A series of recent measurements of the outflow of ionization from the ionosphere have further heightened our awareness of the strength of the ionospheric source of magnetospheric plasmas. In this paper the ionospheric contribution of the polar wind and cleft ion fountain at energies less than 10 eV has been added to the previously measured sources; this total ion outflow has then been used to calculate the resulting ion density in the different internal regions of the earth's magnetosphere: plasmasphere, plasma trough, plasma sheet, and magnetotail lobes. Using estimated volumes for these regions and an ion residence time characteristic of each region, we have found that the observed magnetospheric densities can be attained in all cases with no contribution from the solar wind plasma. In the case of the plasma sheet the ionospherically supplied density is more than enough to match the observations and even suggests an invisible component of low‐energy plasma (< 10 eV) which has never been observed. A detailed comparison between the calculated ionospheric source effects in the plasma sheet and those recently measured by ISEE shows excellent agreement and suggests a direct polar low‐energy ion source for the plasma sheet which has remained unmeasured because of spacecraft potential effects. Although the solar wind is clearly the earth's magnetospheric energy source and energetic solar wind ions are observed in the magnetosphere, these calculations suggest the possibility that the ionospheric source alone is sufficient to supply the entire magnetospheric plasma content under all geomagnetic conditions.

The theta aurora is a remarkable configuration of auroral and polar cap luminosities for which a generally sun‐aligned transpolar arc extends contiguously from the dayside to nightside sectors of the auroral oval. Four individual occurrences of theta aurora over earth's northern hemisphere are examined in detail with the global auroral imaging instrumentation on board the high‐altitude, polar‐orbiting spacecraft DE 1. Simultaneous measurements of fields and plasmas with this high‐altitude spacecraft and its low‐altitude, polar‐orbiting companion, DE 2, are examined in order to establish an overview of auroral and polar cap phenomena associated with the appearance of the theta aurora. For these series of observations, two general states of the polar cap are found corresponding to (1) a bright, well‐developed transpolar arc and (2) a dim or absent transpolar arc. During periods of a relatively bright transpolar arc the plasma convection in the polar cap region associated with the transpolar arc is sunward. Elsewhere over the polar cap the convection is antisunward. The convection pattern over the auroral zones and polar cap is suggestive of the existence of four cells of plasma convection. Field‐aligned electron acceleration into the polar atmosphere and field‐aligned current sheets are present in the transpolar arc plasmas. This electron precipitation and these current sheets are relatively absent over the rest of the polar cap region. The transpolar arc plasmas exhibit similar densities and ion compositions relative to those plasmas observed simultaneously over the poleward zone of the auroral oval. The ion compositions include hot H+, He++, and O+ ions and thus are of both ionospheric and solar wind origins. Principal hot ions in the remainder of the polar cap region are H+ and He++, indicating access from the magnetosheath for these ions. Low‐energy electrons identified with a magnetosheath source are also present in this region. The dominant thermal ions in the polar cap region are O+ ions flowing upward from the ionosphere. These thermal ions are heated along magnetic flux tubes within the transpolar arc plasmas. Pairs of current sheets with oppositely directed current densities occur in the transpolar arc region and with magnitudes similar to those associated with the poleward zones of the auroral oval. The upward currents are carried by electrons accelerated by a field‐aligned potential. Funnel‐shaped auroral hiss and broadband electrostatic noise are associated with the presence of the transpolar arc plasmas. Energetic solar electrons are employed to show that the magnetic field lines threading both the transpolar arc and the poleward zone of the auroral oval are probably closed. In contrast, the accessibility of these electrons to the remainder of the polar cap indicates that these polar regions are characterized by a magnetic topology that is connected directly to field lines within the interplanetary medium. Thus the overall character of the transpolar arc region appears to be very similar to that obser...

[1] Previous studies of the magnetospheric plasma populations have concentrated on the low-energy (1 eV) plasma of the plasmasphere, the more energetic (1-100 keV) plasma of the plasma sheet and ring current, and the high-energy (approximately MeV) plasma of the radiation belts. A compilation of satellite measurements over the past 30 years augmented by recent observations from the Polar-TIDE instrument has revealed a new perspective on a plasma population in the middle magnetosphere. This population consists of ions with energies of a few eV to greater than several hundred eV which display a characteristic bidirectional field-aligned pitch angle distribution. Measurements from the ATS, ISEE, SCATHA, DE, and POLAR satellites establish the characteristics of this ''warm plasma cloak'' of particles that is draped over the nightside region of the plasmasphere and is blown into the morning and early afternoon dayside sector by the sunward convective wind in the magnetosphere. The satellite observations combined with the predictions of an ion trajectory model are used to describe the formation and dynamics of the warm plasma cloak.

Regions of high‐density cold plasma have been observed outside the plasmasphere in the plasma trough region of the magnetosphere. These detached plasma regions may have densities as high as several hundred ions per cubic centimeter in the normally low‐density (∼1 ion/cm³) plasma trough. The detached plasma regions are found across the day side of the magnetosphere, particular concentrations being in the afternoon‐dusk sector. The regions are located at the plasmapause at dusk and progressively farther away for earlier local times in the early afternoon and morning. Detached plasma regions are observed during moderate to disturbed magnetic activity conditions. They exhibit a very complex spatial structure with sizes varying from thousands of kilometers down to less than 50 km in extent. The detached regions may be generated by variations in the magnetospheric convection electric field, which occurs during substorm activity.

The very low energy thermal plasma (~1 eV or several thousand degrees Kelvin) is often overlooked in reviews of magnetospheric plasma populations, usually as a result of energy considerations in which thermal particles are believed unimportant. In reality this cold plasma, which surrounds the earth in a region known as the plasmasphere, has an important direct influence on the characteristics of the ionospheric F region and on the location of regions of plasma turbulence in the magnetosphere, and it is an excellent indirect measure of the intensity of magnetospheric convection. The characteristic morphology and dynamics of the plasmasphere vary with local time and with geomagnetic conditions. On the nightside the plasmapause position changes predicta•bly with changing magnetic activity. Once established at a specific L-shell value, the steep density gradient on the nightside corotares into the dayside, where filling from the ionosphere takes place. In the duskside bulge region the characteristic density profile inside the plasmapause displays a smooth decrease proportional to 1/R •, where R is radial distance. Plasmasphere morphology and dynamics can be understood in terms of a time-varying convection electric-field model of the magnetosphere that includes the bulge region as part of the main circulation pattern of the plasmasphere.