SH11E-2422
Laboratory Study of Non-linear Decay of a Kinetic Shear Alfvén Wave

Monday, 14 December 2015
Poster Hall (Moscone South)
Seth E Dorfman1, Troy A Carter1, Stephen T Vincena1, Patrick Pribyl1, Yu Lin2, Richard Dwayne Sydora3 and Giovanni Rossi1, (1)University of California Los Angeles, Los Angeles, CA, United States, (2)Auburn University at Montgomery, Auburn, AL, United States, (3)University of Alberta, Edmonton, AB, Canada
Abstract:
Alfvén waves, a fundamental mode of magnetized plasmas, are ubiquitous in space plasmas. For example, a cascade of non-linearly interacting Alfvén waves is believed to play a key role in solar wind turbulence. At perpendicular length scales below the ion gyroradius, this takes the form of Kinetic Alfvén Waves (KAWs). Theoretical predictions show that these Alfvén waves may be unstable to various decay instabilities (e.g. [1,2]) even at very low amplitudes (δB/B<10-3) [2]. Given the turbulent nature of solar wind observations and limited spacecraft spacial resolution, laboratory experiments can play a vital role in exploring the key physics responsible.

The present work, conducted at UCLA's Large Plasma Device (LAPD) represents the first fundamental laboratory study of the non-linear Alfvén wave interactions responsible for Alfvén wave decay instabilities. These experiments include the first laboratory observation of the Alfvén-acoustic mode coupling at the heart of the Parametric Decay Instability [3]. More recently, laboratory efforts have focused on the non-linear decay of a KAW into daughter KAWs. In these experiments, a single high-frequency ω/Ωi~0.7 Alfvén wave is launched, resulting in three daughter modes with frequencies and wave numbers that suggest co-propagating KAWs produced by decay of the pump wave. The observed process is parametric in nature, with the frequency of the daughter modes varying as a function of pump mode amplitude. The decay is only observed when there is significant power in the right-hand circularly polarized pump branch; this branch has a phase speed different from the left hand mode at the ω/Ωi where the process is seen. Efforts are underway to fully characterize this set of experiments and compare with decay instabilities predicted by theory and simulations.

[1] JV Hollweg, J. Geophys. Res. 99, 23 431 (1994).
[2] YM Voitenko, Journal of plasma physics 60.03 (1998).
[3] S Dorfman and T Carter, Phys. Rev. Lett. 110, 195001 (2013).

Supported by DOE, NSF, and DOE FES and NASA Eddy Postdoctoral Fellowships

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