Further laboratory measurements of the beam-plasma discharge (BPD) produced when an energetic electron beam traverses an initially neutral gas are presented. The experimental data indicate that the critical beam current Ic at energy V, magnetic field B, and system length L required for ignition obeys the empirical relationship Ic ∝ V3/2/B0.7 PL at pressures P < 2 × 10−5 torr. This relationship appears to primarily describe the accumulation of ambient plasma density, collisionally produced by the beam itself, to a critical value where ωpe² ≈ ωce² at which ignition occurs. Additional measurements of the narrow-band cyclotron-related waves (fce < f < 1.4fce) observed at beam currents below BPD threshold do not clearly establish whether this instability is a necessary precursor to the BPD. The relevance of these experiments to past and planned electron beam experiments in space is briefly discussed.

Further laboratory measurements of the beam-plasma discharge. [for aerospace technology application]

Large-amplitude (10 to 100 μV m−1 Hz−½) natural radio emissions in a wide frequency range (100 kHz up to 2 MHz) are frequently observed on board the AUREOL/ARCAD 3 satellite at high latitude and at altitudes between 400 and 2000 km. The simultaneous measurement of the local cold plasma density allows the identification of cutoff and resonance frequencies. Three different kinds of wave are observed: (1) electrostatic emissions near the local value of the plasma frequency (fp), (2) electromagnetic whistler mode emissions, sometimes associated with type (1) emissions, and (3) electromagnetic Z mode emissions, also associated with type 1 emissions, but occurring more rarely than the whistler mode emissions and then only when fp is greater than the electron cyclotron frequency (fce). These emissions are always associated with high levels of ELF electrostatic turbulence and a high flux of low-energy precipitating electrons, extending in energy down to the lower limit of the detectors (∼100 eV). The statistical distribution of the emissions in geomagnetic coordinates shows an occurrence greater than 80% in the polar cusp region and between 25% and 60% in the nightside auroral zone. A generation mechanism for such emissions is proposed, based on the calculation of the growth rate of the kinetic Cherenkov instability, associated with a beamlike suprathermal tail in the parallel distribution of the bulk electron population. In particular, suprathermal, downward electron beams of about 100–200 eV energy, with a thermal spread of the same order, are found to be responsible for the generation of whistler mode and Z mode HF emissions in a source region extending down to 1000 km of altitude. It is suggested that such intense radio emissions should be considered as one of the energy dissipation processes resulting from magnetosphere-ionosphere coupling, through interdependent electrodynamic mechanisms such as current systems, parallel anomalous resistivity, plasma turbulence, energy diffusion, and heating.

Electrostatic plasma waves and HF auroral hiss generated at low altitude

The first active beam-plasma experiments utilizing the space shuttle were carried out in March 1982 as part of the NASA Office of Space Science mission on the third space shuttle flight. A fast pulse electron generator emitted a 1-keV, 100-mA electron beam in either a continuous (dc) mode or an on/off modulated mode in the ELF to HF frequency range. Stimulated electrostatic and electromagnetic waves and associated plasma effects were measured with the plasma diagnostics package as it was maneuvered through and near the beam by the remote manipulator system. For the dc beam the wave spectrum was electrostatic, was peaked in the 300- to 500-Hz range with a spectral density of 4 × 10−3 V2 m−2 Hz−1, and was unpolarized. Above the nominal lower hybrid resonance frequency the intensity decreased but was polarized. Strong emissions near the electron gyrofrequency and the plasma frequency were occasionally detected. The background spectrum was similar to the dc case for VLF and ELF modulations, but the emissions at the modulation frequency were more electromagnetic and more intense with field strengths of up to 1 Vm−1.

Wave emissions from dc and modulated electron beams on STS 3

The high-latitude spatial distributions of average signal intensities in 12 frequency channels between 4 Hz and 512 kHz as measured by the ac electric field spectrometers on the DE-2 spacecraft are analyzed for 18 mo of measurements. In MLT-INL (magnetic local time-invariant latitude) there are three distinct distributions that can be identified with 4-512 Hz signals from spatial irregularities and Alfven waves, 256-Hz to 4.1-kHz signals from ELF hiss, and 4.1-64 kHz signals from VLF auroral hiss, respectively. Overlap between ELF hiss and spatial irregularity signals occurs in the 256-512 Hz band. VLF hiss signals extend downward in frequency into the 1.0-4.1 kHz band and upward into the frequency range 128-512 kHz. The distinctly different spatial distribution patterns for the three bands, 4-256 Hz, 512-1204 Hz, and 4.1-64 kHz, indicate a lack of any causal relationships between VLF hiss, ELF hiss, and lower-frequency signals from spatial irregularities and Alfven waves.

High-latitude distributions of plasma waves and spatial irregularities from DE 2 alternating current electric field observations

Controlled electron beam experiments in space and supporting laboratory experiments: a review

Polar 5—an electron accelerator experiment within an aurora. 1. Instrumentation and geophysical conditions

Observations of waves artificially stimulated by an electron beam inside a region with auroral precipitation

Three groups of waves appear to be characteristic of the dayside high-lattitude magnetosphere, in view of AC electric field measurements made by polar-orbiting satellites: (1) ULF/ELF broadband noise, whose region of peak intensity is generally found to be consistent with the latitudinal extension of the cusp; (2) VLF hiss, which although present in the cusp region extends outside the established cusp projection; and (3) narrow-band ELF emissions, covering frequencies betwen 200 Hz and 1 kHz, on the equatorward side of the cusp. This classification of the waves is independent of the magnetic disturbance level. Attention is given to the manner in which the appearance of the ULF/ELF noise and the narrow-band ELF emissions can be used to localize the solar cusp position.

J. A. Holtet

Papers

Polar 5—an electron accelerator experiment within an aurora. 1. Instrumentation and geophysical conditions

Observations of waves artificially stimulated by an electron beam inside a region with auroral precipitation

ULF/ELF waves and turbulence in the high‐latitude dayside magnetosphere