J. W. R. Schroeder

Alfvén wave collisions, the fundamental building block of plasma turbulence. IV. Laboratory experiment

Turbulence is a phenomenon found throughout space and astrophysical plasmas. It plays an important role in solar coronal heating, acceleration of the solar wind, and heating of the interstellar medium. Turbulence in these regimes is dominated by Alfven waves. Most turbulence theories have been established using ideal plasma models, such as incompressible MHD. However, there has been no experimental evidence to support the use of such models for weakly to moderately collisional plasmas which are relevant to various space and astrophysical plasma environments. We present the first experiment to measure the nonlinear interaction between two counterpropagating Alfven waves, which is the building block for astrophysical turbulence theories. We present here four distinct tests that demonstrate conclusively that we have indeed measured the daughter Alfven wave generated nonlinearly by a collision between counterpropagating Alfven waves.

Alfvén wave collisions, the fundamental building block of plasma turbulence. III. Theory for experimental design

Turbulence in space and astrophysical plasmas is governed by the nonlinear interactions between counterpropagating Alfven waves. Here, we present the theoretical considerations behind the design of the first laboratory measurement of an Alfven wave collision, the fundamental interaction underlying Alfvenic turbulence. By interacting a relatively large-amplitude, low-frequency Alfven wave with a counterpropagating, smaller-amplitude, higher-frequency Alfven wave, the experiment accomplishes the secular nonlinear transfer of energy to a propagating daughter Alfven wave. The predicted properties of the nonlinearly generated daughter Alfven wave are outlined, providing a suite of tests that can be used to confirm the successful measurement of the nonlinear interaction between counterpropagating Alfven waves in the laboratory.

Measurements of the nonlinear beat wave produced by the interaction of counterpropagating Alfvén waves

Plasma turbulence has been shown to play a critical role in many astrophysical and space environments. In the solar corona and solar wind, this turbulence involves the nonlinear interaction of kinetic Alfven waves. In the Earths magnetosphere, the turbulence is dominated by inertial Alfven wave collisions. Observations of these wave–wave interactions in space and in laboratory plasma environments have shown that, in addition to the nonlinear cascade of energy to small scales, the interaction also produces nonlinear beat waves that have a frequency defined by f3±=|f1±f2|. Although the temporal behavior of the beat wave has been well documented, this paper presents the first detailed analysis of the spatial structure of the nonlinearly generated beat wave.

Direct measurement of electron sloshing of an inertial Alfvén wave

We report laboratory measurements of the parallel current carried by suprathermal electrons associated with inertial Alfven wave excitation in the laboratory. The experiments presented here use a novel wave absorption diagnostic that measures the suprathermal tails of the electron distribution parallel to the mean magnetic field. The diagnostic is used to probe the electron distribution as an inertial Alfven wave propagates along the mean magnetic field. These results identify, for the first time, the periodic motion of suprathermal electrons participating in the inertial Alfven wave itself, in agreement with predictions from linear kinetic theory.

Linear theory and measurements of electron oscillations in an inertial Alfvén wave

The physics of the aurora is one of the foremost unsolved problems of space physics. The mechanisms responsible for accelerating electrons that precipitate onto the ionosphere are not fully understood. For more than three decades, particle interactions with inertial Alfven waves have been proposed as a possible means for accelerating electrons and generating auroras. Inertial Alfven waves have an electric field aligned with the background magnetic field that is expected to cause electron oscillations as well as electron acceleration. Due to the limitations of spacecraft conjunction studies and other multi-spacecraft approaches, it is unlikely that it will ever be possible, through spacecraft observations alone, to confirm definitively these fundamental properties of the inertial Alfven wave by making simultaneous measurements of both the perturbed electron distribution function and the Alfven wave responsible for the perturbations. In this laboratory experiment, the suprathermal tails of the reduced elect...

Analysis of Magnetic Fields in Inertial Alfvén Wave Collisions

Turbulence in astrophysical and space plasmas is dominated by the nonlinear interaction of counter propagating Alfvén waves. Most Alfvén wave turbulence theories have been based on ideal plasma models for Alfvén waves at large scales. However, in the inertial Alfvén wave regime, relevant to magnetospheric plasmas, how the turbulent nonlinear interactions are modified by the dispersive nature of the waves remains to be explored. Here, we present the first laboratory evidence of the nonlinear interaction in the inertial regime. A comparison is made with the theory for MHD Alfvén waves.