TY - JOUR
T1 - Whistler instability driven by the sunward electron deficit in the solar wind
T2 - High-cadence Solar Orbiter observations
AU - Bercic, Laura
AU - Verscharen, Daniel
AU - Owen, Christopher J.
AU - Colomban, L.
AU - Kretzschmar, M.
AU - Chust, T.
AU - Maksimovic, Milan
AU - Kataria, D. O.
AU - Anekallu, C.
AU - Behar, E.
AU - Berthomier, M.
AU - Bruno, R.
AU - Fortunato, V.
AU - Kelly, C. W.
AU - Khotyaintsev, Y.
AU - Lewis, G. R.
AU - Livi, S.
AU - Louarn, P.
AU - Mele, G.
AU - Nicolaou, Georgios
AU - Watson, G.
AU - Wicks, Robert
N1 - Funding information: Solar Orbiter is a space mission of international collaboration between ESA and NASA, operated by ESA. Solar Orbiter Solar Wind Analyser (SWA) data are derived from scientific sensors which have been designed and created, and are operated under funding provided in numerous contracts from the UK Space Agency (UKSA), the UK Science and Technology Facilities Council (STFC), the Agenzia Spaziale Italiana (ASI), the Centre National d’Etudes Spatiales (CNES, France), the Centre National de la Recherche Scientifique (CNRS, France), the Czech contribution to the ESA PRODEX programme and NASA. Solar Orbiter SWA operations work at UCL/MSSL is currently funded under STFC grants ST/T001356/1. L.B., C.J.O., and D.V. are supported by STFC Consolidated Grant ST/S000240/1. D.V. is supported by STFC Ernest Rutherford Fellowship ST/P003826/1. R.T. Wicks is funded by STFC grant ST/V006320/1. The RPW instrument has been designed and funded by CNES, CNRS, the Paris Observatory, The Swedish National Space Agency, ESA-PRODEX and all the participating institutes.
Publisher Copyright:
© 2021 ESO.
PY - 2021/12/14
Y1 - 2021/12/14
N2 - Context. 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. Aims. 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. Methods. 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. Results. 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. Conclusions. We conclude that the sunward deficit acts as a source of quasi-parallel whistler waves in the solar wind. The quasilinear diffusion of the resonant electrons tends to fill the deficit, leading to a reduction in the total electron heat flux.
AB - Context. 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. Aims. 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. Methods. 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. Results. 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. Conclusions. We conclude that the sunward deficit acts as a source of quasi-parallel whistler waves in the solar wind. The quasilinear diffusion of the resonant electrons tends to fill the deficit, leading to a reduction in the total electron heat flux.
KW - Instabilities
KW - Methods: observational
KW - Plasmas
KW - Solar wind
KW - Space vehicles: instruments
KW - Waves
UR - http://www.scopus.com/inward/record.url?scp=85113870191&partnerID=8YFLogxK
U2 - 10.1051/0004-6361/202140970
DO - 10.1051/0004-6361/202140970
M3 - Article
SN - 0004-6361
VL - 656
SP - 1
EP - 12
JO - Astronomy & Astrophysics
JF - Astronomy & Astrophysics
M1 - A31
ER -
