Electron heat flux in the near-Sun environment

¹ Department of Physics and Astronomy, University of Iowa, Iowa City, IA 52242, USA
e-mail: jasper-halekas@uiowa.edu
² Space Sciences Laboratory, University of California, Berkeley, CA 94720, USA
³ Physics Department, University of California, Berkeley, CA 94720, USA
⁴ The Blackett Laboratory, Imperial College London, London, SW7 2AZ, UK
⁵ School of Physics and Astronomy, Queen Mary University of London, London E1 4NS, UK
⁶ Laboratoire de Physique des Plasmas, CNRS, Sorbonne Université, Ecole Polytechnique, Observatoire de Paris, Université Paris-Saclay, Paris 75005, France
⁷ Smithsonian Astrophysical Observatory, Cambridge, MA 02138, USA
⁸ Department of Physics & Astronomy, University of New Hampshire, Durham, NH 03824, USA
⁹ Space Science Center, University of New Hampshire, Durham, NH 03824, USA
¹⁰ Climate and Space Sciences and Engineering, University of Michigan, Ann Arbor, MI 48109, USA
¹¹ Department of Planetary Sciences, University of Arizona, Tucson, AZ 85721, USA
¹² NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
¹³ LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Universite, Université de Paris, 5 place Jules Janssen, 92195 Meudon, France
¹⁴ Astrophysical and Planetary Sciences Department, University of Colorado, Boulder, CO 80309, USA
¹⁵ Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO 80303, USA
¹⁶ Physics Department, Imperial College London, London, SW7 2AZ, UK

Received: 25 August 2020
Accepted: 6 October 2020

Abstract

Aims. We survey the electron heat flux observed by the Parker Solar Probe (PSP) in the near-Sun environment at heliocentric distances of 0.125–0.25 AU.

Methods. We utilized measurements from the Solar Wind Electrons Alphas and Protons and FIELDS experiments to compute the solar wind electron heat flux and its components and to place these in context.

Results. The PSP observations reveal a number of trends in the electron heat flux signatures near the Sun. The magnitude of the heat flux is anticorrelated with solar wind speed, likely as a result of the lower saturation heat flux in the higher-speed wind. When divided by the saturation heat flux, the resulting normalized net heat flux is anticorrelated with plasma beta on all PSP orbits, which is consistent with the operation of collisionless heat flux regulation mechanisms. The net heat flux also decreases in very high beta regions in the vicinity of the heliospheric current sheet, but in most cases of this type the omnidirectional suprathermal electron flux remains at a comparable level or even increases, seemingly inconsistent with disconnection from the Sun. The measured heat flux values appear inconsistent with regulation primarily by collisional mechanisms near the Sun. Instead, the observed heat flux dependence on plasma beta and the distribution of suprathermal electron parameters are both consistent with theoretical instability thresholds associated with oblique whistler and magnetosonic modes.