A large set of bow shock crossings (i.e., 1392) observed by 17 spacecraft has been used to explore the three-dimensional shape and location of the Earth's bow shock and its dependence on solar wind and interplanetary magnetic field (IMF) conditions. This study investigates deviations from gas dynamic flow models associated with the magnetic terms in the magnetohydrodynamic (MHD) equations. Empirical models predicting the statistical position and shape of the bow shock for arbitrary values of the solar wind pressure, IMF, and Alfv6nic Mach number (MA) have been derived. Individual crossings have been rotated into aberrated GSE coordinates to remove asymmetries associated with the earth's orbital •r•otion. Variations due to changes in solar wind dynamic pressure have been taken into consideration by normalizing the observed crossings to the average value (p) = 3.1 nPa. The resulting data set has been used to fit three-dimensional bow shock surfaces and to explore the variations in these surfaces with sonic (Ms), Alfv6nic (MA) and magnetosonic (MMS) Mach numbers. Analysis reveals that among the three Mach numbers, MA provides the best ordering of the least square bow shock curves. The subsolar shock is observed to move Earthward while the flanks flare outward in response to decreasing MA; the net change represents a 6-10% effect. Variations due to changes in the IMF orientation were investigated by rotating the crossings into geocentric interplanetary medium coordinates. Past studies have suggested that the north-south extent of the bow shock surface exceeds the east-west dimension due to asymmetries in the fast mode Mach cone. This study confirms such a north-south versus east-west asymmetry and quantifies its variation with M S, M A, MMS, and IMF orientation. A 2-7% effect is measured, with the asymmetry being more pronounced at low Mach numbers. Combining the bow shock models with the magnetopause model of Roelof and Sibeck (1993), variations in the magnetosheath thickness at different local times are explored. The ratio of the bow shock size to the magnetopause size at the subsolar point is found to be 1. 46; at dawn and dusk, the ratios are found to be 1.89 and 1.93, respectively. The subsolar magnetosheath thickness is used to derive the polytropic index y according to the empirical relation of Spreiter et al. (1966). The resulting y = 2.3 suggests the empirical formula is inadequate to describe the MHD interaction between the solar wind and the magnetosphere. Greenstadt, et al., 1990]. An essential point revealed by many of the previous investigations is that the bow shock formed upstream of the Earth is a highly dynamic boundary, controlled by steady and transient variations in solar wind characteristics. To date, Fairfield's [1971] model enjoys perhaps the widest use as representative of the average shape and position of the bow shock. Fairfield's model, however, is based only on observations made near the ecliptic plane; thus the model is two-dimensional in that it assumes axial symmetry about the solar wi...

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Measurements of electron density and temperature by the Pioneer Venus orbiter electron temperature probe have been employed to examine the characteristics and morphology of ionospheric holes in the antisolar ionosphere of Venus. The holes apparently exist as north‐south pairs which penetrate the ionosphere vertically down to altitudes as low as 160 km. Magnetic field measurements show that the holes are permeated by strong radial fields whose pressure is sufficient to balance the plasma pressure of the surrounding ionosphere. The electron temperature in the holes is substantially cooler than the surrounding ionosphere, except in the lowest density regions of the holes where the temperatures greatly exceed the ionosphere temperature. The low temperatures and the low densities of the holes are consistent with the strong radial magnetic fields which inhibit horizontal transport of plasma and thermal energy from the surrounding ionosphere. Plasma depletion processes associated with magnetotail electric fields may be important in the formation of the holes.

The general problem of the excitation of ion gyroharmonic electrostatic and electromagnetic waves by energetic ions (E ≳ 0.1 MeV) in a thermal plasma as observed in the earth's plasmasphere is considered. The thermal plasma is taken to be predominantly H+ with trace higher Z ion constituents of ionospheric origin such as He+ and O+. This plasma component has a Maxwellian distribution and supports the waves which may be excited by energetic ions with Z ≳ 1. The energetic ions possess a power law distribution in energy with strong pitch angle anisotropy. Coupling of the free energy source of the energetic ions to the waves is obtained via the velocity dependence of the ion gyrofrequency Ωα. Owing to constraints imposed by free energy coupling, wave amplification, and ion cyclotron damping, the dominant growth rates are found for nearly perpendicularly propagating fast magnetosonic waves closely confined to the magnetic equator. Growth is restricted to frequencies immediately below the ion gyroharmonics by the resonance condition and implies line widths of ∼ 10−3Ωα. The fast magnetosonic waves are found to have a significant electromagnetic component which decreases with increasing gyroharmonic number. The polarization becomes almost entirely electrostatic where the fast magnetosonic and ion Bernstein branches merge above the lower hybrid frequency. When it is compared to observations of ion gyroharmonic emissions, the theory implies that there exist a significant number of charge states for Z ≳ 2 MeV ions in the plasmasphere.

Recent observations by the instruments onboard the Pioneer Venus spacecraft have revealed the existence of "holes" or regions of density depletion in the nightside Venus ionosphere associated with regions of radial magnetic fields. The properties of the electrons within the core of these holes are suggestive of an acceleration process along the magnetic field lines as are the Venera 9 and 10 observations of energetic ions in the Venus tail. Given the observational information, we attribute these Venusian plasma depletions to the presence of parallel electric fields similar to those observed in the terrestrial auroral ionosphere. The resulting electric field accelerates electrons down the field lines producing heating of the depleted thermal electron population within the hole and production of ionization below the hole. Simultaneously ionospheric ions are accelerated outward toward the plasmasheet.