A new antenna for varying the perpendicular wavelength of an Alfvén wave over a wide range

Alfvén waves are one of the fundamental modes of magnetized plasmas and are frequently invoked to address aspects of the coronal heating problem. Reflection of Alfvén waves from gradients in Alfvén speed is considered a key mechanism in the heating of coronal holes and the acceleration of the fast solar wind. We carried out experiments on Alfvén wave reflection and found that the coefficient of reflection was smaller than predicted by single fluid magnetohydrodynamic (MHD) theory [1]. In another experiment, we observed a surprising damping of wave energy in the Alfven speed gradient which was also not predicted by MHD theory [2]. Our hypothesis is that two-fluid effects play a role in wave damping and reflection. Since the perpendicular wavelength (λ_⊥) of an Alfvén wave is known to influence two-fluid effects we designed a new antenna capable of varying λ_⊥ of Alfvén waves over a wide range, enabling the study of the role of two-fluid effects on Alfvén wave propagation, damping, and reflection. The design of the antenna is motivated by Gekelman et al [3]. The wave excitation circuit was optimized to increase the amplitude of the Alfvén wave without distorting the waveform. We present data from initial experiments to demonstrate the antenna’s ability to excite a broad range of λ_⊥ values and reveal the sharpness of the perpendicular wave number spectrum across different antenna configurations. In the near future, we plan to perform damping and reflection experiments with the antenna. In the near future, we plan to perform damping and reflection experiments with the antenna.

References

1. Bose et al., ApJ, 971, 72 (2024)

2. Bose et al., ApJ, 882 183 (2019)

3. Gekelman et al., RSI. 90, 083505 (2019)