The Crucial Role of Perpendicular Diffusion in the Longitude Distribution of >10 MeV Solar Energetic Protons

Abstract

Gradual solar proton events are thought to consist of solar components originating near the Sun and interplanetary components associated with interplanetary shocks, and the role of interplanetary shocks is considered to be crucial in supplying particles to regions that are not magnetically connected to the solar source region. We calculate the ratios of the peak intensities for the four energy channels (13–16, 20–25, 32–40, and 40–64 MeV) and compare the ratios observed by multiple spacecraft at different locations. We often find that the ratio of peak intensities observed at different locations in the same event remains almost constant as the energy varies. In other words, the ratio of peak intensities from the different energy channels remains almost constant as the position of the spacecraft changes. The phenomenon implies that in many gradual events, energetic particles observed at different locations are mainly composed of solar components that undergo perpendicular diffusion in both the vicinity of the Sun and the interplanetary space, and that perpendicular diffusion is the main factor enabling energetic particles to be observed in regions without magnetic connection to the solar source region.

Numerical Simulation of Equal Ratio Relations for the Peak Intensities of >10 MeV Energetic Protons

“…The longitudinal difference between the source and the observer is different for different spacecraft and different events, so the observed intensities are different because of the perpendicular diffusion. Therefore, the number is different for STB and SOHO and also different for different events (Wang & Qin 2023). For the 13−16 MeV energy channel, STA observed only one peak in the 2010 August 14, 2011 November 3, and 2012 October 14 events, and the peak appeared approximately 3.62, 6.23, and 6.22 hr after the onset time of the SEPs, respectively.…”

“…In our recent work, we studied the peak intensities of singlepeak events observed by STEREO A (STA), STEREO B (STB), and the Solar and Heliospheric Observatory (SOHO) and calculated the peak ratios of four energy channels (13 −16 MeV, 20−25 MeV, 32−40 MeV, and 40−64 MeV;Wang & Qin 2023). We found that the peak ratios observed at different locations in the same SEP event were very similar.…”

“…Gradual solar proton events (SPEs) typically consist of two different components: solar components originating from the vicinity of the solar corona and interplanetary components associated with the interplanetary shock (Cane et al 1988;Qin & Wang 2015;Wang & Qin 2023). It has been found that gradual SPEs can be observed by multiple spacecraft located at different positions in interplanetary space, and the distribution range of energetic particles is often much larger than that of their solar source regions (Wiedenbeck et al 2013).…”

“…Therefore, for energetic protons with energies ranging from tens of keV to several MeV, continuous acceleration by interplanetary shocks does play an important role in particle intensities. However, for energetic protons with energies above 10 MeV, in most events, there is no significant enhancement in particle intensity when the shock passes the spacecraft location (Wang & Qin 2023). Because interplanetary shocks have a large spatial scale and there are only a limited number of spacecraft available for observation, the lack of ESP events does not necessarily imply that interplanetary shocks have no impact on >10 MeV particle intensity.…”

“…Therefore, the acceleration of interplanetary shock has little impact on the peak intensity observed by the three spacecraft in events that meet equal ratio relations. Based on this observation, we proposed an explanation that in gradual energetic proton events that meet equal ratio relations, the events are mainly composed of solar components, and perpendicular diffusion is the primary factor allowing gradual events to be observed in regions with no magnetic connection to the solar source (Wang & Qin 2023). The phenomenon demonstrates that perpendicular diffusion plays a crucial role in the distribution of particle intensity but cannot be explained by interplanetary shock acceleration.…”