Narrowband oblique whistler-mode waves: comparing properties observed by Parker Solar Probe at <0.3 AU and STEREO at 1 AU

¹ School of Physics and Astronomy, University of Minnesota, 116 Church St. SE Minneapolis, USA
e-mail: cattell@umn.edu
² Department of Physics and Astronomy, University of Iowa, Iowa City, IA 52242, USA
³ Space Sciences Laboratory, University of California, Berkeley, CA 94720-7450, USA
⁴ Climate and Space Sciences and Engineering, University of Michigan, Ann Arbor, MI 48109, USA
⁵ Smithsonian Astrophysical Observatory, Cambridge, MA 02138, USA
⁶ LPC2E, CNRS and University of Orléans, Orléans, France
⁷ Solar System Exploration Division, NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁸ Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO 80303, USA
⁹ LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris, 5 place Jules Janssen, 92195 Meudon, France
¹⁰ Department of Physics, University of California, Berkeley, Berkeley, CA 94709, USA
¹¹ Department of Astrophysical and Planetary Sciences, University of Colorado, Boulder, CO, USA

Received: 28 September 2020
Accepted: 4 January 2021

Abstract

Aims. Large amplitude narrowband obliquely propagating whistler-mode waves at frequencies of ~0.2 f_ce (electron cyclotron frequency) are commonly observed at 1 AU, and they are most consistent with the whistler heat flux fan instability. We want to determine whether similar whistler-mode waves occur inside 0.3 AU and how their properties compare to those at 1 AU.

Methods. We utilized the waveform capture data from the Parker Solar Probe Fields instrument from Encounters 1 through 4 to develop a data base of narrowband whistler waves. The Solar Wind Electrons Alphas and Protons Investigation (SWEAP) instrument, in conjunction with the quasi-thermal noise measurement from Fields, provides the electron heat flux, beta, and other electron parameters.

Results. Parker Solar Probe observations inside ~0.3 AU show that the waves are often more intermittent than at 1 AU, and they are interspersed with electrostatic whistler-Bernstein waves at higher-frequencies. This is likely due to the more variable solar wind observed closer to the Sun. The whistlers usually occur within regions when the magnetic field is more variable and often with small increases in the solar wind speed. The near-Sun whistler-mode waves are also narrowband and large amplitude, and they are associated with beta greater than 1. The association with heat flux and beta is generally consistent with the whistler fan instability. Strong scattering of strahl energy electrons is seen in association with the waves, providing evidence that the waves regulate the electron heat flux.