Propagation of type III solar radio burst exciters and plasma density fluctuations

Azzollini, Francesco (2025) Propagation of type III solar radio burst exciters and plasma density fluctuations. PhD thesis, University of Glasgow.

Abstract

Solar flare accelerated electron beams travel along open magnetic field lines in the solar corona and interplanetary (IP) medium and can interact with the local plasma to produce Langmuir waves and subsequently trigger intense radio emissions known as type III solar radio bursts. These bursts serve as a crucial diagnostic tool for advancing our understanding of electron transport in the inner heliosphere and as potential early indicators of hazardous space weather events. Despite the rapid quasilinear relaxation of electron beams towards a plateau in velocity space, observations suggest significant propagation distances, a challenge referred to as Sturrock’s dilemma. Here, we develop a novel electron transport model by introducing a self-consistently evolving quasilinear time/distance. The resulting nonlinear advection-diffusion equation predicts super-diffusive, ballistic-like expansion of the beam; analytical predictions are consistent with the results of numerical simulations using kinetic equations and can account for some observed characteristics of type III solar radio bursts. A complementary analysis using spacecraft data from SolO/RPW, PSP/RFS, and STEREO A/WAVES enables us to derive the speeds and accelerations of type III exciters from isolated bursts associated with flares of well-characterized angular positions. For the first time, this analysis allows the correction of velocities and accelerations for the angular separation between the spacecraft and the apparent source. The observed rate of change of velocity with heliocentric distance is then compared to theoretical predictions for a beam-plasma structure propagating through a background plasma of decreasing density, with energy loss attributed to the negative shift in velocity space of Langmuir waves and their subsequent absorption by the Maxwellian component of the plasma, shedding light on the mechanisms driving energy dissipation in beam-plasma structures. Additionally, we investigate the impact of compressive waves in the turbulent solar atmosphere on radio wave propagation through the solar corona and solar wind. Using a new anisotropic density fluctuation model from the kinetic scattering theory for type III radio bursts, we infer the plasma velocities needed to explain observed spacecraft signal frequency broadening. At heliocentric distances beyond 10 R⊙, the velocities align with solar wind flows, while closer to the Sun (≲ 10 R⊙), the broadening implies additional radial and transverse speeds consistent, respectively, with sound or proton thermal speeds and non-thermal motions measured via coronal Doppler-line broadening, interpreted as Alfvénic fluctuations. The energy deposition rates due to ion-sound wave damping peak at a heliocentric distance of ∼(1 − 3) R⊙ and are comparable to the rates available from a turbulent cascade of Alfvénic waves at large scales, suggesting a coherent picture of energy transfer, via the cascade or/and parametric decay of Alfvén waves to the small scales where heating takes place.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Additional Information: Supported by funding from the Science and Technology Facilities Council (STFC).
• Subjects: Q Science > QB Astronomy; Q Science > QC Physics
• Colleges/Schools: College of Science and Engineering > School of Physics and Astronomy
• Funder's Name: Science and Technology Facilities Council (STFC)
• Supervisor's Name: Kontar, Professor Eduard
• Date of Award: 2025
• Depositing User: Theses Team
• Unique ID: glathesis:2025-85511
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 09 Oct 2025 13:09
• Last Modified: 20 Oct 2025 06:52
• Thesis DOI: 10.5525/gla.thesis.85511
• URI: https://theses.gla.ac.uk/id/eprint/85511

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