Aran, A. ; Pacheco, D. ; Laurenza, M. ; ManyOtherAuthores, X. ; Rodriguez, L. ; Berghmans, D. ; Magdalenic, J. ; Verbeeck, C. ; Zhukov, A.

published in Astronomy and Astrophysics, 656, pp. L10 (2021)

Abstract: Context. In mid-June 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 onboard 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 (MAG) onboard SolO to evaluate the energetic particle transport conditions and infer the possible acceleration mechanisms through which particles gain energy. We compare > 17 – 20 MeV 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, most likely, the low-energy ions were accelerated via a local second-order Fermi process. The observed GCR decrease on 19 June, together with the 51.8 – 1034.0 keV/nuc 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 a more complex IP structure than a common SIR cannot be discarded, mainly due to the lack of solar wind temperature measurements and of a higher cadence of solar wind velocity observations.

Keyword(s): accelerationofparticles ; Sun:heliosphere ; Sun:solarwind ; Solar Orbiter ; EUI
DOI: 10.1051/0004-6361/202140966
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