Solar wind electrons play an important role in the energy balance of the solar wind acceleration by carrying energy into interplanetary space in the form of electron heat flux. The heat flux is stored in the complex electron velocity distribution functions (VDFs) shaped by expansion, Coulomb collisions, and field-particle interactions. We investigate how the suprathermal electron deficit in the anti-strahl direction, which was recently discovered in the near-Sun solar wind, drives a kinetic instability and creates whistler waves with wave vectors that are quasi-parallel to the direction of the background magnetic field. We combined high-cadence measurements of electron pitch-angle distribution functions and electromagnetic waves provided by Solar Orbiter during its first orbit. Our case study is based on a burst-mode data interval from the Electrostatic Analyser System (SWA-EAS) at a distance of 112 RS (0.52 au) from the Sun, during which several whistler wave packets were detected by Solar Orbiter’s Radio and Plasma Waves (RPW) instrument. The sunward deficit creates kinetic conditions under which the quasi-parallel whistler wave becomes unstable. We directly test our predictions for the existence of these waves through solar wind observations. We find whistler waves that are quasi-parallel and almost circularly polarised, propagating away from the Sun, coinciding with a pronounced sunward deficit in the electron VDF. The cyclotron-resonance condition is fulfilled for electrons moving in the direction opposite to the direction of wave propagation, with energies corresponding to those associated with the sunward deficit. The quasilinear diffusion of the resonant electrons tends to fill the deficit, leading to a reduction in the total electron heat flux.

The Data Processing Unit (DPU) is the “heart” of the plasma suite SWA and is the only interface with the S/C. The DPU is interfaced with EAS, PAS and HIS sensors via SpW dedicated links and is in charge of supporting EAS and PAS with power, functionality control, temporary storage, communication and computational capability and, in addition, supports HIS with communication to the S/C. Its architecture derives from a trade-off analysis aiming to define a system able to perform the needed computational tasks while keeping mass, volume and power within the limits imposed by the constraints. Additionally, the DPU has been designed to be “single fault” tolerant and the “cold-spare” concept has been adopted as redundancy philosophy. It implements data and command interfaces with the S/C via the redundant SpaceWire (SpW) data links and the redundant power input HV-HPC command interface. Two independent, executable SW images represent the overall SWA DPU SW: the Boot SW (BSW) and the Flight SW (FSW). While the BSW manages the basic hardware initialization, the FSW manages TC/TM, controls all the processes related to the state of the sensors, validates and executes TC, acquires, processes, compresses and formats science data prior to downlink for EAS(1&2) and PAS sensors, while HIS autonomously processes its scientific data. In particular, FSW is in charge of data compression, moments calculation and telemetry generation restrictions to keep each sensor within its respective telemetry allocation. Remarkable level of data compression for EAS, which generates the largest data volume, is reached via customized implementation of lossless CCSDS 121.0 based on a “Complex Reordering” mechanism, which avoids periodical jumps among acquisition directions in phase space. Moments of proton and electron velocity distribution functions are computed onboard. Different Look-Up Tables (LUT) for PAS and EAS allow to perform moments calculation modulating the counts in each volume of phase space by a combination of the factors contained in these tables. Finally, since SWA data production greatly changes from normal to burst mode, a book keeping algorithm (BKA) will monitor and control, continuously along the orbit, the amount of burst mode, scheduled or triggered, used against the pro-rata expectation.