The Heliospheric Ambipolar Potential Inferred from Sunward-Propagating Halo Electrons

Abstract

We provide evidence that the sunward-propagating half of the solar wind electron halo distribution evolves without scattering in the inner heliosphere. We assume the particles conserve their total energy and magnetic moment, and perform a "Liouville mapping" on electron pitch angle distributions measured by the Parker Solar Probe SPAN-E instrument. Namely, we show that the distributions are consistent with Liouville's theorem if an appropriate interplanetary potential is chosen. This potential, an outcome of our fitting method, is compared against the radial profiles of proton bulk flow energy. We find that the inferred potential is responsible for nearly 100% of the proton acceleration in the solar wind at heliocentric distances 0.18-0.79 AU. These observations combine to form a coherent physical picture: the same interplanetary potential accounts for the acceleration of the solar wind protons as well as the evolution of the electron halo. In this picture the halo is formed from a sunward-propagating population that originates somewhere in the outer heliosphere by a yet-unknown mechanism.

References in corpus (11)

• The Solar Probe Cup on Parker Solar Probe
• Electrons in the Young Solar Wind: First Results from the Parker Solar Probe
• Whistler wave generation by halo electrons in the solar wind
• Magnetic pumping as a source of particle heating and power-law distributions in the solar wind
• On the role of solar wind expansion as a source of whistler waves: scattering of suprathermal electrons and heat flux regulation in the inner heliosphere
• Whistler instability driven by the sunward electron deficit in the solar wind
• Stability of superthermal strahl electrons in the solar wind
• Statistics of Solar Wind Electron Breakpoint Energies Using Machine Learning Techniques
• The Stability of the Electron Strahl against the Oblique Fast-magnetosonic/Whistler Instability in the Inner Heliosphere
• The Kinetic Expansion of Solar-Wind Electrons: Transport Theory and Predictions for the very Inner Heliosphere
• Stochastic diffusion of electrons interacting with whistler-mode waves in the solar wind