LOW-frequency wave propogation in partially ionized plasmas and a strong density gradient in the Hot hELicon eXperiment (HELIX)

Abstract

Damping of Alfvén waves is one of the most likely mechanisms for ion heating in the solar corona. Ion-neutral collisions have significant but poorly-understood effects on energy transfer and Alfvén wave propagation in partially ionized plasmas, such as those found in the solar chromosphere. Alfvén waves are thought to play a major role in the formation of auroral arcs as well as the generation of parallel electric fields in the Earth's ionosphere. The high densities created in helicon plasmas make them ideal for investigating the propagation and damping of Alfvén waves, which have very long wavelengths and low frequencies (ω/ω <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ci</sub> ≤ 1). In the Hot hELicon eXperiment (HELIX), the neutral density varies strongly with radius, giving access to a wide range of Alfvén dynamics across the plasma column. The ratio of ion-cyclotron collision frequency in the solar atmosphere varies from 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-6</sup> to 10, while in HELIX the ratio varies from about 0.02 to 0.5. A new low-frequency internal wave-launching antenna has been used by the West Virginia University helicon source group to induce azimuthal (sheer) magnetic field perturbations close to the high-density core and across the strong density gradient region. A small-scale, translatable magnetic sense coil is used to measure the propagation behavior of these low-frequency waves. Cross-correlation measurements are used to understand how wave phase speed changes across the plasma column and wavelet analysis is applied to characterize how the density affects the wave power and coupling. This work builds on previous low-frequency wave launching studies conducted in HELIX and extends them with more precise analysis techniques. For the first time, Laser Induced Fluorescence (LIF) measurements of the perturbed and unperturbed plasma column are compared the show the heating effect of the Alfvén waves.