Wave motion in small scale features in the lower solar atmosphere

Abstract

This thesis concerns itself with the wave motion of small scale features in the lower solar atmosphere and their potential for the transportation of energy for heating.

Using high-cadence imaging observations from the Hydrogen-alpha Rapid Dynamics camera imaging system on the Dunn Solar Telescope, an investigation of the statistical properties of transverse oscillations in spicules captured above the solar limb is presented. At five equally separated atmospheric heights, spanning approximately 4900-7500 km, a total of 15,959 individual wave events have been detected, with a mean displacement amplitude of 151 km (σ = 124 km), a mean period of 54 s (σ = 45 s), and a mean projected velocity amplitude of 21 km s⁻¹ (σ = 13 km s⁻¹). Both the mean displacement and velocity amplitudes increase with height above the solar limb, ranging from 132 km (σ = 111 km) and 17.7 km s⁻¹ (σ = 10.6 km s⁻¹) at ≈ 4900 km, and 168 km (σ = 125 km) and 26.3 km s⁻¹ (σ = 14.1 km s⁻¹) at ≈ 7500 km, respectively. Following the examination of neighbouring oscillations in time and space, 45% of the waves are found to be upwardly propagating, 49% to be downwardly propagating, and 6% to be standing, with mean absolute phase velocities for the propagating waves on the order of 75 − 150 km s⁻¹. While the energy flux of the waves propagating downwards does not appear to depend on height, the energy flux of the upwardly propagating waves decreases with atmospheric height at a rate of −13, 200 ± 6500 W m⁻²/Mm. As a result, this decrease in energy flux as the waves propagate upwards may provide significant thermal input into the local plasma.

The magnetic geometry of the solar atmosphere, combined with projection effects, makes it difficult to accurately map the propagation of ubiquitous waves in fibrillar structures. The Interferometric BIdimensional Spectrometer at the Dunn Solar Telescope was employed to capture high-resolution Hα spectral scans of a sunspot, with the transverse oscillations of a prominent super-penumbral fibril examined in depth. The oscillations are reprojected from the helioprojective Cartesian frame to a new frame of reference oriented along the average fibril axis through non-linear force-free field extrapolations. The fibril was found to be carrying an elliptically polarised, propagating kink oscillation with a period of 430 s and a phase velocity of 69 ± 4 km s-1. The oscillation is damped as it propagates away from the sunspot with a damping length of approximately 9.2 Mm, resulting in the energy flux decreasing at a rate on the order of 460 W m^-2/Mm. The Hα line width is examined and found to increase with distance from the sunspot, a potential sign of a temperature increase. Different linear and non-linear mechanisms are investigated for the damping of the wave energy flux, but a first-order approximation of their combined effects is insufficient to recreate the observed damping length by a factor of at least 3. It is anticipated that the reprojection methodology demonstrated in this thesis will aid with future studies of transverse waves within fibrillar structures.

Date of Award	Jul 2025
Original language	English
Awarding Institution	Queen's University Belfast
Sponsors	The Leverhulme Trust
Supervisor	David Jess (Supervisor), Michail Mathioudakis (Supervisor) & Samuel Grant (Supervisor)