Geometry of Magnetic Fluctuations near the Sun from PSP

TL;DR

This study uses Parker Solar Probe data to analyze magnetic fluctuation anisotropy near the Sun, revealing scale-dependent evolution and differences from 1 au observations, with implications for space plasma processes.

Contribution

First measurements of magnetic fluctuation geometry and anisotropy in the inner heliosphere across fluid to kinetic scales using PSP data, highlighting new anisotropy behaviors.

Findings

Outer scale fluctuations are quasi-parallel to the magnetic field.

Inertial range fluctuations are less field-aligned than near Earth.

Dissipation range fluctuations are nearly perpendicular to the magnetic field.

Abstract

Solar wind magnetic fluctuations exhibit anisotropy due to the presence of a mean magnetic field in the form of the Parker spiral. Close to the Sun, direct measurements were not available until the recently launched Parker Solar Probe (PSP) mission. The nature of anisotropy and geometry of the magnetic fluctuations play a fundamental role in dissipation processes and in the transport of energetic particles in space. Using PSP data, we present measurements of geometry and anisotropy of the inner heliosphere magnetic fluctuations, from fluid to kinetic scales. The results are surprising and different from 1 au observations. We find that fluctuations evolve characteristically with size scale. However, unlike 1 au solar wind, at the outer scale, the fluctuations are dominated by wavevectors quasi-parallel to the local magnetic field. In the inertial range, average wave vectors become less field-aligned, but still remain more field aligned than near-Earth solar wind. In the dissipation range, the wavevectors become almost perpendicular to the local magnetic field in the dissipation range, to a much higher degree than those indicated by 1 au observations. We propose that this reduced degree of anisotropy in the outer scale and inertial range is due to the nature of large-scale forcing outside the solar corona.