Ground‐based optical and digital ionosonde measurements were conducted at Thule, Greenland to measure ionospheric structure and dynamics in the nighttime polar cap F layer. These observations showed the existence of large‐scale (800–1000 km) plasma patches drifting in the antisunward direction during a moderately disturbed (Kp ≥ 4) period. Simultaneous Dynamics Explorer (DE‐B) low‐altitude plasma instrument (LAPI) measurements show that these patches with peak densities of ∼106 el cm−3 are not locally produced by structured particle precipitation. The LAPI measurements show a uniform precipitation of polar rain electrons over the polar cap. The combined measurements provide a comprehensive description of patch structure and dynamics. They are produced near or equatorward of the dayside auroral zone and convect across the polar cap in the antisunward direction. Gradients within the large scale, drifting patches are subject to structuring by convective instabilities. UHF scintillation and spaced receiver measurements are used to map the resulting irregularity distribution within the patches.

Satellite in situ measurements made by the Dynamics Explorer 2 (DE 2) satellite were utilized to describe the nature of plasma structuring at high latitudes caused by the gradient drift instability process. Specifically, by using noon‐midnight and dawn‐dusk orbits of the DE 2 satellite it was found possible to study the simultaneous density and electric field spectra of convecting large‐scale (approximately hundreds of kilometers) plasma density enhancements in the polar cap known as “patches”) in directions parallel and perpendicular to their antisunward convection. Distinct differences were noted in the behavior of the ac and dc electric field structure and short‐scale (<125 m) density irregularities in these two mutually orthogonal directions perpendicular to the geomagnetic field. However, since these two orthogonal directions were not sampled simultaneously, the observed differences cannot be unequivocally related to the direction of convection. Structured plasma density enhancements in the auroral oval (known as “blobs”) were found to have considerable power spectral density at these short scales in the presence of significant Pedersen and Hall conductances in the 10‐ to 20‐mho range. While density irregularity amplitudes (ΔN/N)rms were found to be as large as 15–20% using 8‐s samples of the DE 2 data, the corresponding dc electric field fluctuation ΔE was found to be less than a few millivolts per meter for both patches and blobs. This (ΔN/N)RMS vis‐a‐vis ΔE behavior for the gradient drift process provided a fairly dramatic contrast with velocity shear driven processes where the ΔE magnitudes were found to be at least an order of magnitude larger for the same levels of density irregularities. The electric field spectra for the moderate shear category discussed by Basu et al. (1988a) were also found to have a significantly different spectral index as compared to such spectra associated with the gradient drift process. The results of this paper together with those of Basu et al. (1988a) provide fairly conclusive evidence for the existence of at least two generic classes of instabilities operating in the high‐latitude ionosphere: one driven by large‐scale density gradients in a homogeneous convection field with respect to the neutrals and the other driven by the structured convection field itself in an ambient ionosphere where density fluctuations are ubiquitous.

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.

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The general scientific objective of the ASPERA-3 experiment is to study the solar windatmosphere interaction and to characterize the plasma and neutral gas environment within the space near Mars through the use of energetic neutral atom (ENA) imaging and measuring local ion and electron plasma. The ASPERA-3 instrument comprises four sensors: two ENA sensors, one electron spectrometer, and one ion spectrometer. The Neutral Particle Imager (NPI) provides measurements of the integral ENA flux (0.1-60 keV) with no mass and energy resolution, but high angular resolution. The measurement principle is based on registering products (secondary ions, sputtered neutrals, reflected neutrals) of the ENA interaction with a graphite-coated surface. The Neutral Particle Detector (NPD) provides measurements of the ENA flux, resolving velocity (the hydrogen energy range is 0.1-10 keV) and mass (H and O) with a coarse angular resolution. The measurement principle is based on the surface reflection technique. The Electron Spectrometer (ELS) is a standard top-hat electrostatic analyzer in a very compact design which covers the energy range 0.01-20 keV. These three sensors are located on a scanning platform which provides scanning through 180 • of rotation. The instrument also contains an ion mass analyzer (IMA). Mechanically IMA is a separate unit connected by a cable to