It is suggested that a low‐frequency ‘gradient drift’ instability may be important for the formation of striations in barium ion clouds released in the ionosphere above the E layer. The theory predicts that the trailing edge (with respect to the neutrals) of the plasma cloud will be unstable while the leading edge is stable, in qualitative agreement with observations. The growth time is relatively independent of wavelength and is of order d/Uo, where d is the typical density gradient length and Uo is the velocity of the cloud with respect to neutrals. Predicted growth times varying from seconds in the auroral zone at high latitude to minutes at midlatitudes are also in agreement with observation. Wavelengths of waves with zero phase velocity are found to be given approximately by λ = 2πd, which can be verified by optical observations.

The stability of crossed-field electron beams is treated for arbitrary values of the parameter q=ωp2/ωc2 = (plasma frequency/cyclotron frequency)2. The theory bridges a gap between existing theories applicable for q«1 and for q=1. For wavelengths longer than about five beam thicknesses an instability occurs with a growth rate not exceeding ∼¼qωc. It can be shorted out by conducting walls, or else one can prevent long wavelengths in closed (cylindrical or toroidal) beam geometries. For wavelengths shorter than roughly 6q beam thicknesses a cyclotron instability occurs with a growth rate ∼½qωc exp(−2/q). This becomes practically unimportant for q⪝0.2. Observations support these theoretical results.

The limitation on the current that a sphere at a large positive potential ϕ can draw from a surrounding collisionless plasma in a magnetic field is investigated. The model assumes that for eϕ ≫ kTe the ions are excluded from the region of high potential surrounding the sphere. This region increases in size as ϕ increases, thus presenting an increased cross‐sectional area through which electrons can be collected from the plasma along magnetic field lines. We suggest that there is a critical density at which the electron cloud surrounding the positive body is sufficiently turbulent that the electrons are free to diffuse radially across magnetic field lines and thereby reach the sphere. This model yields a saturation current I ∝ ϕ/lnϕ, which scales directly with the density, thermal velocity, and square of the body size. At high background electron concentrations a space‐charge‐limited flow model that predicts I ∝ ϕ6/7 is more applicable, whereas the more pessimistic limit of I ∝ ϕ1/2, which follows from the conservation of the single particle constants of motion in time invariant electric and magnetic fields, is appropriate when the background electron concentration is low.

ABSTRACT.The electromagnetic dispersion relation for two counterstreaming ion beams of arbitrary relative strength flowing parallel to a dc magnetic field is derived. The beams flow through a stationary electron background and the dispersion relation in the fluid approximation is unaffected by the electron thermal pressure. Magnetic effects on the ion beams are included but the electrons are treated as a magnetized fluid, m -*0. 6The dispersion relation is solved with a zero net current condition applied and the regions of instability in the k-U space (U is the relative velocity between the two ion beams) are presented. These results are extensions of Kovner's analysis for weak beams. The parameters are then chosen to be applicable for parallel shocks. We find that unstable waves with zero group velocity in the shock frame can exist near the leading edge v, of the shock for upstream Alfven Mach 'number's" greater than-5-r5-. -It-i-s suggested that this mechanism could generate sufficient turbulence within the shock layer to scatter the incoming ions and create the required dissipation for intermediate strength shocks.-111-

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