This is the first monograph to describe the historical development of ideas concerning the plasmasphere by the pioneering researchers themselves. The plasmasphere is a cold thermal plasma cloud encircling the Earth, terminating abruptly at a radial distance of 30,000 km over a sharp discontinuity known as the plasmapause. The volume commences with an account of the difficulties met in USSR by Gringauz to publish his early discoveries from Soviet rocket measurements, and the contemporaneous breakthroughs by Carpenter in the USA from ground-based whistler measurements. The authors then update our picture of the plasmasphere by presenting experimental and observational results of the past three decades, and mathematical and physical theories proposed to explain its formation. The volume will be invaluable for researchers in space physics, and will also appeal to those interested in the history of science.

Abstract.As an inner magnetospheric phenomenon the plasmapause region is of interest for a number of reasons, one being the occurrence there of geophysically important interactions between the plasmas of the hot plasma sheet and of the cool plasmasphere. There is a need for a conceptual framework within which to examine and discuss these interactions and their consequences, and we therefore suggest that the plasmapause region be called the Plasmasphere Boundary Layer, or PBL. Such a term has been slow to emerge because of the complexity and variability of the plasma populations that can exist near the plasmapause and because of the variety of criteria used to identify the plasmapause in experimental data. Furthermore, and quite importantly in our view, a substantial obstacle to the consideration of the plasmapause region as a boundary layer has been the longstanding tendency of textbooks on space physics to limit introductory material on the plasmapause phenomenon to zeroth order descriptions in terms of ideal MHD theory, thus implying that the plasmasphere is relatively well understood. A textbook may introduce the concept of shielding of the inner magnetosphere from perturbing convection electric fields, but attention is not usually paid to the variety of physical processes reported to occur in the PBL, such as heating, instabilities, and fast longitudinal flows, processes which must play roles in plasmasphere dynamics in concert with the flow regimes associated with the major dynamo sources of electric fields. We believe that through the use of the PBL concept in future textbook discussions of the plasmasphere and in scientific communications, much progress can be made on longstanding questions about the physics involved in the formation of the plasmapause and in the cycles of erosion and recovery of the plasmasphere.

Data acquired during several multiday periods in 1982 at ground stations Siple, Halley, and Kerguelen and on satellites DE 1, ISEE 1, and GEOS 2 have been used to investigate thermal plasma structure and dynamics in the duskside plasmasphere bulge region of the Earth. The distribution of thermal plasma in the dusk bulge sector is difficult to describe realistically, in part because of the time integral manner in which the thermal plasma distribution depends upon the effects of bulk cross‐B flow and interchange plasma flows along B. While relatively simple MHD models can be useful for qualitatively predicting certain effects of enhanced convection on a quiet plasmasphere, such as an initial sunward entrainment of the outer regions, they are of limited value in predicting the duskside thermal plasma structures that are observed. Furthermore, use of such models can be misleading if one fails to realize that they do not address the question of the formation of the steep plasmapause profile or provide for a possible role of instabilities or other irreversible processes in plasmapause formation. Our specific findings, which are based both upon the present case studies and upon earlier work, include the following: (1) during active periods the plasmasphere appears to become divided into two entities, a main plasmasphere and a duskside bulge region. The latter consists of outlying or outward extending plasmas that are the products of erosion of the main plasmasphere; (2) in the aftermath of an increase in convection activity, the main plasmasphere tends (from a statistical point of view) to become roughly circular in equatorial cross section, with only a slight bulge at dusk; (3) the abrupt westward edge of the duskside bulge observed from whistlers represents a state in the evolution of sunward extending streamers; (4) in the aftermath of a weak magnetic storm, 10 to 30% of the plasma “removed” from the outer plasmasphere appears to remain in the afternoon‐dusk sector beyond the main plasmasphere. This suggests that plasma flow from the afternoon‐dusk magnetosphere into the boundary layers is to some extent impeded, possibly through a mechanism that partially decouples the high altitude and ionospheric‐level flow regimes; (5) outlying dense plasma structures may circulate in the outer duskside magnetosphere for many days following an increase in convection, unless there is extremely deep quieting; (6) a day‐night plasmatrough boundary may be identified in equatorial satellite data; (7) factor‐of‐2‐to‐10 density irregularities appear near the plasmapause in the postdusk sector in the aftermath of weak magnetic storms; (8) during the refilling of the plasmatrough from the ionosphere at L = 4.6, predominantly bidirectional field aligned and equatorially trapped light ion pitch angle distributions give way to a predominantly isotropic distribution (as seen by DE 1) when the plasma density reaches a level a factor of about 3 below the saturated plasmasphere level; (9) some outlying dense plasma structures are effectively det...

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[1] Among the objectives of the Radio Plasma Imager (RPI) on IMAGE is the observation of the Earth's plasmasphere from the satellite's polar orbit, with apogee %8 R E geocentric distance and perigee near 1200 km altitude. This objective is here pursued by (1) remote sounding from high-altitude regions outside the main plasmasphere, (2) sounding within the plasmasphere, and (3) in situ passive measurements of natural wave activity. During sounding of the plasmasphere from both outside and inside the plasmapause, RPI echoes that follow non-field-aligned ray paths are usually not the discrete traces on range-versus-frequency records (plasmagrams) that are predicted by raytracing simulations in smooth magnetospheric density models. Instead, such RPI echoes exhibit various amounts of spreading, from %0.5 R E to %2 R E in virtual range (range assuming free-space speed of light propagation). The range spreading is attributed to scattering from, partial reflection from, and propagation along geomagnetic field-aligned electron density irregularities. There exists a substantial body of theoretical work on mechanisms that might explain the appearance of such irregularities both within and beyond the plasmasphere. That the spread-producing irregularities are field-aligned is suggested by the efficiency with which RPI excites discrete echoes that propagate along the geomagnetic field, sometimes into both hemispheres. The spatial distributions and scale sizes of the spread-producing irregularities remain to be investigated. The RPI echo data, however, coupled with earlier evidence from topside sounders and whistler mode instruments, suggest that they can have cross-field scale sizes within a range from %200 m to over 10 km and electron densities within %10% of background. RPI is found capable of detecting plasmapause locations from distances of %3 R E or more. When minimal signal integration is used, the location and range of density values of a steep plasmapause can be determined from distances of order 1 R E , and echoes can at times be returned from points extending inward from the plasmapause to locations where the electron density reaches %3000 cm À3 , which is usually at L < 3.

Lightning‐induced electron precipitation (LEP) events are studied using the Trimpi effect, in which the precipitation‐induced ionization enhancements in the lower ionosphere (D region) give rise to rapid perturbations of subionospheric VLF signals. In 1983, the phase and amplitude of signals from the NPM transmitter in Hawaii (23.4 kHz) and the Omega transmitter in Argentina (12.9 kHz) were measured at Palmer, Antarctica (L ∼ 2.4), together with the magnetospheric whistler background. The long baseline and over‐sea great circle paths from these two sources make it possible for the observed perturbations to be interpreted using a single waveguide mode theory. Analytical expressions are used to relate the magnitude of the phase perturbations to differential changes in ionospheric reflection height along a segment of the propagation path. The predicted relationship between relative perturbation sizes on the two different signals is compared with measurements. From this information, the whistler‐induced flux levels are inferred to be in the 10−4 − 10−2 erg cm−2 s−1 range and the precipitation regions are inferred to be roughly “circular” in shape, rather than elongated along L shells. Measured amplitude changes tended to be small (∼ 0.5 dB) and negative, as expected from a single‐mode theory, but the ratios of simultaneous amplitude and phase perturbations were slightly larger than the theory predicts, probably due to the effects of an additional mode(s). An assessment of the relative detectability of amplitude versus phase perturbations favors phase perturbations by ∼ 10 dB, irrespective of the detection scheme used.

It is well known that coherent VLF signals injected into the magnetosphere from Siple Station, Antarctica, often show temporal growth of 20-30 dB and emission triggering, as observed at the conjugate point, near Roberval, Quebec.In a new kind of experiment it has been found that when the input power to the transmitting antenna is reduced below a 'threshold' value Pt, growth and triggering cease. Below Pt the output is proportional to the input. In one typical experiment the peak received power increased by 24 dB when the input power to the antenna crossed the threshold value of 1000 W (estimated radiated power of 10 W). The value of Pt varied widely depending on the duct involved and on the time, changing as much as 10 dB in less than 1 hour. As the power was lowered during multipath propagation, the last duct to be cut off was found to terminate nearly overhead at Siple, as was expected, assuming all ducts to be equally active. Minimum radiated power for growth and triggering was 1 W. One possible explanation for the threshold effect is background noise (e.g., plasmaspheric hiss) that prevents the instability from getting started.Another is a drop in temporal growth rate to below zero at low signal level.Measurement of Pt might possibly serve as a groundbased diagnostic for magnetospheric flux levels, assuming calibration by satellite particle detectors.