Speaker
Description
Energy spectra of solar energetic particles provide valuable insights into particle acceleration processes. However, also transport effects have been found to potentially alter the spectra, especially in the case of solar energetic electron (SEE) events, which commonly show broken power-law shapes. We analyze the energy spectra of the 50 most intense SEE events measured by Solar Orbiter’s Energetic Particle Detector (EPD) between December 2020 and December 2022. These measurements provide new opportunities to understand the physics shaping SEE spectra due to EPD's unprecedented energy resolution and the spacecraft's varying distance to the Sun. We investigate the shape of SEE peak-intensity spectra by fitting them with various mathematical models. We find five different spectral shapes in our sample: a single power-law (3 events), a double power-law (8 events), a double power-law with exponential cutoff (1 event) and two types of triple power-laws, which have not been observed before: a knee-knee (KK, 10 events) and an ankle-knee (AK, 16 events) triple power-law. Surprisingly, no significant correlations with radial distance were identified, but the observed spectral shapes show an ordering with the longitudinal separation between the spacecraft and the associated solar flare.
A comparison of our results with different transport modeling studies suggest that the two breaks of the KK triple power-law spectra arise from distinct effects, Langmuir-wave generation and pitch-angle scattering respectively, while the AK triple-power law could be due to Langmuir-wave generation including part of the electron beam gaining energy as Langmuir waves are absorbed. We also find evidence for the double power-law events being formed by a merging of the first and second break of KK triple power-laws.
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