Letter to the Editor

Quiet-time low energy ion spectra observed on Solar Orbiter during solar minimum

¹ Johns Hopkins Univ. Applied Physics Laboratory, Laurel, MD, USA
e-mail: glenn.mason@jhuapl.edu
² Institut für Experimentelle und Angewandte Physik, Christian-Albrechts-Universität zu Kiel, Kiel, Germany
³ ESA-ESAC, Madrid, Spain European Space Agency (ESA/ESAC), Villafranca del Castillo, 28692 Madrid, Spain
⁴ Space Research Group, Universidad de Alcalá, Alcalá de Henares, Spain
⁵ Now at Univ. Colorado/LASP, Boulder, CO, USA
⁶ Now at: German Aerospace Center (DLR), Dept. of Extrasolar Planets and Atmospheres, Berlin, Germany
⁷ Now at: DSI Datensicherheit GmbH, Rodendamm 34, 28816 Stuhr, Germany
⁸ Now at: Deutsches Elektronen-Synchrotron (DESY), Platanenallee 6, 15738 Zeuthen, Germany
⁹ Now at: Max-Planck-Institute for Solar System Research, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany

Received: 11 February 2021
Accepted: 16 March 2021

Abstract

Context. The Solar Orbiter spacecraft cruised in the inner heliosphere during Feb. 2020 – Jan. 2021, moving between ∼0.5–1.0 au radial distance. The Energetic Particle Detector suite operated continuously during this period.

Aims. The Suprathermal Ion Spectrograph and High Energy Telescope observations made during intervals in between transient intensity increases were used to determine the low energy ion spectra and composition during quiet times.

Methods. Energetic particle spectra and major ion components, including ³He, were measured over the range ∼0.1–100 MeV nucleon⁻¹. The radial dependence of 4.4 MeV nucleon^{−1 4}He and O was measured. A short interval of extremely low intensities (“super-quiet”) was also studied.

Results. Spectra measured during the quiet period showed transitions, including galactic cosmic rays (> 50 MeV nucleon⁻¹), anomalous cosmic rays (a few to ∼50 MeV nucleon⁻¹), and a steeply rising “turn-up” spectrum below a few MeV nucleon⁻¹ whose composition resembled impulsive, ³He-rich solar energetic particle events. The radial dependence had large uncertainties but was consistent with a small gradient. During the super-quiet interval, the higher energy components remained similar to the quiet period, while the approximately flat low energy ⁴He spectrum extended downward, reaching ∼300 keV nucleon⁻¹ before transitioning to a steeply rising spectrum.