Peak-intensity energy spectra of intense solar energetic electron events measured with Solar Orbiter in 2020-2022

The Sun is the most efficient particle accelerator in the solar system, capable of accelerating particles such as electrons and protons to relativistic energies. Solar Energetic Particles (SEPs) are known to be accelerated both at solar flare reconnection sites and by shocks driven by coronal mass ejections. One way to distinguish between these two SEP acceleration mechanisms is through their energy spectra, either fluence or peak intensity.
While the spectral breaks commonly observed in solar energetic electron (SEE) spectra may represent signatures of the acceleration process, several transport-related effects have also been proposed as their origin. In this study, we analyse the energy spectra of intense SEE events measured by Solar Orbiter’s Energetic Particle Detector (EPD) between December 2020 and December 2022. EPD’s unprecedented energy resolution enables us to identify spectral features with greater detail than previously possible.
We investigate the shape of SEE spectra by fitting them with a range of mathematical models. Our results are compared with previous studies, and we explore possible connections to transport-related effects. In addition, we examine potential correlations between spectral features and parameters such as radial distance or properties of the associated solar events.
Our analysis reveals four distinct spectral shapes: single power-law, double power-law, and two types of triple power-law spectra, namely knee–knee (KK) and ankle–knee (AK) forms. No significant correlations with radial distance are found. However, the observed spectral shapes exhibit an ordering with respect to the longitudinal separation between the spacecraft and the associated solar flare.
We conclude that multiple processes likely contribute to shaping SEE spectra. Our results suggest that the two breaks observed in KK triple power-law spectra arise from distinct physical effects, namely Langmuir-wave generation and pitch-angle scattering. Furthermore, the break in double power-law spectra may represent a merger of the first and second breaks seen in KK triple power-law spectra.