Unusually long path length for a nearly scatter-free solar particle event observed by Solar Orbiter at 0.43 au

¹ Institute of Experimental and Applied Physics, Christian-Albrechts-University Kiel, Leibnizstraße 11, 24118 Kiel, Germany
e-mail: wimmer@physik.uni-kiel.de
² Universidad de Alcalá, Space Research Group (SRG-UAH), Plaza de San Diego s/n, 28801 Alcalá de Henares, Madrid, Spain
³ Johns Hopkins University Applied Physics Laboratory, 11100 Johns Hopkins Rd, Laurel, MD 20723, USA
⁴ Physikalisch-Meteorologisches Observatorium Davos/World Radiation Center, Dorfstrasse 33, 7260 Davos Dorf, Switzerland
⁵ Institut für Teilchenphysik und Astrophysik, Eidgenössische Technische Hochschule Zürich, Wolfgang-Pauli-Str. 27, 8093 Zürich, Switzerland
⁶ School of Earth and Space Sciences, No. 2 Yifu Building, Peking University, No.5 Yiheyuan Road, Haidian District, Beijing 100871, PR China
⁷ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
⁸ NASA Goddard Space Flight Center, Heliophysics Science Division, Mail code 672, Greenbelt, MD 20771, USA
⁹ Department of Physics, The Catholic University of America, 620 Michigan Ave., N.E. Washington, DC 20064, USA
¹⁰ Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK
¹¹ Southwest Research Institute, 6220 Culebra Road, San Antonio, TX 78238, USA
¹² National Observatory of Athens, Institute for Astronomy, Astrophysics, Space Applications and Remote Sensing (IAASARS), Vas. Pavlou & I. Metaxa, GR-15 236 Penteli, Greece
¹³ Solar-Terrestrial Centre of Excellence – SIDC, Royal Observatory of Belgium, Ringlaan -3- Av. Circulaire, 1180 Brussels, Belgium

Received: 4 March 2023
Accepted: 21 August 2023

Abstract

Context. After their acceleration and release at the Sun, solar energetic particles (SEPs) are injected into the interplanetary medium and are bound to the interplanetary magnetic field (IMF) by the Lorentz force. The expansion of the IMF close to the Sun focuses the particle pitch-angle distribution, and scattering counteracts this focusing. Solar Orbiter observed an unusual solar particle event on 9 April 2022 when it was at 0.43 astronomical units (au) from the Sun.

Aims. We show that the inferred IMF along which the SEPs traveled was about three times longer than the nominal length of the Parker spiral and provide an explanation for this apparently long path.

Methods. We used velocity dispersion analysis (VDA) information to infer the spiral length along which the electrons and ions traveled and infer their solar release times and arrival direction.

Results. The path length inferred from VDA is approximately three times longer than the nominal Parker spiral. Nevertheless, the pitch-angle distribution of the particles of this event is highly anisotropic, and the electrons and ions appear to be streaming along the same IMF structures. The angular width of the streaming population is estimated to be approximately 30 degrees. The highly anisotropic ion beam was observed for more than 12 h. This may be due to the low level of fluctuations in the IMF, which in turn is very probably due to this event being inside an interplanetary coronal mass ejection The slow and small rotation in the IMF suggests a flux-rope structure. Small flux dropouts are associated with very small changes in pitch angle, which may be explained by different flux tubes connecting to different locations in the flare region.

Conclusions. The unusually long path length along which the electrons and ions have propagated virtually scatter-free together with the short-term flux dropouts offer excellent opportunities to study the transport of SEPs within interplanetary structures. The 9 April 2022 solar particle event offers an especially rich number of unique observations that can be used to limit SEP transport models.