Engineering topics

Bergmann, R.

Publications and source records attributed to Bergmann, R..

5 records

NASA NTRS1989

Effects of ion two-stream instability on auroral ion heating

The ion two-stream instability develops between upflowing H(+) and O(+) in the auroral acceleration region, if the two species are accelerated to different velocities in a semistationary electrostatic potential. Parallel propagating modes are unstable only for small relative drifts, while oblique modes remain unstable at larger drifts and, consequently, higher altitudes. The effects of oblique modes on ion heating are investigated, and it is found that O(+) heats primarily by trapping in the direction of wave propagation during the early, coherent phase of wave growth in the temporal evolution problem. After undergoing trapping along the magnetic field direction, H(+) heats primarily by quasi-linear diffusion when the spectrum broadens nonlinearly. The parallel ion distributions evolve by formation of a high-energy (low-energy) tail on oxygen (hydrogen) along with bulk slowing of hydrogen, which results in more energetic oxygen than hydrogen. Total heating is greatest in the direction of wave propagation, and this effect is greater for hydrogen than for oxygen.

Roth, I.↗

NASA NTRS1988

Linear stability of the H(+) - O(+) two-stream interaction in a magnetized plasma

The Bergman and Lotko (1986) analysis of the linear stability of the H(+) - O(+) two-stream interactions for parallel propagating wave modes is extended to an analysis of the stability of wave modes that propagate at an angle to the terrestrial magnetic field. The analaysis provides an insight into the manner in which the oblique modes linearly couple and become unstable; the conditions under which the particular couplings are important; the qualitative behavior of the frequency, wave vector, and growth rate of the most unstable mode; and the changes to these introduced by finite ion temperatures. Consideration is also given to the relative importance of obliquely propagating and parallel propagating waves for the condition when the velocity difference between two ion beams is below the upper limit.

Bergmann, R.↗

NASA NTRS1987

Decay of electrostatic hydrogen cyclotron waves into ion acoustic modes in auroral field lines

The coherent three-wave decay of a linearly unstable electrostatic hydrogen cyclotron (EHC) wave into stable EHC and ion acoustic modes is considered. The general problem of the three weakly interacting electrostatic normal modes in a Maxwellian plasma is discussed. EHC is examined in a fluid description, and the results are used to guide a similar study in a Vlasov plasma system intended to model the aurora acceleration region parameters. The time dependence of the decay in a simple three-wave interaction is presented in order to show how wave saturation can arise.

Bergmann, R.↗

NASA NTRS1986

Transition to unstable ion flow in parallel electric fields

The stability of ionospheric O(+)-H(+) outflows accelerated by a nonambipolar parallel electric field is considered under conditions where the ion motion initially develops adiabatically and the ambient plasma is vertically stratified with an effective temperature that increases with altitude. Such conditions are expected near the bottom of the auroral acceleration region where ion and electron streaming instabilities first develop. It is shown for a particular equilibrium profile that the differentially accelerated ion flows become unstable within about 100 km from their entry point in the acceleration region. At O(+)/H(+) density ratios less than about 9, the instability is dominated by a violent H(+)-O(+) two-stream interaction which couples the O(+) and H(+) acoustic modes, and which mediates a transition to nonadiabatic acceleration. At higher altitudes and/or larger O(+)/H(+) density ratios, a much weaker resonant instability exists, which is driven by the relative drift between electrons and O(+) or H(+) ions. The results suggest that the H(+)-O(+) two-stream instability may be a viable mechanism for heating upflowing auroral ions.

Bergmann, R.↗

NASA NTRS1984

Electrostatic ion (hydrogen) cyclotron and ion acoustic wave instabilities in regions of upward field-aligned current and upward ion beams

An investigation is made into the stability of electrostatic hydrogen ion cyclotron and ion acoustic waves in a model plasma where an ion beam, population 2, and oppositely directed drifting electrons pass through a stationary ion background, population 1. The excited wave properties are then compared with the characteristics of the unstable modes observed on the S3-3 satellite. Three temperature regimes are studied: (1) Te greater than Ti2 much greater than Ti1, (2) Ti2 greater than Te not less than Ti1, and (3) Te approximately equal to Ti1 greater than Ti2. It is found that the ion beam acts as a free energy source only in regime 1. This regime is also highly unstable to the electrons as a free energy source. Unstable modes in regimes 2 and 3 seem to best satisfy the electrostatic hydrogen cyclotron wave (EHC) properties at 1 earth radius. For these cases the electrons are the free energy source, the beam supplies damping.

Bergmann, R.↗