First year of energetic particle measurements in the inner heliosphere with Solar Orbiter’s Energetic Particle Detector

¹ Institute of Experimental and Applied Physics, Kiel University, 24118 Kiel, Germany
² European Space Astronomy Center, Villanueva de la Cañada, 28692 Madrid, Spain
³ Universidad de Alcalá, Space Research Group, 28805 Alcalá de Henares, Spain
⁴ Johns Hopkins University Applied Physics Laboratory, Laurel, MD, USA
⁵ Now at Paradox Cat GmbH, Brienner Str. 53, 80333 München, Germany
⁶ Department of Physics, Imperial College London, organ dyLondon SW7 2AZ, UK
⁷ Departament de Física Quàntica i Astrofísica, Institut de Ciències del Cosmos (ICCUB), Universitat de Barcelona (UB-IEEC), Barcelona, Spain
⁸ Now at DSI Datensicherheit GmbH, Rodendamm 34, 28816 Stuhr, Germany
⁹ Department of Physics and Astronomy, University of Turku, Turku, Finland
¹⁰ Département d’Astrophysique, Commissariat l’énergie atomique et aux énergies alternatives, CEA, Saclay, France
¹¹ Now at Deutsches Elektronen-Synchrotron (DESY), Platanenallee 6, 15738 Zeuthen, Germany
¹² Heliophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
¹³ National Observatory of Athens, Institute for Astronomy, Astrophysics, Space Applications and Remote Sensing, Athens, Greece
¹⁴ Now at German Aerospace Center (DLR), Department of Extrasolar Planets and Atmospheres, Berlin, Germany
¹⁵ Now at Max-Planck-Institute for Solar System Research, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
¹⁶ Now at Univ. Colorado/LASP, Boulder, CO, USA

Received: 30 March 2021
Accepted: 10 July 2021

Abstract

Context. Solar Orbiter strives 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 in operation, 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⁻¹ for ions). We present an overview of the initial results from the first year of operations and we provide a first assessment of issues and limitations. In addition, we present areas where EPD excels and provides opportunities for significant scientific progress in understanding how our Sun shapes the heliosphere.

Methods. We used the solar particle events observed by Solar Orbiter on 21 July and between 10 and 11 December 2020 to discuss the capabilities, along with updates and open issues related to EPD on Solar Orbiter. We also give some words of caution and caveats related to the use of EPD-derived data.

Results. During this first year of operations 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 that were measured with a combination of all four EPD sensors, namely: the SupraThermal Electron and Proton sensor (STEP), the Electron Proton Telescope (EPT), the Suprathermal Ion Spectrograph (SIS), and the High-Energy Telescope (HET) as well as the associated energy spectra for electrons measured with STEP and EPT. We illustrate the capabilities of the EPD suite using the 10 and 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 and 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 we 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.