Whistler instability driven by the sunward electron deficit in the solar
wind: High-cadence Solar Orbiter observations
Abstract
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.