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Whistler instability driven by the sunward electron deficit in the solar wind: High-cadence Solar Orbiter observations
  • +19
  • Laura Bercic,
  • Daniel Verscharen,
  • Christopher Owen,
  • Lucas Colomban,
  • Matthieu Kretzschmar,
  • Thomas Chust,
  • Milan Maksimovic,
  • D Kataria,
  • Chandrasekhar Anekallu,
  • Etienne Behar,
  • Matthieu Berthomier,
  • Roberto Bruno,
  • Vito Fortunato,
  • Christopher Kelly,
  • Yuri Khotyaintsev,
  • Gethyn Lewis,
  • Stefano Livi,
  • Philippe Louarn,
  • Gennaro Mele,
  • Georgios Nicolaou,
  • Gillian Watson,
  • Robert Wicks
Laura Bercic
Mullard Space Science Laboratory, University College London

Corresponding Author:l.bercic@ucl.ac.uk

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Daniel Verscharen
University of New Hampshire Main Campus
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Christopher Owen
University College London, Mullard Space Science Laboratory
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Lucas Colomban
Laboratoire de Physique et Chimie de l'Environnement et de l'Espace
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Matthieu Kretzschmar
CNRS and University of Orléans
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Thomas Chust
Laboratoire de Physique des Plasmas (UMR7648)
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Milan Maksimovic
LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris
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D Kataria
University College London
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Chandrasekhar Anekallu
University College London
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Etienne Behar
IRF Swedish Institute of Space Physics Kiruna
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Matthieu Berthomier
LPP
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Roberto Bruno
INAF-IFSI
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Vito Fortunato
Planetek
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Christopher Kelly
Mullard Space Science Laboratory
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Yuri Khotyaintsev
Swedish Institute of Space Physics (IRF)
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Gethyn Lewis
University College London
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Stefano Livi
SwRI
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Philippe Louarn
IRAP, CNRS
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Gennaro Mele
Leonardo
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Georgios Nicolaou
UTSA
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Gillian Watson
University College London
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Robert Wicks
Northumbria University
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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.