The “SEP Clock”: A Discussion of First Proton Arrival Times in Wide-Spread Solar Energetic Particle Events

• Published in: Solar physics Vol. 299; no. 9; p. 126
• Main Authors: Posner, A., Richardson, I. G., Strauss, R. D.-T.
• Format: Journal Article
• Language: English
• Published: Dordrecht Springer Netherlands 01-09-2024; Springer Nature B.V
• Subjects: Astronomy; Astrophysics and Astroparticles; Atmospheric Sciences; Energetic particles; Longitude; Observations and Techniques; Observatories; Physics; Physics and Astronomy; Protons; SOHO Mission; Solar longitude; Space Exploration and Astronautics; Space Sciences (including Extraterrestrial Physics; Spacecraft; Solar energetic particles; Solar radio bursts; Particle transport
• ISSN: 0038-0938; 1573-093X
• DOI: 10.1007/s11207-024-02350-7

This work analyzes the appearance of wide-spread deka-MeV solar energetic proton (SEP) events, in particular the arrival of the first protons within ≈ 4.5 – 45 MeV measured at Earth–Sun L1, and their relationship with their relative solar source longitude. The definition of “wide-spread SEP event” for this study refers to events that are observed as a 25 MeV proton intensity increase at near 1 AU locations that are separated by at least 130
∘
in solar longitude. Many of these events are seen at all three of the spacecraft, STEREO (Solar-Terrestrial Relations Observatory) A, STEREO B, and SOHO (Solar and Heliospheric Observatory), and may therefore extend far beyond 130
∘
in longitude around the Sun. A large subset of these events have already been part of a study by Richardson et al. (
Solar Phys
.,
289
, 3059, 2014). The event source region identifications draw from this study; more recent events have also been added. Our focus is on answering two specific questions: (1) What is the maximum longitude over which SEP protons show energy dispersion, i.e., a clear sign of arrival of higher-energy protons before those of lower energy? (2) What implications can be drawn from the ensemble of events observed regarding either direct magnetic connectivity to shocks and/or cross-field transport from the site of the eruption in the onset phase of the event?