George Gatling

Laboratory investigation of boundary layer processes due to strong spatial inhomogeneity

Laboratory experiments have been conducted to simulate the dynamics of highly localized magnetospheric boundary layers. These regions, such as the plasma sheet boundary layer and the magnetopause, are primary regions of solar wind mass, energy, and momentum transport into the near-Earth space environment. During periods of solar activity, the boundary layers can become compressed to scale lengths less than an ion gyroradius. Theoretical predictions indicate that the plasma can respond to relax these highly stressed conditions through the generation of instabilities in the lower hybrid frequency range. The experiments reported here document the characteristics of waves associated with these instabilities.

Whistler wave propagation and whistler wave antenna radiation resistance measurements

Whistler waves are a common feature of ionospheric and magnetospheric plasmas. While the linear behavior of these waves is generally well understood, a number of interesting observations indicate that much remains to be learned about the nonlinear characteristics of the mode. For example, in space, very low frequency (VLF) emissions triggered by whistler modes launched from ground-based transmitters have been observed. Emission is assumed to come from transverse currents formed by counterstreaming electrons that are phase bunched by the triggering signal. In the laboratory, it has been shown that with increasing amplitude of the driving signal applied to an antenna, the whistler mode radiation pattern forms a duct with diameter of the order of the parallel wavelength. The ducted waves were observed to propagate virtually undamped along the length of the plasma column. These observations have prompted an Naval Research Laboratorys (NRL) Space Physics Simulation Chamber study of whistler wave dynamics. The goals are to investigate whistler wave ducting, self-focusing, and amplification, and to study nonlinear whistler-plasma interactions.

Direct observation of microparticle gyromotion in a magnetized direct current glow discharge dusty plasma

Laboratory observations of oscillatory motion of charged microparticles have been made in an argon dc glow discharge plasma created within a strong dc magnetic field. Measurements of the oscillation frequency and amplitude are consistent with the gyromotion of magnetized dust grains under the ambient plasma conditions. The measurements provide an effective method for the noninvasive determination of the charge on the observed microparticles. The observations also seem to indicate that the neutral drag force on the dust grains may be smaller than anticipated from the classical estimation.

Advances in Impedance Probe Applications and Design in the NRL Space Physics Simulation Chamber

Impedance probe measurements are made at ionospheric (F-region) plasma conditions (n_e ≈ 10⁴-10⁶ cm^-3 and λ_D ≈ 1-10 cm) created in the Space Physics Simulation Chamber at the Naval Research Laboratory. Measurements of probe-plasma impedance are used to provide comparative data and identify possible problems, with the goal of developing flight-capable diagnostics for sounding rocket experiments. The ability of the diagnostic technique to infer electron density and plasma potential in an ionospheric plasma is demonstrated with laboratory measurements. In addition, preliminary data from prototype instruments built in our laboratory are presented.

Laboratory investigation of the propagation and ducting of whistler-waves

Summary form only given. There have been a number of interesting in situ and laboratory observations of whistler wave propagation and stimulated emissions over the past few decades. For example, Stenzel [1975] reported on the self-ducting of large amplitude whistler waves in a laboratory plasma. Those experiments showed that with increasing amplitude, the radiation pattern from a small dipole antenna becomes increasingly narrow, and ultimately forms a duct with diameter of the order of the parallel wavelength. The ducted waves were observed to propagate virtually undamped along the length of the plasma column. In the space environment, observations of artificially stimulated VLF emissions triggered in the magnetosphere by whistler modes from VLF transmitters have been reported by Stiles and Helliwell [1975]. Emission radiation is assumed to come from the transverse currents formed by counter-streaming electrons that have been temporarily phase bunched by the constant frequency triggering signal. These observations have prompted a new NRL Space Physics Simulation Chamber investigation of whistler wave dynamics in a simulated radiation belt environment. The ultimate goals of these experiments are to understand and quantify ducting, self-focusing, and amplification of whistler waves, to investigate nonlinear whistler-plasma interactions, and to study the secondary emission of whistler waves. The initial experiments concentrates on the ducting of whistler waves in pre-existing density depletions and enhancements. Density structures with controllable scale size and depth is created using methods previously developed for a Space Chamber investigation of the dynamics of magnetospheric boundary layers.

Observation of microparticle gyromotion in a magnetized DC glow discharge dusty plasma

Summary form only given. Laboratory observations of the motion of charged microparticles have been made in an argon DC glow discharge plasma created within a strong DC magnetic field. The experimental configuration consists of an anode-cathode pair centered between a pair of neodymium iron boride permanent magnets. The cylindrical axis of the resulting plasma column is directed vertically (i.e., along the direction of the gravitational force). Depending upon the orientation of the magnets, the magnetic field can be directed either upward or downward, with a field strength of approximately 2.5 kG. A pair of Helmholtz magnetic field coils external to the vacuum chamber allows the magnetic field to be varied by approximately /spl plusmn/75 G in the experimental region. Alumina microparticles (/spl sim/1.2 /spl mu/m) placed directly on the grounded cathode provide the source of charged dust in the plasma. Individual dust grains suspended in the plasma can be observed moving in an oscillatory fashion. Measurements of the oscillation frequency, spatial amplitude, and scalings with magnetic field strength and transverse particle velocity have been made. The measurements are consistent with the expected gyromotion of magnetized dust grains under the ambient plasma conditions and the data are shown to provide an effective method for the noninvasive determination of the dust grain charge.