Issue |
A&A
Volume 639, July 2020
|
|
---|---|---|
Article Number | A105 | |
Number of page(s) | 9 | |
Section | The Sun and the Heliosphere | |
DOI | https://doi.org/10.1051/0004-6361/201937338 | |
Published online | 20 July 2020 |
3D propagation of relativistic solar protons through interplanetary space
1
Jeremiah Horrocks Institute, University of Central Lancashire, Preston PR1 2HE, UK
e-mail: sdalla@uclan.ac.uk
2
NASA Goddard Space Flight Center, Greenbelt, USA
3
University of Arizona, Tucson, USA
4
University of Helsinki, Helsinki, Finland
5
Met Office, UK
Received:
17
December
2019
Accepted:
19
May
2020
Context. Solar energetic particles (SEPs) with energy in the GeV range can propagate to Earth from their acceleration region near the Sun and produce ground level enhancements (GLEs). The traditional approach to interpreting and modelling GLE observations assumes particle propagation which is only parallel to the magnetic field lines of interplanetary space, that is, spatially 1D propagation. Recent measurements by PAMELA have characterised SEP properties at 1 AU for the ∼100 MeV–1 GeV range at high spectral resolution.
Aims. We model the transport of GLE-energy solar protons using a 3D approach to assess the effect of the heliospheric current sheet (HCS) and drifts associated to the gradient and curvature of the Parker spiral. We derive 1 AU observables and compare the simulation results with data from PAMELA.
Methods. We use a 3D test particle model including a HCS. Monoenergetic populations are studied first to obtain a qualitative picture of propagation patterns and numbers of crossings of the 1 AU sphere. Simulations for power law injection are used to derive intensity profiles and fluence spectra at 1 AU. A simulation for a specific event, GLE 71, is used for comparison purposes with PAMELA data.
Results. Spatial patterns of 1 AU crossings and the average number of crossings per particle are strongly influenced by 3D effects, with significant differences between periods of A+ and A− polarities. The decay time constant of 1 AU intensity profiles varies depending on the position of the observer and it is not a simple function of the mean free path as in 1D models. Energy dependent leakage from the injection flux tube is particularly important for GLE energy particles, resulting in a rollover in the spectrum.
Key words: Sun: particle emission / Sun: heliosphere / Sun: activity / magnetic fields
© S. Dalla et al. 2020
Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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