Stability of superthermal strahl electrons in the solar wind

Abstract

We present a kinetic stability analysis of the solar wind electron distribution function consisting of the Maxwellian core and the magnetic-field aligned strahl, a superthermal electron beam propagating away from the sun. We use an electron strahl distribution function obtained as a solution of a weakly collisional drift-kinetic equation, representative of a strahl affected by Coulomb collisions but unadulterated by possible broadening from turbulence. This distribution function is essentially non-Maxwellian and varies with the heliospheric distance. The stability analysis is performed with the Vlasov-Maxwell linear solver LEOPARD. We find that depending on the heliospheric distance, the core-strahl electron distribution becomes unstable with respect to sunward-propagating kinetic-Alfvén, magnetosonic, and whistler modes, in a broad range of propagation angles. The wavenumbers of the unstable modes are close to the ion inertial scales, and the radial distances at which the instabilities first appear are on the order of 1 AU. However, we have not detected any instabilities driven by resonant wave interactions with the superthermal strahl electrons. Instead, the observed instabilities are triggered by a relative drift between the electron and ion cores necessary to maintain zero electric current in the solar wind frame (ion frame). Contrary to strahl distributions modeled by shifted Maxwellians, the electron strahl obtained as a solution of the kinetic equation is stable. Our results are consistent with the previous studies based on a more restricted solution for the electron strahl.

The Stability of the Electron Strahl against the Oblique Fast-magnetosonic/Whistler Instability in the Inner Heliosphere

“…Assuming that our PSP and Helios data are representative and that our Eqs. ( 2) and (3) reflect the thresholds accurately, our findings suggest that the strahl is not scattered by the oblique FM/W instability in the inner heliosphere, in agreement with the arguments presented by Horaites et al (2018a) and Schroeder et al (2021).…”

“…It is unknown at which heliocentric distance the local strahl scattering sets in. Based on a linear stability analysis, Horaites et al (2018a) and Schroeder et al (2021) argue against the action of a strahl-driven instability in the inner heliosphere at all. Parker Solar Probe (PSP) data from encounters 4 and 5 suggest that the strahl, at times, drives the oblique FM/W wave unstable at heliocentric distances below 54r s (Halekas et al 2021), where r s is the solar radius.…”

Temperature Anisotropy Instabilities Stimulated by the Solar Wind Suprathermal Populations

“…This is another consequence of the presence of suprathermal populations, which can indirectly contribute to the accumulation of quasistable states in between these thresholds, near and along the isotropy condition A ≃ 1, as shown by the observations, not only for protons (Kasper et al, 2002;Bale et al, 2009) but also for electrons (Štverák et al, 2008). Other instabilities induced by beams or drifting populations (Verscharen et al, 2019;López et al, 2020;Micera et al, 2020;Schroeder et al, 2021), may show less systematic behavior when modeled by Kappa distributions (Shaaban et al, 2018a;. Present resultss and recent advances in modeling the dispersion and stability of these anisotropic populations (López et al, 2021) should motivate future investigations to decode even more complex spectra of wave instabilities, as triggered by the interplay of various kinetic anisotropies of solar wind plasma populations, e.g., temperature anisotropy and relative drifts (Shaaban et al, 2018b;Vasko et al, 2020), but also density gradients in phase space (Page et al, 2021).…”

The Kinetic Expansion of Solar-Wind Electrons: Transport Theory and Predictions for the very Inner Heliosphere

“…For the understanding of local strahl scattering, it is important to model the evolution of the electron VDF up to large heliocentric distances where the effect of the Parker-spiral geometry of the interplanetary magnetic field is noticeable (Horaites et al 2018a;Schroeder et al 2021;Halekas et al 2021). Many previous studies for the radial kinetic evolution are based on a simplified radial magnetic-field geometry.…”