High-cadence measurements of electron pitch-angle distributions from Solar Orbiter SWA-EAS burst mode operations

Small-scale kinetic processes associated with turbulence, plasma instabilities, magnetic reconnection, etc., play a major role in dissipating energy and governing the large-scale evolution of the solar wind. However, a general impediment to improving the understanding of the kinetic physics of the solar wind is in the limitations on measurement cadences of particle instruments, which are usually several orders of magnitude below the equivalent cadences of field instruments. Nevertheless, knowledge of the details of the particle velocity distribution functions (VDFs) at sub-second cadence is required to make progress in this area. This is particularly true for the electron VDFs, which play a significant role in the overall energetics of the solar wind through their transmission of heat flux from the Sun. In this paper, we detail and illustrate a novel measurement scheme deployed on Solar Orbiter's Solar Wind Analyser Electron Analyser System (SWA-EAS), which allows for 2D pitch angle distributions (PAD) to be returned over short periods (5-10 min) at a cadence of 0.125 s. This is achieved through the use of a B-field vector shared by the magnetometer (MAG) instrument to steer the SWA-EAS system to record only that part of the full SWA-EAS field-of-view needed to construct the PAD. We provide an example of early observations using this scheme to illustrate that it is working well. Given that the electrons are usually gyrotropic, these measurements provide a new tool with which to derive details of the electron VDFs at high cadence for the study of the solar wind's kinetic processes.

Automated summary

Small-scale kinetic processes like turbulence, plasma instabilities, and magnetic reconnection are crucial for dissipating energy and shaping the large-scale evolution of the solar wind. The limitations on measurement frequencies of particle instruments have hindered the understanding of solar wind kinetic physics. The new measurement scheme discussed in this paper offers a way to obtain detailed electron velocity distribution functions in real-time, providing a new tool for studying the kinetic processes of the solar wind.