Scattering of solar energetic electrons in interplanetary space

Abstract

Context. Solar energetic electrons are observed to arrive between 10 and 30 min later at 1 AU compared to the expectation based on their production in a solar flare and the travel time along the Parker spiral. Both a delayed release of electrons from the Sun and scattering of the electrons along their path are discussed as possible underlying mechanisms. Aims. We investigate to what extent scattering of energetic electrons in interplanetary space influences the arrival times of electrons at a solar distance of 1 AU, as a function of electron energy and for different scattering models. Methods. A kinetic model for electrons in interplanetary space is used to study the propagation of solar-flare electrons injected into the corona. The electrons are scattered by resonant interaction with a whistler-wave spectrum that is based on observed magnetic field fluctuation spectra in the solar wind. The arrival times of the electrons at 1 AU is determined by the electron flux exceeding a given threshold value.Results. The simulation results show a significant influence of the scattering on electron arrival times. Electrons with energies in the range of several tens of keV are delayed by up to about one minute for a pure pitch-angle scattering model. It is demonstrated that this simplification is not applicable, and the full quasi-linear diffusion equation needs to be considered. This reduces the delays to values below 30 s. Conclusions. It follows from these numerical studies that scattering of electrons in interplanetary space due to resonant interaction with whistler waves cannot explain the observed delays of 600 s, unless an unrealistic wave spectrum is assumed in interplanetary space.

Evolution of the Solar Flare Energetic Electrons in the Inhomogeneous Inner Heliosphere

“…Kontar and Pécseli, 2002;Ziebell et al, 2011). Both the electrons and Langmuir waves exchange energy through the resonant interaction ω pe = kv described by quasilinear terms [first term on the right hand sides (Drummond and Pines, 1962;Vedenov, Velikhov, and Sagdeev, 1962)]…”

“…However, these electrons are scatter-free only in respect to Langmuir waves and could be affected by other plasma waves (e.g. Vocks and Mann, 2009;Bian and Kontar, 2010;Threlfall, McClements, and de Moortel, 2011, Bian and Kontar, 2011, Tan et al, 2011. The simulations by Reid and Kontar (2010) suggest that the suppression of Langmuir waves changes the electron beam transport.…”

Switchbacks in the Young Solar Wind: Electron Evolution Observed inside Switchbacks between 0.125 au and 0.25 au

“…In the solar wind, various scattering mechanisms are in play to alter the electron velocity distribution function (eVDF) (Cuperman et al 1972;Hollweg 1974;Vocks & Mann 2009;Bale et al 2013;Horaites et al 2019;Halekas et al 2020;Micera et al 2020;Berčič et al 2021;). These include, but are not limited to, Coulomb collisions (Scudder & Olbert 1979a, 1979bSalem et al 2003;Landi et al 2012;, electrostatic modes (Gary 1978;Roberg-Clark et al 2018;López et al 2020), quasi-parallel whistler-mode waves (Gary et al 1975(Gary et al , 1994(Gary et al , 1999Saeed et al 2017;Shaaban et al 2018), oblique whistler waves/magnetosonic instabilities (Horaites et al 2018;Vasko et al 2019;Verscharen et al 2019;Micera et al 2020), and firehose instability (Innocenti et al 2020).…”

Electron-Driven Instabilities in the Solar Wind

“…Moreover, the majority of the fast-magnetosonic/whistler waves observed farther away from the Sun have a quasiparallel direction of propagation with respect to the magnetic field (Kretzschmar et al, 2021). Therefore, other mechanisms than the self-induced scattering of strahl electrons by the oblique fast-magnetosonic/whistler instability may thus be needed to explain the observed scattering of strahl electrons into the halo population (e.g., the interaction with pre-existing fast-magnetosonic/whistler waves; Vocks et al, 2005;Vocks and Mann, 2009;Pierrard et al, 2011;Jagarlamudi et al, 2021;Cattell and Vo, 2021;. Moreover, Bernstein and ion-acoustic waves become more dominant than fast-magnetosonic/whistler waves in the very inner heliosphere, suggesting a transition into an electrostatic regime which could affect the electron distributions near the Sun (Mozer et al, 2021b;Malaspina et al, 2021).…”