Quantifying the diffusion of suprathermal electrons by whistler waves between 0.2 and 1 AU with Solar Orbiter and Parker Solar Probe

Abstract

The evolution of the solar wind electron distribution function with heliocentric distance exhibits different features that are still unexplained, in particular, the increase of the Strahl pitch angle width. Wave-particle interactions between electrons and whistler waves are often proposed to explain these phenomena. We aim at quantifying the effect of whistler waves on suprathermal electrons as a function of heliocentric distance. We first perform a statistical analysis of whistler waves (occurrence and properties) observed by Solar Orbiter and Parker Solar Probe between 0.2 and 1 AU. The wave characteristics are then used to compute the diffusion coefficients in the framework of quasi-linear theory. These coefficients are integrated in order to deduce the overall effect of whistler waves on electrons along their propagation. 110,000 whistler wave packets are detected and characterized in the plasma frame. Most waves are aligned with the magnetic field and only about 0.5% of them have a propagation angle greater than 45{\deg}. Beyond 0.3 AU, almost exclusively anti-sunward waves (some of them are found sunward but are within switchbacks with a change of sign of the radial component of the background magnetic) are observed. These waves are therefore Strahl-aligned and not counter-streaming. At 0.2 AU we find both Strahl-aligned and counter-streaming waves. Beyond 0.3 AU, the integrated diffusion coefficients show that the observed waves can explain the measured Strahl pitch angle evolution and are effective in isotropizing the halo. Strahl diffusion is mainly due to whistler waves with an angle of propagation between 15{\deg} and 45{\deg}. Near 0.2 AU, counter-streaming whistler waves can diffuse the Strahl electrons more efficiently than the Strahl-aligned waves by two orders of magnitude.