Auroral Electron Acceleration

Here we describe the results in our recent paper in Nature Communications 12:3103 (2021),
Laboratory measurements of the physics of auroral electron acceleration by Alfven waves

The paper has also been featured as an Editor's Highlight for Nature Communications in the research area of “Astronomy and Planetary Science.”

Technological and Scientific Advancements

In addition to the unique experimental conditions made possible by using the Large Plasma Device (LAPD) at UCLA's Basic Plasma Science Facility, the research team developed a number of cutting-edge instruments and techniques needed to perform this challenging experiment. Below, we enumerate each of these advancements, providing details about their capabilities:
  1. Arbitrary Spatial Waveform Antenna (Thuecks et al., 2009): Early work involved postdoctoral researcher Scott Bounds' and graduate student Derek Theucks' contributions to the design of the unique Arbitrary Spatial Waveform (ASW) antenna, in which each of 48 elements can be driven independently to tune the spatial pattern of the wave. This powerful tool for exploring the physics of Alfven waves in the experiments allowed control of the power of the Alfven waves in wavevector space in both the kinetic and inertial regimes, providing the critical control needed to test the Whistler Wave Absorption Diagnostic. Ultimately, the flexible design limited the peak antenna power, making it difficult to measure the very small, desired signal of accelerated electrons, and so the new Sigma antenna was designed for the final electron acceleration experiment.
  2. Wave Absorption Techniques (Skiff et al., 1993): The single most challenging aspect of the project to demonstrate experimentally the acceleration of electrons by Alfven waves is to measure the small population of electrons in the tail of the electron velocity distribution. A very sensitive technique is needed to detect this small population of electrons moving within a narrow range of velocities, requiring a sensitivity often greater than one in a thousand. Wave absorption techniques, in which a wave passing through the plasma of ions and electrons is partly absorbed, can be used to connect the fraction of the wave transmitted through the plasma to the population of electrons with a particular speed. Professor Fred Skiff brought his expertise on sophisticated wave absorption techniques to the design and construction of a new diagnostic, the Whistler Wave Absorption Diagnostic, needed to measure the accelerated electrons under the particular plasma conditions relevant to the physics of the auroral magnetosphere.
  3. Whistler Wave Absorption Diagnostic (Thuecks et al., 2012): Graduate student Derek Theucks worked on the first implementation of wave absorption techniques relevant to the overdense plasma conditions corresponding to the auroral magnetosphere, in which whistler waves serve as an appropriate probe of the electrons in the tail of the velocity distribution. The resulting Whistler Wave Absorption Diagnostic was constructed and tested, demonstrating the critical capability of measuring the small population of electrons that will experience resonant acceleration by the inertial Alfven waves launched in this experiment.
  4. Elsasser probes (Drake et al., 2011): In addition to measuring the accelerated electrons, the successful demonstration of auroral electron acceleration in the laboratory requires the careful measurement of the electric and magnetic fields associated with the Alfven wave at the same point in the experiment. Postdoctoral researcher Jan Drake lead the development and testing of a new ``Elsasser'' probe that can measure both fields simultaneously, named after a scientist that created a method to determine the direction of Alfven waves by combining electric and magnetic field measurements.
  5. Sigma Antenna (Schroeder et al., 2021): In the final experiment, graduate student Jim Schroeder designed and constructed the "Sigma" antenna (named because its design looks like the capital Greek letter Sigma) to generate high-power Alfven waves with a sufficiently small wavelength across the magnetic field to efficiently accelerate electrons.
  6. Field-Particle Correlation Technique (Klein and Howes, 2016; Howes et al., 2017; Klein et al.,2017): The final piece of the puzzle to demonstrate the acceleration of electrons---showing that the variation of the acceleration as a function of the electron velocity agrees with predictions for Landau damping---was the use of the field-particle correlation technique, a new analysis method that can be used to determine the energization of particles using spatially coincident particle and electric field measurements. Although this innovative technique had been used successfully to determine rates of particle energization in numerical simulations of plasma turbulence (Klein et al., 2017, 2020) and Magnetospheric Multiscale (MMS) spacecraft measurements of Earth's turbulent magnetosheath (Chen et al., 2019), this project is the first application of the technique to explore particle acceleration in a laboratory plasma.


  • Chen, C. H. K., Klein, K. G., and Howes, G. G. (2019). Evidence for electron Landau damping in space plasma turbulence. Nature Comm., 10:740.
  • Drake, D. J., Kletzing, C. A., Skiff, F., Howes, G. G., and Vincena, S. (2011). Design and use of an Elsasser probe for analysis of Alfv´en wave fields according to wave direction. Rev. Sci. Instrum., 82:103505.
  • Howes, G. G., Klein, K. G., and Li, T. C. (2017). Diagnosing collisionless energy transfer using field-particle correlations: Vlasov-Poisson plasmas. J. Plasma Phys., 83:705830102.
  • Klein, K. G. and Howes, G. G. (2016). Measuring Collisionless Damping in Heliospheric Plasmas using Field-Particle Correlations. Astrophys. J. Lett., 826:L30.
  • Klein, K. G., Howes, G. G., and TenBarge, J. M. (2017). Diagnosing collisionless enegy transfer using field-particle correlations: gyrokinetic turbulence. J. Plasma Phys., 83:535830401.
  • Klein, K. G., Howes, G. G., TenBarge, J. M., and Valentini, F. (2020). Diagnosing collisionless energy transfer using field-particle correlations: Alfven-ion cyclotron turbulence. J. Plasma Phys., 86:905860402.
  • Schroeder, J. W. R., Howes, G. G., Kletzing, C. A., Skiff, F., Carter, T. A., Vincena, S., and Dorfman, S. (2021). Laboratory measurements of the physics of auroral electron acceleration by Alfven waves. Nature Comm. 12:3103
  • Skiff, F., Boyd, D. A., and Colborn, J. A. (1993). Measurements of electron parallel-momentum distributions using cyclotron wave transmission. Phys. Fluids B, 5:2445.
  • Thuecks, D. J., Kletzing, C. A., Skiff, F., Bounds, S. R., and Vincena, S. (2009). Tests of collision operators using laboratory measurements of shear Alfv´en wave dispersion and damping. Phys. Plasmas, 16:052110.
  • Thuecks, D. J., Skiff, F., and Kletzing, C. A. (2012). Measurements of parallel electron velocity distributions using whistler wave absorption. Rev. Sci. Instr., 83:083503.