Lorfing, Camille Yasmina; (2024) Solar electron beams and Langmuir waves: simulations and observations of a match made in the heliosphere. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

How do solar energetic electrons interact with plasma waves and what are the signatures of these interactions in the electron spectra? The first study in this thesis simulates the transport of solar energetic electron beams in the heliospheric plasma under different initial beam density and velocity spectral index scenarios. We study the wave-particle interactions during the beam’s journey, focusing on what electron velocities resonate with the background plasma of the solar wind to grow Langmuir waves. We find that the velocity of the electrons that are responsible for Langmuir wave growth decreases with heliocentric distance as r−0.5. We also find that a higher velocity spectral index leads to a lower maximum electron velocity interacting with Langmuir waves, and vice versa for the initial beam density. This provides us with a new tool to understand the transport effects on the electron beam kinetics and arrival time at Earth, and their space weather implications. The second study in this thesis associates Solar Orbiter (European Space Agency) in-situ and remote sensing data for 42 events between 0.5 and 1 au. It answers an outstanding science question of the Solar Orbiter science activity plan. Analysing solar electron flux, type III radio dynamic spectra, Langmuir wave intensity, electron spectra, pitch angle distributions, X-Ray spectra and extreme UV images, this solar study is the first to combine measurements from all four in-situ instruments onboard Solar Orbiter and the first one to link different electron populations measured by different instruments onboard the spacecraft. The very high temporal resolution of the Solar Orbiter in-situ data allows us to observe for the first time the appearance of a plateau in the electron distribution function that results from wave-particle interactions. This effect was theorised and shown by numerous simulation work but never observed in-situ due to the poor temporal resolution of previous missions. We also show that the wave-particle interactions are responsible for the appearance of breaks in the electron spectrum at the same energies where these interactions happen. This study presents 3 example events to illustrate the signatures of wave-particle interactions in electron spectra in the deca-keV range. The third study presented in this thesis is a pilot study focusing on on the relationship between different metrics that link Langmuir waves and the type III solar radio bursts they produce. Using exclusively Solar Orbiter’s RPW instrument for 17 events between 0.5 and 1 au, we compare the frequency of the maximum of the solar radio emission, the maximum of the Langmuir wave intensity and of the type III solar radio burst, and look at the evolution of these metrics with heliocentric distance. If most metrics display a low negative correlation coefficient around -0.2, the maximum radio emission and Langmuir wave intensities seem to be more strongly correlated at 0.52 independently of distance from the solar surface. The last part of this project looks at the electron density fluctuations and the Langmuir wave intensity fluctuations and aims to identify a proportionality relation between both. We obtain promising results for the fractional density and Langmuir wave spectrum fluctuations for 3 events between July 2020 and April 2022. The conclusions reached in this study are preliminary and it is necessary to extend the analysis to a greater number of events that reach Solar Orbiter’s closest approach.

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