Investigating electrons in SEP events observed by Solar Orbiter/EPD/STEP and Solar Orbiter/SWA/EAS with Velocity Dispersion Analysis

The Sun constantly emits a stream of charged particles, e.g. electrons and protons which is called the solar wind. In addition to this low energetic background, Solar Energetic Particle (SEP) events occur and are observed by Solar Orbiter in the inner heliosphere. Here, we are interested in the electron component of SEP events. The Electron Analyzer System (EAS) and the SupraThermal Electron Proton (STEP) sensor on Solar Orbiter measure electrons in the energy range from 1 eV to 5 keV and 2 keV to 60 keV, respectively. The field of view of STEP overlaps with the field of view of the EAS 1 sensor head. SEP events are identified in the STEP data and compared with the electron signal in the EAS data. We utilize this overlap to evaluate the electron measurements in STEP and EAS for at least one selected SEP event. During electron SEP events and times where most of the higher energy bins in EAS 1 are populated, the one dimensional differential energy flux spectra show an overlap within the respective uncertainties. SEP events typically show a velocity dispersion. In a Velocity Dispersion Analysis (VDA), for each energy channel an onset time for the event is determined. Due to the reduced quantum efficiency in the highest energy channels of EAS, higher fluxes are required to detect an SEP event in EAS than in STEP. To increase the signal to noise ratio for the SEP events, EAS bins in all three measurement dimensions, i.e. azimuth, elevation and energy, are chosen depending on the pitch angle coverage of the event. Anisotropic SEP events cover fewer instrumental bins in EAS than isotropic events. To test the uncertainty of the onset times depending on the method several approaches are compared, including a manual identification and the Poisson CUSUM method on the EAS Level 1 count data. The selected event illustrates the importance of considering an energy dependent minimal detection threshold in VDA since VDA relies on the assumption that the earliest detected particles for each energy are indeed the first particles that reach the spacecraft. The VDA is then applied to the STEP data and the results are compared with the EAS results. The instrumental and quantum efficiency driven onset times influence the approximation of the release time of the accelerated particles at the acceleration time, i.e. in the solar corona while neglecting transport effects along the way to the spacecraft. All in all combining EAS and STEP gives us several advantages. (1) It allows us to evaluate the calibration of both instruments. (2) With EAS the VDA is extended to lower energies. (3) In addition the full 360° field of view of EAS helps us to evaluate the anisotropy of SEP events outside the field of view of STEP which are strong enough to produce a signal in the higher EAS energy bins.