Interplay among Arched Plasma Eruptions, Global Oscillations, and Broad Spectra of Alfvén Waves

Abstract:

Arched magnetized structures that carry electrical current ubiquitously exist in solar and heliospheric plasmas. Varieties of plasma waves and instabilities (e.g., fast waves, kink, sausage, Kelvin-Helmholtz instabilities) and associated processes (e.g., enhanced damping due to phase-mixing of Alfvén waves) have been at the forefront of contemporary research in solar and heliospheric physics. We present results on related topics from a laboratory experiment on arched magnetized plasmas (plasma β ≈ 10^-3, Lundquist number ≈ 10²–10⁵, plasma radius/ion-gyroradius ≈ 20, B ≈ 1000 Gauss at footpoints). The arched plasma is created using a lanthanum hexaboride plasma source and it evolves in an ambient magnetized plasma produced by another source. The experiment runs continuously with a 1/2 Hz repetition rate. The plasma and wave parameters are recorded with a good resolution using movable Langmuir and three-axis magnetic-loop probes in 3D. Images of the plasma are recorded using a CCD camera. In the upgraded experiment, the main focus is on the direct measurement of propagation and damping characteristics of global kink-mode oscillations and fast waves. The relative magnitudes of the parameters of the arched and ambient plasma were varied to simulate a variety of eruptive conditions relevant to the Sun. Recent results reveal fascinating interplay among global oscillations of the arched plasma and fast waves. Transverse gradients in Alfvén speed across the arched plasma have been observed to excite broad spectra of fast Alfvén waves that carry away energy from large scale oscillations in the arched plasma. These observations are consistent with predictions of the phase mixing of fast Alfvén waves in an inhomogeneous magnetized plasma that effectively enhances damping of large scale oscillations.

(Work supported by National Science Foundation, USA under award number 1619551)