Solar energetic particle event onsets at different heliolongitudes: The effect of turbulence in Parker spiral geometry

Abstract

Context. Solar energetic particles (SEPs), accelerated during solar eruptions, are observed to rapidly reach a wide heliolongitudinal range in the interplanetary space. Turbulence-associated SEP propagation across the mean Parker spiral direction has been suggested to contribute to this phenomenon. Aims. We study SEP propagation in turbulent magnetic fields to evaluate SEP spatial distribution in the heliosphere, their path lengths, and the overall evolution of SEP intensities at 1 au. Methods. We use full-orbit test particle simulations of 100-MeV protons in a novel analytic model of the turbulent heliospheric magnetic field, where the turbulence is dominated by modes that are transverse and 2D with respect to the Parker spiral direction. Results. We find that by propagating along meandering field lines, SEPs reach a 60°-wide heliolongitudinal range at 1 au within an hour of their injection for the turbulence parameters considered. The SEP onset times are asymmetric with respect to the location connected to the source along the Parker spiral, with the earliest arrival times being 15° westwards from the well-connected Parker spiral longitude. The inferred path length of the first arriving particles is 1.5−1.8 au within 30° of the well-connected longitude; 20−30% longer than the length of the random-walking field lines, increasing monotonously at longitudes further away; and 30−50% longer than the Parker spiral. The global maximum intensity is reached 15° west from the well-connected longitude an hour after the SEP injection. Subsequently, the SEP distribution broadens, consistent with diffusive spreading of SEPs across the field lines. Conclusions. Our results indicate that magnetic field line meandering can explain rapid access of SEPs to wide longitudinal ranges, as well as several other features of SEP event intensity evolution.

Variation in Path Lengths of Turbulent Magnetic Field Lines and Solar Energetic Particles

“…Their work confirms that the peak field line path length is reasonably well described by the simple expression of Chhiber et al (2021a), which corresponds to Equation (9) in the present work. Their corresponding results for 100 MeV proton path lengths (Laitinen et al 2023b) are also comparable to the results in our Table 1 for 1 MeV protons with b/B 0 = 0.75, when accounting for the 0.17 au path length difference along the mean field lines.…”

“…But farther from the Sun, as this process weakens, pitch-angle scattering drives the distribution to larger values of the pitch angle (Ruffolo & Khumlumlert 1995). Simulated path length distributions have also been presented for field lines by Laitinen et al (2023a) and Li & Bian (2023) and for energetic particles by Laitinen et al (2023b). Various studies recognize that while nominal path lengths are computed relative to a typical Parker spiral magnetic field (Parker 1958), it is worth noting that inside an interplanetary magnetic flux rope (e.g., inside an interplanetary coronal mass ejection), path lengths of field lines and particles can be much longer (e.g., Larson et al 1997;Wimmer-Schweingruber et al 2023).…”

On the Onset Delays of Solar Energetic Electrons and Protons: Evidence for a Common Accelerator

“…For a fixed source this usually implies more efficient particle transport toward the west of the best-connected HMF line, as these particles are transported away from the Sun, leading to asymmetrical distributions in terms of longitude. In this context, see also the recent simulations of Laitinen et al (2023). (2017), and Bruno & Richardson (2021).…”

Statistical Study of the Correlation between Solar Energetic Particles and Properties of Active Regions

“…This can be related to the fact that more complex ARs generate stronger flares (in terms of their soft X-ray class) that statistically result in faster and wider CMEs. The CME width was recently indicated to be an important parameter for SEP forecasting (Torres et al 2022); on the other hand, the recent work by Laitinen et al (2023) indicated that SEPs can arrive from a wide range of longitudes, even without a wide particle source. We also notice that the AR records close to the western limb and behind it are extrapolated records, and for them the AR class is assumed to be unchanged from the last reliable observations.…”