Characteristics of electron velocity distribution functions in the solar wind derived from the Helios Plasma Experiment

Abstract

The shapes of three typical examples of electron distribution functions, which have been observed by Helios 2 in the solar wind, are analyzed and compared with theoretical predictions. We have considered a distribution function with a "narrow strahl" (narrow beam), which is extremely anisotropic and skewed with respect to the magnetic field direction at particle energies above 100 eV, a distribution function with a "broad strahl" (broad beam), which is less anisotropic and skewed, and finally a nearly isotropic distribution function which, however, shows a slight bidirectional anisotropy. The main results are as follows: (1) For each distribution function we may discern a "break," i.e., a sudden change in the slope of the distribution function, separating the "core" at lower energies from the "halo" at larger energies. For the anisotropic distributions a significant break is observed in velocity directions opposite to the strahl and perpendicular to it but not along the strahl. Here the energy of the break (breakpoint energy) may be determined both by the interplanetary electrostatic potential and by collisions. In contrast, for the nearly isotropic distribution function, a significant break is observed for all velocity directions, and the breakpoint energy may be determined by collisions only. (2) The strahl observed at larger energies in the anisotropic distribution functions can be qualitatively explained by existing theoretical approaches describing the propagation of electrons in the solar wind. However, at least for the distribution function with the broad strahl as well as for the nearly isotropic distribution function, the halo electrons should be scattered by unknown anomalous scattering processes, which do not show a strong energy dependence. (3) For the anisotropic distribution functions we find a velocity shift between the peak of each distribution function and the solar wind bulk velocity, which is typically 100 km s -• to 300 km s -•. This shift is drastically reduced compared to the shift predicted by exospheric theory, indicating strong frictional processes between electrons and ions. However the results do not settle the question whether this friction is provided by the combined action of wave-particle interactions and Coulomb collisions or by Coulomb collisions only. For the nearly isotropic distribution function this shift is probably not significantly different from zero. In this case it may be determined by some anomalous processes and/or trapping in closed magnetic field structures. (4) For the anisotropic distribution functions the heat flux is carried mainly by the strahl. For the nearly isotropic distribution function most of the heat flux is carried by the core electrons. For this distribution, part of the halo electrons carry heat flux in the opposite direction, and the total heat flux is probably not significantly different from zero. (5) The pitch angle distribution in the energy regime of the halo may provide some indications for the global structure of the magnetic field. 1.For the ...

QUIET-TIME SUPRATHERMAL (∼0.1–1.5 keV) ELECTRONS IN THE SOLAR WIND

“…In both solar cycles, the density ratio between the quiet-time strahl and halo electrons roughly anti-correlates (correlates) with n p (v p , except during the solar maximum of cycle 24), consistent with previous observations that the strahl is predominantly observed in fast solar wind (e.g., Pilipp et al 1987). This suggests that the generation of halo electrons could be related to the properties of solar wind, as well as its propagation, in the IPM.…”

“…On the other hand, both the strahl n and halo n have no association with the solar wind core population, while the halo n strongly correlates with the strahl n. These results support the idea that the strahl/halo have a different origin from the solar wind core: the strahl could originate from the Sun, e.g., due to the escaping electrons from the hot corona, while the halo may be due to some processes (e.g., scattering) acting on the strahl in the IPM (e.g., Montgomery et al 1968;Feldman et al 1975;Rosenbauer et al 1977;Pilipp et al 1987;Pierrard et al 2001). For the 0.1-1.5 keV strahl electrons at quiet times, κ ranges from 4.3 to 15.3 (from 4.6 to 16.6) in solar cycle 23 (24), anticorrelated with the sunspot number (see Figure 7).…”

“…The highly anisotropic strahl could result from the escape of thermal electrons from the hot (∼10 6 K) solar corona (e.g., Feldman et al 1975;Salem et al 2007) that carries heat flux outward, while the isotropic halo may be due to the scattering of the strahl in the interplanetary medium (IPM; e.g., Montgomery et al 1968;Feldman et al 1975;Rosenbauer et al 1977;Pilipp et al 1987;Pierrard et al 2001). Combining the measurements from the Helios, WIND, and Ulysses spacecraft, Maksimovic et al (2005) and Stverak et al (2009) reported that the relative halo (strahl) number density to the total electron density increases (decreases) with the heliocentric distance, while the relative core density remains roughly constant, supporting the above scenarios.…”

“…The solar wind electrons observed near 1 au consist of three components: a thermal (∼10 eV) Maxwellian core that contains ∼90%-95% of the plasma density, a much hotter (∼50-80 eV) halo/strahl that contains ∼5%-10% of the plasma density (e.g., Montgomery et al 1968;Vasyliunas 1968;Feldman et al 1975;Rosenbauer et al 1977;Pilipp et al 1987;Maksimovic et al 2005;Zouganelis 2008), and a power-law ( µ b -J E with b~2.4) superhalo at energies above ∼2 keV (Lin 1998;Wang et al 2012;Yang et al 2015). The halo population is observed to have a Maxwellian/Kappa spectrum and isotropic angular distribution.…”

Solar wind electron halo depletions at 90° pitch angle

“…Up to energies of -2 keV two suprathermal electron populations can usually be distinguished [e.g., Feldman et al, 1975;Rosenbauer et al, 1977;Pilipp et al, 1987]: (1)an intense beam known as the "strahl" directed outward from the Sun along the heliospheric magnetic field, and (2) a more tenuous and roughly isotropic component known as the "halo". The strahl results from a competition between (a) focusing associated with conservation of total particle energy and magnetic moment as suprathermal electrons from the solar corona move outward through the diverging heliospheric magnetic field and (b) defocusing associated with scattering.…”