Excitation and dissipation of Alfvén waves and current-driven instabilities in arched magnetized plasmas*

Arched magnetoplasma structures that carry electrical current ubiquitously exist in solar and heliospheric plasmas. Varieties of Alfvén waves and instabilities (e.g., fast waves, Kink and Kelvin-­Helmholtz instabilities) have been detected in recent space and ground based observations. Contemporary research on these topics is at the forefront of solar physics due to their remote diagnostic capabilities. After briefly reviewing the progress made in this area, results from a recently upgraded UCLA experiment on arched magnetized plasmas (plasma β ≈ 10^-3, Lundquist number ≈ 10²–10⁵, plasma radius/ion-gyroradius ≈ 20, B ≈ 1000 Gauss at footpoints) will be presented. The arched plasma was created using a lanthanum hexaboride (LaB₆) plasma source and it evolved in an ambient magnetoplasma produced by another LaB₆ source. The experiment runs continuously with a 0.5 Hz repetition rate. The plasma and wave parameters were recorded with a good resolution using movable Langmuir and three-axis magnetic-loop probes in 3D. Images of the plasma were recorded using a fast-CCD camera. In the upgraded experiment, the main focus is on the direct measurement of propagation and damping characteristics of fast waves and kink-mode oscillations. The kink-mode oscillations were observed as transverse oscillations across the symmetry plane of the arched plasma. The relative magnitudes of the parameters of the arched and ambient plasma were varied to simulate a variety of conditions relevant to coronal loops and solar prominences. Recent results reveal fascinating interplay among fast waves and global oscillations of the arched plasma. We also examine theoretical models of kink-modes and Alfvén waves in the presence of an electrical current in a magnetized plasma structure.

References: (1) Tripathi and Gekelman, Phys. Rev. Lett. 105, 075005 (2010); (2) Tripathi and Gekelman, Solar Phys. 286, 479 (2013)