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 fast decrease in the electron heat flux and the increase in the Strahl pitch angle width. Wave-particle interactions between electrons and whistler waves are often proposed to explain these phenomena. We aim to quantify the effect of whistler waves on suprathermal electrons as a function of heliocentric distance. We first performed 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 were then used to compute the diffusion coefficients for solar wind suprathermal electrons in the framework of quasi-linear theory. These coefficients were integrated to deduce the overall effect of whistler waves on electrons along their propagation. About $110,000$ whistler wave packets were detected and characterized in the plasma frame, including their direction of propagation with respect to the background magnetic field and their radial direction of propagation. Most waves are aligned with the magnetic field and only $ 0.5 <!PCT!>$ of them have a propagation angle greater than $45^ Beyond 0.3 AU, it is almost exclusively quasi-parallel waves propagating anti-sunward (some of them are found sunward but are within switchbacks with a change of sign of the radial component of the background magnetic) that are observed. Thus, these waves are found to be Strahl-aligned and not counter-streaming. At 0.2 AU, we find both Strahl-aligned and counter-streaming quasi-parallel whistler waves. Beyond 0.3 AU, the integrated diffusion coefficients show that the observed waves are sufficient to explain the measured Strahl pitch angle evolution and effective in isotropizing the halo. Strahl diffusion is mainly attributed to whistler waves with a propagation angle of $ although their origin has not yet been fully determined. Near 0.2 AU, counter-streaming whistler waves are able to diffuse the Strahl electrons more efficiently than the Strahl-aligned waves by two orders of magnitude.

Enhanced Efficiency of Solar Wind Electron Interaction with Whistlers Caused by Switchback-related Magnetic Dips

“…Accounting for Doppler shift, whistlers observed within magnetic dips typically have relative frequencies ω/Ω e in the plasma frame between 0.05 and 0.2 (Agapitov et al 2020;Karbashewski et al 2023;Colomban et al 2023Colomban et al , 2024. Ω e = 2πf ce = eB 0 /m e is the unsigned angular cyclotron frequency.…”

“…Inside the magnetic dips, sunward or counterpropagating waves are often observed. However, Solar Orbiter only observed antisunward whistlers around 100R S (Kretzschmar et al 2021;Colomban et al 2024), suggesting that PSP is crossing a region of local generation of whistlers in the young solar wind.…”

“…However, their occurrence rate is low in the free solar wind (Jagarlamudi et al 2021;Kretzschmar et al 2021). On the other hand, quasiparallel (less than 30°) and slightly oblique (up to 45°) whistlers are more frequently observed and may play an important role (Colomban et al 2024). Nevertheless, simulations of whistler instabilities suggest that these waves do not operate in isolation (López et al 2020;Micera et al 2020Micera et al , 2021.…”

“…Observations from spacecraft at 1 au revealed two types of whistlers in the solar wind. Low-amplitude whistlers with quasi-parallel propagation angles were observed by Artemis, Cluster, and Solar Orbiter during slow and quiet winds (Lacombe et al 2014;Stansby et al 2016;Tong et al 2019aTong et al , 2019bColomban et al 2024), while Stereo and Wind reported occurrences of high-amplitude and obliquely propagating whistlers often near stream interaction regions and interplanetary shocks (Breneman et al 2010;Wilson et al 2013;Cattell et al 2020).…”