review, we discuss some numerical experiments performed with the TFM, to study the various wave components excited in the auroral regions which propagate through the thermosphere and lower atmosphere, and to elucidate the properties of realistic source geometries. The model is applied to the interpretation of satellite measurements. Gravity waves observed in the thermosphere of Venus are also discussed.

Satellite and rocket studies of relativistic electrons and their influence on the middle atmosphere

Abstract Magnetospheric electrons from hundreds of keV to over 10MeV in energy have been systematically measured at geostationary altitude (6.6 R E ) for well over a decade. We find evidence of significant diurnal, solar-rotational (27-day), annual, and solar-cycle (11-yr) variations in the fluxes of the relativistic electron component. We have also used low-altitude satellite data and sounding rocket measurements to characterize the location and strength of the relativistic electron precipitation into the atmosphere. We conclude that the magnetospheric electrons, when dumped into the middle atmosphere, represent a very significant ionization source which affects the pattern of conductivity, electric fields, and atmospheric chemistry. These measurements—when combined with global atmospheric modeling—suggest that relativistic electrons provide a robust coupling mechanism to impose long-term solar wind and magnetospheric variability onto the Earths deep atmospheric regions. A strong 11-yr cycle of relativistic electron effects is found in available atmospheric data sets.

Low energy neutral atoms in the magnetosphere

We report observations of low energy neutral atoms (LENA) from the solar wind and the ionosphere, obtained by the LENA Imager on the IMAGE spacecraft. The LENA Imager detects and images LENAs arriving at the spacecraft from within a 90° field of view (8° × 8° pixels), swept through 360° every two minutes by spacecraft spin. Neutral atoms arriving at the sensor are converted to negative ions by a conversion surface. The resulting negative ions are separated in energy (3 bins, 10–250 eV) and arrival direction (±45°). They are then accelerated, detected, and time-of-flight mass analyzed. The solar wind and the ionosphere both emit measurable neutral atom fluxes, the latter responding rapidly to to variations of the former.

Thermosphere and F-region plasma dynamics in the equatorial region

Abstract The dynamics of the equatorial thermosphere and the F-region ionospheric plasma are reviewed highlighting several features observed with in-situ satellite and ground-based experiments. Attention is given to the midnight temperature maximum (MTM) and related phenomena and to recent results on zonal neutral and plasma flows at F-region heights. The midnight temperature maximum and its midnight pressure bulge above 250 km altitude lead to neutral wind variations which significantly affect the F-region equilibrium height and its airglow emissions. During magnetically active periods, enhanced meridional winds from the poles lead to strong meridional intensity gradients (MIGs) in the atomic oxygen emission at 6300A; MIGs have been used to estimate the magnitude of meridional wind gradients during active periods, and these estimates are consistent with measurements using incoherent scatter radar and optical Fabry-Perot interferometry. The pressure gradients which drive the thermospheric wind have been estimated using averaged density and temperature data, and the results have been used to check the consistency of the current data base in terms of the momentum equation. New analyses of the AE-E data are presented as further evidence of the effect of the MTM on the latitude-local time distribution of the meridional wind reversal. The tidal decomposition studies of the neutral temperature and of both ion and neutral flows are reviewed. The zonal plasma flow is found to be closely coupled to the zonal neutral wind as a consequence of the F-region dynamo, and more recently, the F-region dynamo has been found to play an important role in an anomaly in the latitudinal distribution of the equatorial zonal plasma flow.

Local time and altitude variation of equatorial thermosphere midnight density maximum (MDM): San Marco drag balance measurements

We present the first study of the local time and altitude variation of the MDM, the density component of the equatorial midnight pressure bulge, an important feature in the nighttime motions of the ionosphere-thermosphere system in the equatorial region. The neutral density data of the San Marco 3 (SM3) and San Marco 5 (SM5) satellites were averaged to obtain 24 hr density variations from April to December in 1971 and 1988. From these variations, the MDM amplitude as a function of altitude and local time was obtained at altitudes from 220 to 400 km at the geographic equator. We show the first evidence of downward phase propagation of the MDM together with a vertical structure not observed before, which may suggest an ionospheric interaction or perhaps a viscous dissipation effect. In 1971, the MDM propagates rather gradually from about 350 to 220 km in about two hours. In 1988, there appear two regions in which the MDM time shows little change with height with a discontinuous jump in between the two regions. The net effect is still a downward displacement of the MDM with local time. The results show significant seasonal and solar activity effects. During low solar activity periods (1971), the MDM occurs earlier in equinox than in solstice for all altitudes, consistent with the seasonal variation of the midnight temperature maximum (MTM). However, with higher solar activity (1988) and at altitudes below 340 km, the MDM occurs earlier in solstice than in equinox, raising new questions on solar activity effects on the relative phases of the tidal oscillations of the neutral density and the neutral temperature.

Neutral atom imaging mass spectrograph

We describe a concept for an instrument to measure 2-D space plasma distributions by remote sensing of neutral atoms. The instrument measures in one dimension, and from a spinning spacecraft one obtains 2-D (line-of-sight) maps of the neutral flux. Because we want to employ this instrument for measurements in the magnetosphere, the main species of interest are neutral H and O atoms with kinetic energies ranging from about 10 eV to 1 keV. The instrument makes use of a low-work-function surface to convert neutral atoms efficiently to negative ions. The ions are then accelerated away from the surface and brought to an intermediate focus by a large aperture lens. After further acceleration, the ions are deflected by a spherical electrostatic analyzer into a time-of-flight mass spectrometer. Mass resolution of the device is sufficient to resolve H, D, He, and O. Energy and azimuth angle information are obtained by position imaging of the secondary electrons produced at the carbon foil. The large geometric factor combined with simultaneous angle-energy-mass measurement eliminates the need for cycling and provides the necessary high sensitivity for imaging at short time intervals. On a spinning spacecraft this instrument is capable of producing 2-D maps of low-energy neutral atom fluxes.

Evidence for orographic wave heating in the equatorial thermosphere at solar maximum

Fabry Perot interferometer observations of equatorial 630 nm airglow thermospheric winds and temperatures were made at Arequipa, Peru (16.5 S, 71.5 W) during the solar maximum years of 1989 and 1990. Examination of the thermospheric winds measured from Doppler shifts of the 630 nm airglow emission in the two directions east and west of Arequipa shows that the zonal wind is often significantly weaker over the Andes (east) than over the Pacific (west) in winter. Between 19 LT and 24 LT, the typical reductions in speed found were ∼40 to 70 m/s. In the same time period the temperature results show elevated values of ∼200-500 K for the three directions (north, east, and zenith) over mountainous terrain compared with those of the two directions (west and south) over water. This effect fades near equinoctial periods. Examination of the standard deviation of the temperature ohservations for the five directions showed an increase in this value above the measurement error hy a factor of three for the direction toward the Andes (East) but not for the direction over water (South) ; this elevation is consistent with wave activity as a source of the heating. Viscous dissipation of waves propagating into the thermosphere region from below is suggested as an explanation for the localized regions of heating.

Mass spectrograph for imaging low-energy neutral atoms

We describe an instrument concept for measuring low-energy neutral H and O atoms with kinetic energies ranging from about 10 eV to several hundred. The instrument makes use of a low work function surface to convert neutral atoms to negative ions. These ions are then accelerated away from the surface and brought to an intermediate focus by a large aperture lens. After deflection in a spherical electrostatic analyzer, the ions are postaccelerated to ~25-keV final energy into a carbon-foil time-of-flight mass analyzer. Mass resolution is adequate to resolve H, D, He, and O. Energy and azimuth angle information is obtained by means of position imaging the secondary electrons produced at the carbon foil. A large geometric factor combined with simultaneous angle-energy-mass imaging that eliminates the need for duty cycles provide the necessary high sensitivity. From a spinning spacecraft this instrument is capable of producing a 2-D map of low-energy neutral atom fluxes.

Optical interferometric studies of the nighttime equatorial thermosphere: Enhanced temperatures and zonal wind gradients

Fabry-Perot interferometric observations at 630 nm of equatorial thermospheric winds and temperatures in the four cardinal directions and zenith from Arequipa, Peru, during local winter for moderate and high solar fluxes showed elevated temperatures over the Andes Mountains that persisted through the night. The difference in temperature between east and west observations was typically ∼100 to 200 K for moderate flux values and as high as 400 K at solar maximum. Correlated with these localized heating regions were differences in the zonal thermospheric wind of 50 to 70 m/s for observations to the west and to the east of the Arequipa observatory. Also noted in these periods for the region over the Andes was the increased variance of the temperature values above the measurement error. These effects of increased variability and localized heating were not observed at solar minimum. The lack of a significant local time dependence in the diurnal variation of the temperature enhancements suggests that the origin of the heating cannot be related to the coupling of the electrodynamics of the ionosphere to the thermosphere. Instead the hypothesis is advanced that gravity wave energy from the surface penetrates into the thermosphere, where viscous dissipation causes the heating. Such wave activity would also explain the increased variability of the temperatures for the thermosphere regions over mountainous terrain.