The main trough in the topside ionosphere has been studied using the thermal positive ion and electron densities measured on 15,000 orbits over a 3-year period (1969)(1970)(1971)(1972) by means of spherical electrostatic analyzers aboard the Isis I and Injun 5 satellites in the 560-to 3600-km altitude range. The trough is found to be a persistent feature at night with an occurrence frequency of approximately 95%. The occurrence frequency decreases to approximately 60% near the dawn-dusk meridian and to approximately 48% near local noon. At altitudes below about 1500 km during quiet to moderate conditions (Kp _< 3o) the trough equatorward boundary is found at L = 3.5 + 0.5 near midnight and L = 12.5 + 1.0 near local noon. The equatorward edge of the trough in the nighttime sector lies near the L shell where the plasmapause has been observed. Near local noon the trough occurs at the equatorward edge of the magnetospheric cleft and is at significantly higher L values than those reported for the dayside plasmapause. The seasonal variation of the trough location at a given local time is negligible except near sunrise. With increasing altitude between 1500 and 3600 km the equatorial boundary of the trough moves to continually lower latitudes during the night hours. The equatorward trough wall becomes a dominant feature of the trough, often extending from 15 ø to 20 ø in width during quiet magnetic periods. The poleward edge of the trough becomes less well marked with increasing altitude, often being defined only by a sharp spike in ionization extending over a few degrees within the auroral zone. At altitudes above 1500 km on the dayside, two independent troughs or density gradients are observed. The trough at high latitudes located at the equatorward edge of the cusp gradually decreases in amplitude with increasing altitude. This trough is tentatively identified as the dayside analog of the high-latitude trough observed on the nightside by Ogo 6. Its development is attributed to the depleting effects of enhanced ion chemical reactions in the presence of convective electric fields. The lower latitude dayside trough, observed between L = 2 and 6, results from the partial depletion of flux tubes in the outer plasmasphere.

Thermal ion density irregularities at high latitudes have been investigated by using results from the spherical electrostatic analyzer flown on the polar‐orbiting satellite Isis 1. The size of the irregularities is found to be highly variable, dimensions typically ranging from 1 to 140 km. Results from 3500 orbits in the top side ionosphere during the first year of operation have been examined. The daily and seasonal behavior of the equatorial boundary of the high‐latitude irregularity zone and its dependence on magnetic activity have been established for the northern and southern hemispheres. The daily variation in the location of the boundary of the irregularity zone is found to be 14°±2° for any level of magnetic activity; the boundary is at the highest latitude near local noon. The irregularities consistently extend across the pole; there is no upper boundary. Systematic differences are found in the mean location of the equatorial boundary in the northern and southern hemispheres. In the south the boundary is on the average 4.8°±0.6° closer to the pole than in the north. Good agreement is found between the direct measurements of the irregularity boundary and scintillations, severe top side spread F and ≤300‐eV electron precipitation boundaries, and trough and plasmapause positions on the night side. Significant differences found to exist between the irregularity boundary and plasmapause positions on the day side are considered to be due to their different origins.

Through the use of a reductive perturbation technique, solitary kinetic Alfvén waves (KAWs) are investigated in a low but finite b (particle-to-magnetic pressure ratio) dense electron-positron-ion plasma where electrons and positrons are degenerate. The degenerate plasma model considered here permits the existence of sub-Alfvénic compressive solitary KAWs. The influence of r (equilibrium positron-to-ion density ratio), s F (electron-to-positron Fermi temperature ratio), b and obliqueness parameter l z on various characteristics of solitary KAWs are examined through numerical plots. We have shown that there exists a critical value of l z at which a soliton width attains its maximum value which decreases with an increase in r and s. F It is also found that solitons with a higher energy propagate more obliquely in the direction of an ambient magnetic field. The results of the present investigation may be useful for understanding low frequency nonlinear electromagnetic wave propagation in magnetized electron-positron-ion plasmas in dense stars. Specifically, the relevance of our investigation to a pulsar magnetosphere is emphasized.

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Thermal positive ion densities measured with a spherical electrostatic analyzer aboard Ogo I on approximately 25 orbits between September and December 1964 have been studied. The charged-particle distributions exhibit a highly variable character within and beyond the plasmapause boundary. The presence of the plasmapause, though quite evident in most cases, is not obvious in some, whereas on a few orbits more than one steep density decrease is observed. The density gradient at the plasmapause varies between 3 X 10 -• and 6.0 ions/cm3/km. The magnitude of the density gradient is inversely related to the plasmapause L position and is consistently about an order of magnitude higher for the afternoon sector orbits than for the nightside orbits. Within the plasmasphere irregularities are observed on the density distributions in the form of troughs and humps, whereas beyond the plasmapause the irregularities exhibit 'wavelike' structures with wavelengths in the range of 0.1-0.3 L. The amplitude of these fluctuations is most commonly about a factor of 2 relative to background magnetospheric density; however, excursions from 20% to a factor of 20 are observed. These irregularities do not show any correlation with the level of magnetic activity. A delay of 3-9 hours is found in the movement of the plasmapause following an increase in the level of magnetic activity on the nightside. Similar studies in the afternoon sector show that the plasmapause location correlates best with the magnetic activity that existed when the sampled plasma was in the formative nightside region rather than with the magnetic activity immediately preceding the plasmapause measurement. Direct measurements of the spatial distribution, temperature, and energy spectrum of positive thermal ions in the magnetosphere have been made by a spherical electrostatic analyzer (SEA) on Ogo 1. Results of charged-particle experiments employing widely differing techniques on the same satellite [Taylor et al., 1965; Brinton et al., 1968; Vasyliunas, 1968] and on other satellites [Gringauz, 1963; Taylor et al., 1968; Binsack, 1967; Serbu and Mater, 1966] have been used to map the spatial distributions of thermal-and low-energy particles under conditions of varying magnetic activity. The results show that the particle concentration decreases gradually with altitude Within the plasmasphere until the outer edge, 'the plasmapause,' is reached. At the plasmapause the charged-particle concentration shows a steep drop of an order of magnitude or higher. The satellite measurements of charged-particle distributions are very similar to the equatorial electron-density profiles resulting from whistler studies by Carpenter [1963]. The plasmapause is identified with the knee in the electron-density profile [Taylor et al., 1965]. Satellite results are still very limited and are sometimes conflicting. Further, the physical processes responsible for the production, movement, and maintenance of the plasmapause boundary are not yet fully understood.