Solar wind electron dynamics revealed by high-time resolution observations of 3D electron velocity distribution functions captured by Solar Orbiter SWA.

The Electron Analyser System (EAS), a dual sensor system within the Solar Wind Analyser (SWA) suite, is capable of measuring a full 3D velocity distribution function (VDF) of solar wind electrons with energies of a few eV to ~5 keV in an accumulation time of 1 second.  At full energy and angular resolution, telemetry constraints limit the maximum EAS normal mode data return to 1 3D VDF every 10 seconds.  However, the SWA DPU includes a rolling buffer capable of storing 1-second full electron VDF data for a period of 5 minutes.  Over the last year, during periods in which the telemetry allocation is high, we have been able to freeze this buffer and return this data up to three times per hour, generating a large dataset of very high time resolution electron measurements.  Moreover, a trigger detection algorithm is at times operated by the Radio and Plasma Waves (RPW)  instrument, combining data from the magnetometer and the SWA Proton and Alpha Sensor in order to detect the passage of a shock passed the spacecraft.    Although the operation of the algorithm has suffered from a number of technical issues, receipt of a positive flag is used by SWA to freeze the EAS rolling buffer and add the resulting data to the telemetry stream.  This operation has again generated a significant dataset of high resolution electron data which is often associated with a solar wind transient, such as a shock or a current sheet.

In this presentation, we present case studies of periods in which SWA-EAS returned this high resolution data, bringing out new features of electron dynamics that are not captured by normal resolution instruments on Solar Orbiter or other missions.  For example, we examine the development of electron distributions across the shock, revealing how the ‘flat-top’ nature of the downstream distribution develops as a function of pitch angle.  Moreover, even the manually triggered events can be used to reveal how the electron distribution may vary with less dramatic, but more regular, variations in the solar wind, such as those driven by waves or instabilities.