M. E. Koepke

Velocity‐shear‐driven ion‐cyclotron waves and associated transverse ion heating

Recent sounding rocket experiments, such as SCIFER, AMICIST, and ARCS-4, and satellite data from FAST, Freja, DE-2, and HILAT, provide compelling evidence of a correlation between small-scale spatial plasma inhomogeneities, broadband low-frequency waves, and transversely heated ions. These naturally arising, localized inhomogeneities can lead to sheared cross-magnetic-field plasma flows, a situation that has been shown to have potential for instability growth. Experiments performed in the Naval Research Laboratorys Space Physics Simulation Chamber demonstrate that broadband waves in the ion-cyclotron frequency range can be driven solely by a transverse, localized electric field, without the dissipation of a field-aligned current. Significant perpendicular ion energization resulting from these waves has been measured. Detailed comparisons with both theoretical predictions and space observations of electrostatic waves found in the presence of sheared cross-magnetic-field plasma flow are made.

Dispersive properties of a magnetized plasma with a field‐aligned drift and inhomogeneous transverse flow

Electrostatic fluctuations driven by the combination of a magnetic‐field‐aligned electron current and a localized transverse electric field are investigated. Characteristic parameters, such as scale length and magnitude of the sheared E×B velocity, magnitude of the magnetic‐field‐aligned current, and temperature ratio τ≡Ti/Te are varied to include conditions associated with electrostatic waves driven entirely by magnetic‐field‐aligned current, driven entirely by transverse electric field, and driven by a combination of magnetic‐field‐aligned current and transverse electric field. It is shown that, in contrast to the homogeneous case of current‐driven modes, the modes in the presence of a transverse‐velocity shear can be unstable in a wider range of temperature ratio τ and they are broadband in frequency. Using a simplified model, numerical solutions of the nonlocal dispersion relation, and physical arguments, cases of stabilization and destabilization due to the inhomogeneous energy‐density driven instabi...

Control of ion temperature anisotropy in a helicon plasma

Laser induced fluorescence measurements of the parallel and perpendicular ion temperatures in a helicon source indicate the existence of a substantial ion temperature anisotropy, . The magnitude of the ion temperature anisotropy depends linearly on the source magnetic field. The parallel ion temperature is independent of magnetic field strength while the perpendicular temperature increases linearly with increasing magnetic field. Bohm-like particle confinement is proposed as an explanation for the linear dependence on magnetic field of the perpendicular ion temperature. In the helicon mode, the ion temperature components are independent of RF driving frequency and power and show a trend towards isotropy at high neutral fill pressures.

Observation of ion-cyclotron turbulence at small values of magnetic-field-aligned current

Fluctuations in plasma density are observed in the ion-cyclotron frequency range in a Q-machine plasma column at values of magnetic-field-aligned current significantly below the critical value for current-driven electrostatic ion-cyclotron (CDEIC) waves. These fluctuations arise from the inhomogeneous energy-density driven (IEDD) instability, are relatively broadband, and can have large amplitude compared to CDEIC fluctuations. These results impact the interpretation of ion heating and acceleration in the ionosphere, especially at low-altitude where magnetic- field-aligned current densities estimated to be large enough to be supercritical to current-driven or beam-driven electrostatic ion-cyclotron waves are rarely detected.

Laboratory simulation of broadband ELF waves in the auroral ionosphere

Space-relevant observational signatures, such as frequency spectra, phase velocities, excitation thresholds, and ion heating, associated with electrostatic ion-cyclotron waves excited by the inhomogeneous energy-density driven (IEDD) instability in a laboratory experiment are presented. A comparison is made between these waves and the broadband ELF waves recently observed in the auroral ionosphere with sounding rockets and satellites. The measurements of broadband spectra, electron-Landau-resonant phase velocities, low excitation threshold value of parallel electron drift speed, and significant perpendicular ion heating suggest that attributing the broadband ELF waves to the IEDD instability mechanism is justified.

Perpendicular ion heating by velocity‐shear‐driven waves

Perpendicular ion heating resulting from velocity-shear-driven ion-cyclotron waves has been measured for the first time. The experiment was performed in the Naval Research Laboratorys Space Physics Simulation Chamber (SPSC) under plasma conditions approaching those in the natural space environment. Sheared cross-field flow is induced by a controllable, inhomogeneous, transverse, DC electric field (LE ∼ (1–2)ρi) created without drawing significant levels of magnetic-field aligned current. Mode frequency data suggest that the most efficient heating occurs when the Doppler shifted frequency in the ion frame is located near a harmonic of the ion-cyclotron frequency.

Velocity-shear-induced ion-cyclotron turbulence: Laboratory identification and space applications

Laboratory measurements are reported that identify a new class of plasma oscillation driven by the inhomogeneity in wave energy density caused by transverse‐velocity shear [Ganguli et al., Phys. Fluids 31, 823 (1988)]. The experiments concentrate on a branch of oscillation in the ion‐cyclotron range of frequencies that results from the coupling of the magnetic‐field‐aligned current and the inhomogeneous dc electric field localized in a layer thicker than the ion gyroradius. The observed transition between the well‐known current‐driven electrostatic ion‐cyclotron mode and this inhomogeneous energy–density‐driven mode is related to the ratio of the azimuthal and axial Doppler shifts. The mode characteristics associated with the two instabilities have significantly different properties. For conditions of large transverse‐velocity shear, turbulence is generated with a broadband, spiky spectrum around the ion‐cyclotron frequency at small values of the magnetic‐field‐aligned current. The experimental identification is reinforced with numerical results from a nonlocal eigenvalue condition.

Contributions of Q-machine experiments to understanding auroral particle acceleration processes

Experiments performed over the past 40 years on Q-machine plasma devices [N. Rynn and N. D’Angelo, Rev. Sci. Instrum. 31, 1326 (1960)] have contributed significantly to the basic understanding of plasma behavior. Many of these laboratory results are relevant to plasma processes in the Earth’s ionosphere and magnetosphere. Here are reviewed examples in which Q-machine experiments contributed to understanding particle acceleration in the Earth’s auroral energization region by discovering unexpected behavior, developing physical insight, benchmarking theoretical models, and establishing observational signatures relevant to space plasmas. Magnetic-field-aligned (parallel) electric fields, solitary structures, ion–cyclotron waves, ion–acoustic waves, Kelvin–Helmholtz waves, and lower-hybrid waves are discussed. The legacy of these contributions is a tribute to the Q-machine design.

Shear driven waves in the induced magnetosphere of Mars

We present measurements of oscillations in the electron density, ion density and ion velocity in the induced magnetosphere of Mars. The fundamental frequency of the oscillations is a few millihertz, but higher harmonics are present in the spectrum. The oscillations are observed in a region where there is a velocity shear in the plasma flow. The fundamental frequency is in agreement with computational results from an ideal-MHD model. An interpretation based on velocity-shear instabilities is described.

Inhomogeneous transverse electric fields and wave generation in the auroral region: A statistical study

We use data from the Freja satellite to investigate the importance of localized transverse DC electric fields for the generation of broadband waves responsible for ion heating in the auroral region ...