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

Abstract

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 Alfv en waves have been proposed as a possible means for accelerating electrons and generating auroras. Inertial Alfv en 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 Alfv en wave by making simultaneous measurements of both the perturbed electron distribution function and the Alfv en wave responsible for the perturbations. In this laboratory experiment, the suprathermal tails of the reduced electron distribution function parallel to the mean magnetic field are measured with high precision as inertial Alfv en waves simultaneously propagate through the plasma. The results of this experiment identify, for the first time, the oscillations of suprathermal electrons associated with an inertial Alfv en wave. Despite complications due to boundary conditions and the finite size of the experiment, a linear model is produced that replicates the measured response of the electron distribution function. These results verify one of the fundamental properties of the inertial Alfv en wave, and they are also a prerequisite for future attempts to measure the acceleration of electrons by inertial Alfv en waves. Published by AIP Publishing.

Laboratory measurements of the physics of auroral electron acceleration by Alfvén waves

“…2b, we plot measurements of the total perturbation δg e (v z ) = g e (v z ) − g e0 (v z ) over 2π of Alfvén wave phase, showing a signal along the vertical axis dominated by the fundamental mode of oscillations g e1 (v z ) at the frequency ω of the Alfvén wave. A previously derived analytical solution to the linearized Boltzmann equation 44,45 , shown in Fig. 2c, reproduces the dominant features of this pattern using only g e1 (v z ).…”

“…High-precision measurements of the suprathermal tails of the parallel electron velocity distribution g e (v z ) were performed using the WWAD [39][40][41][42][43] . This measurement technique provides sufficient time resolution and precision to determine variations of g e (v z ) as the Alfvén wave phase advances 44 (Supplementary Methods 3). The wave absorption technique is unable to provide measurements of g e (v z ) through the lower-velocity bulk of the distribution; however, because resonant acceleration affects suprathermal electrons, we are still able to explore this phenomenon with the available measurements (Supplementary Methods 3).…”

Laboratory space physics: Investigating the physics of space plasmas in the laboratory

“…153,[188][189][190][191] To test this hypothesis, recent experiments on the LAPD have launched inertial Alfvén waves down the 16 m cylindrical plasma column and measured the resulting perturbations of the parallel electron velocity distribution function using a novel whistler wave absorption diagnostic. 224 These auroral electron acceleration experiments have found good agreement with the predicted linear perturbation of the parallel electron distribution function, [225][226][227] establishing the critical foundation for future experiments to directly measure the acceleration of the electrons by inertial Alfvén waves. With enhanced capabilities, laboratory experiments are likely to contribute increasingly to our understanding of the mechanisms for particle acceleration in space and astrophysical plasmas.…”

“…224,453,454 Whistler wave absorption, for example, has been used in LAPD experiments to study the physics of electron acceleration by Alfvén waves in the auroral regions. [225][226][227] A number of other non-perturbative approaches have also been applied in the laboratory to determine various aspects of the velocity space dynamics of plasmas, including Rutherford scattering diagnostics applied in the Madison Symmetric Torus, 129 charge-exchange recombination spectroscopy, [455][456][457][458][459] continuous wave cavity ring-down spectroscopy, 460,461 and nonlinear optical tagging. 462,463 Improvements in conventional diagnostic techniques and the development and refinement of innovative ideas for the measurement of laboratory plasmas will no doubt boost the impact of laboratory experiments on studies at the frontier of heliophysics and astrophysics.…”

Shifting and splitting of resonance lines due to dynamical friction in plasmas

“…( 10)) in a set-up similar to that of Ref. [42] for the case of resonant ions or using whistler mode wave absorption for resonant electrons [43].…”