Abstract
Abstract The Parker Solar Probe (PSP) spacecraft is performing the first in situ exploration of the solar wind within 0.2 au of the Sun. Initial observations confirmed the Alfvénic nature of aligned fluctuations of the magnetic field B and velocity V in solar wind plasma close to the Sun, in domains of nearly constant magnetic field magnitude ∣ B ∣, i.e., approximate magnetic pressure balance. Such domains are interrupted by particularly strong fluctuations, including but not limited to radial field (polarity) reversals, known as switchbacks. It has been proposed that nonlinear Kelvin–Helmholtz instabilities form near magnetic boundaries in the nascent solar wind leading to extensive shear-driven dynamics, strong turbulent fluctuations including switchbacks, and mixing layers that involve domains of approximate magnetic pressure balance. In this work we identify and analyze various aspects of such domains using data from the first five PSP solar encounters. The filling fraction of domains, a measure of Alfvénicity, varies from median values of 90% within 0.2 au to 38% outside 0.9 au, with strong fluctuations. We find an inverse association between the mean domain duration and plasma β. We examine whether the mean domain duration is also related to the crossing time of spatial structures frozen into the solar wind flow for extreme cases of the aspect ratio. Our results are inconsistent with long, thin domains aligned along the radial or Parker spiral direction, and compatible with isotropic domains, which is consistent with prior observations of isotropic density fluctuations or flocculae in the solar wind.
Highlights
Fluctuating magnetic fields are prevalent in space plasmas
The domain boundaries typically involve major changes in the components of B, often but not always including switchbacks. These boundaries may correspond to sharp jumps in the magnetic field vector as indicated by a high partial variance of increments (PVI; Greco et al 2008), and the waiting time distribution between high PVI events in Parker Solar Probe (PSP) data has been analyzed by Chhiber et al (2020)
Much attention has been given to switchbacks, jets and largeamplitude, high cross-helicity fluctuations observed in this region with vigorous ongoing debate concerning the origin and expected evolution of such structures (Squire et al 2020; Fisk & Kasper 2020; Ruffolo et al 2020; Zank et al 2020; Schwadron & McComas 2021; Drake et al 2021)
Summary
Fluctuating magnetic fields are prevalent in space plasmas. In the solar wind, such fluctuations play a key role in the acceleration (Drury 1983) and transport (Jokipii 1966) of energetic charged particles, including solar energetic particles, which are an important component of space weather effects of solar storms on human activity (Knipp 2011). Previous work has remarked upon domains of nearly constant |B| punctuated by sharp changes in PSP data (Ruffolo et al 2020) They interpreted the domains of Alfvenic turbulence with nearly constant |B| as (possibly merged) mixing layers, which exhibit sharp boundaries as topological defects across which the dynamics have not yet balanced the magnetic pressure. The domain boundaries typically involve major changes in the components of B, often but not always including switchbacks These boundaries may correspond to sharp jumps in the magnetic field vector as indicated by a high partial variance of increments (PVI; Greco et al 2008), and the waiting time distribution between high PVI events in PSP data has been analyzed by Chhiber et al (2020). We investigate the filling fraction of domains as a measure of Alfvenicity, and test simple frozen-in models of their aspect ratio by comparing the mean domain duration with the solar wind velocity relative to the spacecraft
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