The first year of energetic particle measurements in the inner heliosphere with Solar Orbiter's Energetic Particle Detector


Context. Solar Orbiter promises to unveil how the Sun controls and shapes the heliosphere and fills it with energetic particle radiation. To this end, its Energetic Particle Detector (EPD) has now been operating and providing excellent data for just over a year. Aims. EPD measures suprathermal and energetic particles in the energy range from a few keV up to (near-) relativistic energies (few MeV for electrons and about 500 MeV/nuc for ions). We present an overview of initial results from the first year of operations and provide a first assessment of issues and limitations, but also present areas where EPD excels and provides opportunities for significant scientific progress in understanding how our Sun shapes the heliosphere. Methods. We use the solar particle events observed by Solar Orbiter on 21 July and 10/11 December 2020 to discuss the capabilities, updates on them, and open issues of EPD on Solar Orbiter. We also give some words of caution and caveats about how to use data from the EPD. Results. During this first year of the Solar Orbiter mission, EPD has recorded several particle events at distances between 0.5 and 1 au from the Sun. We present dynamic and time-averaged energy spectra for ions which were measured with a combination of all four EPD sensors (the SupraThermal Electron and Proton sensor (STEP), the Electron Proton Telescope (EPT), the Supathermal Ion Spectrograph (SIS), and the High- Energy Telescope (HET)) and associated energy spectra for electrons measured with STEP and EPT.We illustrate the capabilities of the EPD suite using the 10/11 December 2020 solar particle event. This event showed an enrichment of heavy ions as well as 3He for which we also present dynamic spectra measured with SIS. The high anisotropy of electrons at the onset of the event and its temporal evolution is also shown using data from these sensors. We discuss the ongoing in-flight calibration and a few open instrumental issues using data from the 21 July and the 10/11 December 2020 events and give guidelines and examples for the usage of the EPD data. We explain how spacecraft operations may affect EPD data and present a list of such time periods in the appendix. A list of the most significant particle enhancements as observed by EPT during this first year is also provided.