Letter to the Editor

Evidence for local particle acceleration in the first recurrent galactic cosmic ray depression observed by Solar Orbiter

The ion event on 19 June 2020^⋆

¹ Dept. de Física Quàntica i Astrofísica, Institut de Ciències del Cosmos (ICCUB), University de Barcelona (UB-IEEC), Barcelona, Spain
e-mail: angels.aran@fqa.ub.edu
² Institute of Experimental and Applied Physics, Kiel University, 24118 Kiel, Germany
³ INAF/Istituto di Astrofisica e Planetologia Spaziali, Via del Fosso del Cavaliere 100, 00133 Roma, Italy
⁴ Centre for mathematical Plasma-Astrophysics, Dept. of Mathematics, KU Leuven, Celestijnenlaan 200B, 3001 Leuven, Belgium
⁵ Heliophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁶ Dept. of Astronomy, University of Maryland, Maryland, MD 20742, USA
⁷ Solar-Terrestrial Centre of Excellence – SIDC, Royal Observatory of Belgium, 1180 Brussels, Belgium
⁸ Now at Paradox Cat GmbH, 80333 Mnchen, Germany
⁹ George Mason University, Fairfax, VA 22030, USA
¹⁰ Universidad de Alcalá, Space Research Group, 28805 Alcalá de Henares, Spain
¹¹ Swedish Institute of Space Physics (IRF), Uppsala, Sweden
¹² Space Plasma Physics, Department of Physics and Astronomy, Uppsala University, Uppsala 75120, Sweden
¹³ LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris, 5 place Jules Janssen, 92195 Meudon, France
¹⁴ Radboud Radio Lab, Dept. of Astrophysics, Radboud University, Nijmegen, The Netherlands
¹⁵ Goddard Planetary Heliophysics Institute, University of Maryland, Baltimore County, Baltimore, MD, USA
¹⁶ Institute of Physics, University of Maria Curie-Skłodowska, ul. Radziszewskiego 10, 20-031 Lublin, Poland
¹⁷ Johns Hopkins University Applied Physics Laboratory, Laurel, MD, USA
¹⁸ Dept. of Physics, Imperial College London, London SW7 2AZ, UK
¹⁹ Now at DSI Datensicherheit GmbH, Rodendamm 34, 28816 Stuhr, Germany
²⁰ Institut de Recherche en Astrophysique et Planétologie, 9 avenue du Colonel Roche, BP 4346, 31028 Toulouse Cedex 4, France
²¹ European Space Agency (ESA), European Space Astronomy Centre (ESAC), Villanueva de la Cañada, Madrid, Spain
²² Now at Deutsches Elektronen-Synchrotron (DESY), Platanenallee 6, 15738 Zeuthen, Germany
²³ National Observatory of Athens, Institute for Astronomy, Astrophysics, Space Applications and Remote Sensing, Athens, Greece
²⁴ Now at German Aerospace Center (DLR), Dept. of Extrasolar Planets and Atmospheres, Berlin, Germany
²⁵ Dept. of Space and Climate Physics, University College London, Holmbury St Mary, Dorking RH5 6NT, UK
²⁶ Now at Max-Planck-Institute for Solar System Research, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
²⁷ Now at University of Colorado/LASP, Boulder, CO, USA
²⁸ Skobeltsyn Institute of Nuclear Physics, Moscow State University, Moscow, Russia

Received: 31 March 2021
Accepted: 1 August 2021

Abstract

Context. In mid-June 2020, the Solar Orbiter (SolO) mission reached its first perihelion at 0.51 au and started its cruise phase, with most of the in situ instruments operating continuously.

Aims. We present the in situ particle measurements of the first proton event observed after the first perihelion obtained by the Energetic Particle Detector (EPD) suite on board SolO. The potential solar and interplanetary (IP) sources of these particles are investigated.

Methods. Ion observations from ∼20 keV to ∼1 MeV are combined with available solar wind data from the Radio and Plasma Waves (RPW) instrument and magnetic field data from the magnetometer on board SolO to evaluate the energetic particle transport conditions and infer the possible acceleration mechanisms through which particles gain energy. We compare > 17–20 MeV ion count rate measurements for two solar rotations, along with the solar wind plasma data available from the Solar Wind Analyser (SWA) and RPW instruments, in order to infer the origin of the observed galactic cosmic ray (GCR) depressions.

Results. The lack of an observed electron event and of velocity dispersion at various low-energy ion channels and the observed IP structure indicate a local IP source for the low-energy particles. From the analysis of the anisotropy of particle intensities, we conclude that the low-energy ions were most likely accelerated via a local second-order Fermi process. The observed GCR decrease on 19 June, together with the 51.8–1034.0 keV nuc⁻¹ ion enhancement, was due to a solar wind stream interaction region (SIR). The observation of a similar GCR decrease in the next solar rotation favours this interpretation and constitutes the first observation of a recurrent GCR decrease by SolO. The analysis of the recurrence times of this SIR suggests that it is the same SIR responsible for the ⁴He events previously measured in April and May. Finally, we point out that an IP structure more complex than a common SIR cannot be discarded, mainly due to the lack of solar wind temperature measurements and the lack of a higher cadence of solar wind velocity observations.