The magnetic equator provides a unique location for thermal plasma and plasma wave measurements. Plasma populations are found to be confined within a few degrees latitude of the equator, particularly the ions. The equatorially trapped ion population is found to be primarily hydrogen, and we find little evidence for preferential heating of heavier ions. Helium is occasionally found to be heated along with the protons, and forms about 10% of the equatorially trapped populations at such times, similar to the percentage of He+ in the cold, core plasma of the plasmasphere. One case of a heated O+ component was found; at the 0.1% level it generally comprises in the outer plasmasphere core plasma. The heated H+ ions can be characterized by a bi‐Maxwellian with kT∥ = 0.5–1.0 eV, and kT = 5–50 eV, with a density of 10–100 cm−3. The total plasma density, as inferred from the plasma wave instrument measurements of the upper hybrid resonance (UHR), is relatively constant with latitude, occasionally showing a local minimum at the magnetic equator, even though the ion flux has increased substantially. The first measurements of the equatorially trapped plasma and coincident UHR measurements show that the trapped plasma is a feature of the plasmapause region, found at total plasma densities of 20–200 cm−3. The warm, trapped plasma is found in conjunction with equatorial noise, a plasma wave feature found at frequencies near 100 Hz, with a broad spectrum generally found between the proton gyrofrequency at the low frequency edge and the geometric mean gyrofrequency at the high frequency edge. This latter frequency is generally the lower hybrid resonance (LHR) for a proton‐electron plasma. Sharp spatial boundaries are occasionally found with latitude, delimiting the equatorially trapped plasma. In such cases, the equator is a region with a relative minimum in density, and it appears that field‐aligned ions found at higher latitudes are “bounced” from these boundaries, indicating a positive plasma potential of a volt or two in the equatorial region.

Measurements from the Retarding Ion Mass Spectrometer (RIMS) on Dynamics Explorer (DE) have, for the first time, revealed a supersonic polar wind (Mach number is greater than 1) along polar cap field lines. The observations reported were obtained on the nightside (22:30 to 23:30 MLT) from 65° to 81° invariant latitude and at altitudes near 2 RE. Fitting the data using a thin‐sheath model gives a range of temperatures of 0.1 to 0.2 eV with corresponding flow velocities of 25 to 16 km s−1 over the estimated range of spacecraft potential of +3 to +5 V. For these values the Mach number ranged from 5.1 to 2.6 (with a most likely value of 3). Characteristics of the H+ flow are in general agreement with those predicted by "classical" polar wind theory, but high variability of the He+/H+ ratio was observed.

Abstract. Electrostatic charging of a spacecraft modifies the distribution of electrons and ions before the particles enter the sensors mounted on the spacecraft body. The floating potential of magnetospheric satellites in sunlight very often reaches several tens of volts, making measurements of the cold (several eV) component of the ambient ions impossible. The plasma electron data become contaminated by large fluxes of photoelectrons attracted back into the sensors.The Cluster spacecraft are equipped with emitters of the liquid metal ion source type, producing indium ions at 5 to 9 keV energy at currents of some tens of microampere. This current shifts the equilibrium potential of the spacecraft to moderately positive values. The design and principles of the operation of the instrument for active spacecraft potential control (ASPOC) are presented in detail.Experience with spacecraft potential control from the commissioning phase and the first two months of the operational phase are now available. The instrument is operated with constant ion current for most of the time, but tests have been carried out with varying currents and a "feedback" mode with the instrument EFW, which measures the spacecraft potential . That has been reduced to values according to expectations. In addition, the low energy electron measurements show substantially reduced fluxes of photoelectrons as expected. The flux decrease in photoelectrons returning to the spacecraft, however, occurs at the expense of an enCorrespondence to: K. Torkar (klaus.torkar@oeaw.ac.at) larged sheath around the spacecraft which causes problems for boom-mounted probes.

Plasma observations made at 5.5R E. tric field experiment, and then briefly describby the USCD detectors on the SCATHA satellite ing the pertinent characteristics of the magnereveal the existence of a thermal plasma popula-tometer and mass spectrometers. Following the tion trapped within a few degrees latitude of the instrument descriptions, two events are described magnetic equator, defined here as the minimum B in detail. This is followed by a brief destripsurface.These measurements were restricted to tion of the general nature of the equatorial the 1000 to 2000 LT sector by the spacecraft or-observations on SCATHA and the discussion. bit.The ions show a higher degree of anisotropy than the electrons, with a FWHM of 10" to 25", Spacecraft and Instruments narrowing with increasing energy. The electron distribution shows a width of 20" to 60", again The Air Force P78-2 (SCATHA) satellite was narrowing with increasing energy. The 20-to launched on January 30, 1979, reaching its final, 100-eV ion fluxes typically show temperatures in near geosynchronous, orbit on February 7, 1979. the320-to 50-eV range, and densities of 1-10 The cylindrical spacecraft rotates at a relativecm ß The electron population typically extends ly slow 1 rpm about an axis lying in the orbital from 50 to 500 eV, with temperatures of 100-200 plane and nominally perpendicular to the earth-eV and densities also in the 1-to 10-ca -3 range. sun line. Perigee was at 5.3 R E , apogee at 7.8 Lower energy field-aligned populations are R E , and the orbital plane was inclined by about occasionally found in both ions and electrons at 8". The satellite drifts eastward about 5.

Abstract-A new pseudocolor mapping strategy for use with spectral imagery is presented. This strategy is based on a principal components analysis of spectral data, and capitalizes on the similarities between three-color human vision and high-dimensional hyperspectral datasets. The mapping is closely related to three-dimensional versions of scatter plots that are commonly used in remote sensing to visualize the data cloud. The transformation results in final images where the color assigned to each pixel is solely determined by the position within the data cloud. Materials with similar spectral characteristics are presented in similar hues, and basic classification and clustering decisions can be made by the observer. Final images tend to have large regions of desaturated pixels that make the image more readily interpretable. The data cloud is shown here to be conical in nature, and materials with common spectral signatures radiate from the origin of the cone, which is not (in general) at the origin of the spectral data. A supervised method for locating the origin of the cone based on identification of clusters in the data is presented, and the effects of proper origin orientation are illustrated.Index Terms-Hyperspectral imagery, multidimensional imagery display, spectral imagery.