Wide longitudinal distribution of interplanetary electrons following the 7 February 2010 solar event: Observations and transport modeling

Abstract

We analyze 65–105 keV electrons in the 7 February 2010 solar electron event observed simultaneously by STEREO‐A, STEREO‐B, and ACE. A method to reconstruct the full‐electron pitch angle distributions from the four Solar Electron and Proton Telescope sensors on STEREO‐A/B and the Solar Electron and Proton Telescope instrument on ACE in the energy range of approximately 60–300 keV for periods of incomplete angular coverage is presented. A transport modeling based on numerical solutions of a three‐dimensional particle propagation model which includes pitch angle scattering and focused transport is applied to the intensity and anisotropy profiles measured on all three spacecraft. Based on an analysis of intensity gradients observed between the three spacecraft, we find that the lateral transport of the electrons occurs partially close to the Sun, due to effects of nonradial divergence of magnetic field lines or particle diffusion, and partially in the interplanetary medium. For the mean free paths characterizing the electron diffusion parallel and perpendicular to the interplanetary magnetic field, we derive values of λ∥∼ 0.1 AU and λ⟂∼ 0.01 AU. In comparison with results from other particle events which we had previously analyzed in a similar manner we discuss whether the diffusion mean free paths parallel and perpendicular to the average magnetic field might be related with each other, and whether the particle transport perpendicular to the average magnetic field is more likely due to particles following meandering magnetic field lines, or due to particles being scattered off individual field lines.

Statistical survey of widely spread out solar electron events observed with STEREO and ACE with special attention to anisotropies

“…Interestingly, two of the three events that lack a type II radio burst are class (2) events, suggesting that a shock may not be the key ingredient for the wide SEP distribution close to the Sun. This also was the conclusion by Dröge et al (2014), who studied one of these events in detail, that of 7 February 2010, and suggested that the lateral transport of the SEPs in this event occurs partially close to the Sun and partially in the IP medium. Furthermore, all of the events in our sample are accompanied by EIT waves (see Table 1).…”

“…To investigate the eventto-event variations and the specific dominance of the different mechanisms, detailed modeling of the events is required (e.g. Dröge et al 2010;Kallenrode et al 1992;Dröge et al 2014).…”

“…Omnidirectional intensities were calculated by integrating second-order polynomial fits to the pitchangle-dependent intensities I(μ) using five-minute averages of the data. To stabilize the fit during periods of poor pitch-angle coverage an artificial point was added to the pitch-angle distribution to fill the uncovered range (cf., Dröge et al 2014). The four panels below show the magnetic field latitudinal and azimuthal angles, the magnetic field magnitude, and the solar wind speed.…”

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

“…These approaches have support in full-orbit particle simulations (Giacalone and Jokipii, 1999) and galactic cosmic ray observations (Burger et al, 2000;Potgieter et al, 2014). However, several recent observational studies suggest faster propagation of SEPs across the mean field than predicted by the current theoretical understanding: they often require a ratio of the cross-field diffusion coefficient to the parallel one of order k ⊥ /k k ∼ 0.1 À 1 (Zhang et al, 2003;Dresing et al, 2012;Dröge et al, 2014) 1 , whereas values k ⊥ /k k ≲ 0.01 are more consistent with the interplanetary turbulence conditions at 1 AU (Burlaga and Turner, 1976;Pei et al, 2010;Laitinen et al, 2016).…”

Solar flares, coronal mass ejections and solar energetic particle event characteristics

“…The interpretations offered for such widespread events include: (a) the propagation of CME-driven shocks able to inject SEPs over broad angular regions (Rouillard et al 2012;Lario et al 2016); (b) the effective propagation of particles perpendicular to the direction of the interplanetary magnetic field (IMF), i.e. cross-field diffusion (Dresing et al 2012;Dröge et al 2014); (c) the perpendicular transport of particles through processes other than diffusion (Wiedenbeck et al 2012). Hence, this wealth of observations from different vantage points within the heliosphere has provided significant insight and further tested the proposed mechanisms for the injection, propagation and acceleration of the SEP events.…”