The effect of turbulence strength on meandering field lines and Solar Energetic Particle event extents

Abstract

-Insights into the processes of Solar Energetic Particle (SEP) propagation are essential for understanding how solar eruptions affect the radiation environment of near-Earth space. SEP propagation is influenced by turbulent magnetic fields in the solar wind, resulting in stochastic transport of the particles from their acceleration site to Earth. While the conventional approach for SEP modelling focuses mainly on the transport of particles along the mean Parker spiral magnetic field, multi-spacecraft observations suggest that the cross-field propagation shapes the SEP fluxes at Earth strongly. However, adding cross-field transport of SEPs as spatial diffusion has been shown to be insufficient in modelling the SEP events without use of unrealistically large cross-field diffusion coefficients. Recently, Laitinen et al. [ApJL 773 (2013b); A&A 591 (2016)] demonstrated that the early-time propagation of energetic particles across the mean field direction in turbulent fields is not diffusive, with the particles propagating along meandering field lines. This early-time transport mode results in fast access of the particles across the mean field direction, in agreement with the SEP observations. In this work, we study the propagation of SEPs within the new transport paradigm, and demonstrate the significance of turbulence strength on the evolution of the SEP radiation environment near Earth. We calculate the transport parameters consistently using a turbulence transport model, parametrised by the SEP parallel scattering mean free path at 1 AU, l à k , and show that the parallel and cross-field transport are connected, with conditions resulting in slow parallel transport corresponding to wider events. We find a scaling s ';max ∝ð1=l à k Þ 1=4 for the Gaussian fitting of the longitudinal distribution of maximum intensities. The longitudes with highest intensities are shifted towards the west for strong scattering conditions. Our results emphasise the importance of understanding both the SEP transport and the interplanetary turbulence conditions for modelling and predicting the SEP radiation environment at Earth.

Comparing Long-duration Gamma-Ray Flares and High-energy Solar Energetic Particles

“…Some studies reported that the centers of the derived SEP distributions tend to lie west of the flare location (Lario et al 2006(Lario et al , 2013Cohen et al 2017), although this trend was not confirmed by other analyses (e.g., Richardson et al (2014)). Lario et al (2014) suggested that this displacement would be likely due to the fact that the maximum peak intensity is observed at some helioradial distance for which the eruption occurs eastward with respect to the footpoints of the nominal field line connecting the spacecraft with the Sun.…”

Radio observatories and instrumentation used in space weather science and operations

“…Despite efforts in forecasting SEPs, which sometimes involve solar radio bursts (Balch, 1999, Zucca et al, 2017, SEP arrival time prediction remains one of the most difficult aspects of space weather. Much of the difficulty is in determining SEP origin (either flare or shock see e.g., Kahler, 2007), and determining the complicated particle injection and transport physics of SEPs through the heliosphere (e.g., Agueda & Lario, 2016;Lario et al, 2017;Dresing et al, 2018;Laitinen et al, 2018).…”

On the Pitch-angle-dependent Perpendicular Diffusion Coefficients of Solar Energetic Protons in the Inner Heliosphere

“…Due to the significance of pitch-angle scattering in the transport of SEPs, many studies focus on the pitch-angle diffusion coefficient D μμ and its role in their diffusion parallel to some mean field (e.g., Karimabadi et al 1992;Reames 1999;He & Wan 2012;Agueda & Vainio 2013). To date, however, fewer studies have focused on the pitch-angledependent Fokker-Planck perpendicular diffusion coefficient D ⊥ (see, however, Qin & Shalchi 2014;Wang et al 2017), which enters directly into the Skilling (1971) equation solved in computational studies of SEP transport and, thus, could potentially play a crucial role in studies of space weather (e.g., Schwadron et al 2014;Laitinen et al 2018). These particle transport simulations, however, have shown that calculated SEP intensities and anisotropies are sensitive to the magnitude as well as the pitch-angle dependence of this quantity (Zhang et al 2009;Dresing et al 2012;Laitinen et al 2013Laitinen et al , 2018Strauss & Fichtner 2015;Strauss et al 2017).…”

“…To date, however, fewer studies have focused on the pitch-angledependent Fokker-Planck perpendicular diffusion coefficient D ⊥ (see, however, Qin & Shalchi 2014;Wang et al 2017), which enters directly into the Skilling (1971) equation solved in computational studies of SEP transport and, thus, could potentially play a crucial role in studies of space weather (e.g., Schwadron et al 2014;Laitinen et al 2018). These particle transport simulations, however, have shown that calculated SEP intensities and anisotropies are sensitive to the magnitude as well as the pitch-angle dependence of this quantity (Zhang et al 2009;Dresing et al 2012;Laitinen et al 2013Laitinen et al , 2018Strauss & Fichtner 2015;Strauss et al 2017). Furthermore, it has been shown that particle transport across magnetic discontinuities is also sensitive to the pitch-angle dependence of D ⊥ (e.g., Strauss & Fichtner 2014, 2015Strauss et al 2016).…”