Measurement of the rate of change of the electron heat flux due to the whistler instability with Solar Orbiter observations

Non-Maxwellian features of the coronal electron population are important for some models of solar wind acceleration processes. Remnants of these features are detectable in spacecraft observations, in particular in the form of field-aligned beams (strahl) and anti-sunward deficits in the electron distribution function. These features are shaped by expansion, collisions, and kinetic effects. Therefore, determining how these processes alter the distribution is important for our understanding of how the solar wind accelerates and evolves. The strahl and deficit contribute to the overall electron heat flux. If the heat flux crosses the threshold for instabilities, the plasma will generate waves which in turn reduce the heat flux via pitch-angle scattering of electrons out of the strahl and/or into the deficit. The work presented here examines an interval observed by Solar Orbiter during which short bandwidth whistler waves are observed by the Radio and Plasma Waves instrument. We apply a method to measure the pitch-angle gradient to high cadence pitch angle distribution (PAD) functions measured by the Electrostatic Analyser System to quantify the rate of change of heat flux from quasilinear theory. The primary part of the measurement technique is based on low-pass filtering of the PAD function with a Hermite-Laguerre transform providing a measurement of the pitch-angle gradient. We compare our quantification of the rate of change of the heat flux with other timescales and processes relevant in the solar wind. We show the potential of our technique to further our understanding of the role of wave-particle interactions in the evolution of the solar wind electrons.