We present measurements of electron temperature Te made by a retarding potential analyzer and a Langmuir probe (both flush‐mounted on the spin‐stabilized satellite Explorer 31) to investigate the variation of Te around the satellite. Most of the time there is a Te variation, which repeats for given ionospheric conditions. The variation is strongly controlled by the angle between the velocity vector and the probe normal, Te usually being enhanced in the near wake of the satellite. Magnetic field control of Te, if it is present, is hidden by the stronger velocity vector control. Our results indicate that the magnitude of the Te enhancement in the wake does not depend on the average ion mass M, although the electron density depletion in the wake is strongly correlated with M.

Direct measurement of the densities of ionic constituents (H+, He+, and O+) and the temperatures of ions and electrons have been obtained from the Ogo 4 planar retarding potential analyzer in the altitude range 400–900 km. Results are presented from day and night passes in the middle and low latitudes near the 1967 fall equinox. The passes are selected to empasize the latitudinal rather than the height dependence of the measurements. The main results can be summarized as follows: (1) Above 800 km at night, there is a deep equatorial trough in He+ and a corresponding rise in O+, suggesting a charge exchange between He+ and O as an important loss mechanism for He+. (2) The dominant ion in the night at these altitudes between ±40° geomagnetic latitudes is H+ followed generally by O+ and He+. Outside this latitude region O+ becomes the dominant constituent, increasing continuously toward the pole. (3) The major ionic constituent in the daytime is O+ throughout the altitude and latitude range of observations. In the height range 400–500 km, the latitudinal variation in O+ shows the well‐known feature of the geomagnetic anomaly. (4) Both electron and ion temperatures generally increase poleward from their low latitude values, attaining maxima between 40 and 50° geomagnetic latitude.

Passive microwave measurements of sea ice were made at 14, 19, 31, and 90 GHz in a series of aircraft flights over the Greenland Sea in April 1977. Brightness temperatures and emissivities are computed for four types of ice: multiyear (MY), first year (FY), young, and new. The results show that (1) emissivity, defined as the ratio of the measured brightness temperature to the physical temperature of the ice surface, is a more fundamental ice parameter and exhibits less sample to sample variation than the brightness temperature; (2) the emissivity of MY ice decreases between 14 and 31 GHz, but changes little or may even increase between 31 and 90 GHz; (3) the emissivity of FY ice is generally 0.95 or greater at all frequencies, except for an occasional decrease at 90 GHz, which may be due to weathering of the FY ice; and (4) the emissivities of young ice and new ice are less than the emissivity of FY ice.

This paper describes the phenomenon of the midlatitude red arc of September 29, 1967, through observations of the properties of the ionospheric plasma. The ion and electron temperatures, ion composition and density, and suprathermal electron flux during this period are measured by retarding potential analyzers near 900 km from Ogo 4 and near 2000 kra from Explorer 31. These parameters show the following changes in the region L • 2.3 to 3.0 during the red arc period, as compared with their values during normal periods: (1) Electron and ion temperatures increase to above 4000øK from a normal value of 2000øK at 900 km, while at 2000 km electron temperature increases to above 5000øK from a normal value of 2500øK. (2) At 900 km the ratio of O*/(H* • He*) changes from I to 5, while the total density remains approximately the same. (3) At 2000 km the ion density decreases by a factor of 10 with the composition remaining all H*. (4) There is no significant increase in the flux of 5-to 10-ev electrons. The relative importance of electric field heating, magnetospheric conduction, and the changes in the neutral composition in the lower atmosphere are examined in the light of these observations. It is concluded that the subauroral red arc is caused by a combination of thermal conduction of energy from the magnetosphere and changes in the neutral compositions in the lower atmosphere. Recent measurements of electron temperatures in the region of subauroral red arcs have given direct support to the concept that the arc is produced primarily by the thermal electron excitation of atomic oxygen [Norton and Findlay, 1969]. The mechanism exciting the arc is therefore related to the mechanism he.ating the ambient plasma. In recent years, a number of models for heating the electron gas have been proposed. These are (1) adc electric field [Megill et al., 1963]; (2) a precipitating flux of low-energy electrons [Dalgarno, 1964]; (3) the thermal conduction of energy from the magnetosphere in the ionosphere along the geomagnetic field lines [Cole, 1965, 1970]. Walker and Rees [1968] investigated the relative merits of the three processes in raising electron temperature to the point of exciting the 600-R red arc of June 21, 1961. They concluded that all satisfied the necessary criteria so far as ability to heat the ambient plasma was concerned.

Ogo 4 observations of the O I (6300‐A) emissions have revealed a global pattern hitherto undetected from the ground‐based observations. It is seen that the postsunset emission of O I (6300 A) in October 1967 is very asymmetrical with respect to the geomagnetic equator in certain longitude regions and shows poor correlation with the electron density measured simultaneously from the same spacecraft. This asymmetry is less marked in the UV airglow, O I (1356 A), which appears to vary as the square of the maximum electron density in the F region. The horizon scan data of the 6300‐A airglow reveal that the latitudinal asymmetry is associated with asymmetry in the height of the O I (6300‐A) emission and hence with the altitude of the F2 peak. From the correlative studies of the airglow and the ionospheric measurements the mechanisms for the UV and the 6300‐A emissions are discussed in terms of the processes involving radiative and dissociative recombination. Theoretical expressions are developed relating the airglow data to the ionospheric parameters, and it is shown that the agreement between observed and calculated emission rates is well within the uncertainty of the measurements.