Shear-driven Transition to Isotropically Turbulent Solar Wind Outside the Alfvén Critical Zone

Abstract

Motivated by prior remote observations of a transition from striated solar coronal structures to more isotropic “flocculated” fluctuations, we propose that the dynamics of the inner solar wind just outside the Alfvén critical zone, and in the vicinity of the first surface, is powered by the relative velocities of adjacent coronal magnetic flux tubes. We suggest that large-amplitude flow contrasts are magnetically constrained at lower altitude but shear-driven dynamics are triggered as such constraints are released above the Alfvén critical zone, as suggested by global magnetohydrodynamic (MHD) simulations that include self-consistent turbulence transport. We argue that this dynamical evolution accounts for features observed by Parker Solar Probe (PSP) near initial perihelia, including magnetic “switchbacks,” and large transverse velocities that are partially corotational and saturate near the local Alfvén speed. Large-scale magnetic increments are more longitudinal than latitudinal, a state unlikely to originate in or below the lower corona. We attribute this to preferentially longitudinal velocity shear from varying degrees of corotation. Supporting evidence includes comparison with a high Mach number three-dimensional compressible MHD simulation of nonlinear shear-driven turbulence, reproducing several observed diagnostics, including characteristic distributions of fluctuations that are qualitatively similar to PSP observations near the first perihelion. The concurrence of evidence from remote sensing observations, in situ measurements, and both global and local simulations supports the idea that the dynamics just above the Alfvén critical zone boost low-frequency plasma turbulence to the level routinely observed throughout the explored solar system.

Characteristic Scales of Magnetic Switchback Patches Near the Sun and Their Possible Association With Solar Supergranulation and Granulation

“…It has been proposed that switchbacks may form as the solar wind evolves, being produced by turbulence or velocity shears (Ruffolo et al 2020;Squire et al 2020). This is supported by the studies of Mozer et al (2020) and Macneil et al (2020) who found that the occurrence of switchbacks increases with radial distance from the Sun r. While we cannot conclude on switchback occurrence at radial distances greater than 60 R s , our analysis suggests that it is unrelated to heliocentric radial distance near the Sun (see Sections 3.1 and 4).…”

Switchbacks: statistical properties and deviations from Alfvénicity

“…This mechanism was first proposed by Landi et al (2005Landi et al ( , 2006. A slightly different version, involving the nonlinear instability of such sheared flows, has been proposed by Ruffolo et al (2020). This mechanism mainly leads to a slow-mode (pressure-balanced) contribution to the switchback.…”

Direct evidence for magnetic reconnection at the boundaries of magnetic switchbacks with Parker Solar Probe

“…Observational studies invoke the formation of these structures either by processes occurring deep in the solar atmosphere (Dudok de Wit et al 2020;Krasnoselskikh et al 2020;Macneil et al 2020;Woodham et al 2021) or directly in the solar wind (Ruffolo et al 2020). These two types of origins have also been explored on the numerical and theoretical side: on the one hand, via interchange reconnection in the corona (Fisk & Kasper 2020;Drake et al 2021) or jets in the solar atmosphere (Roberts et al 2018;Sterling & Moore 2020;He et al 2020); on the other hand, via waves growth generated by the expansion of the structure away from the Sun (Squire et al 2020) or shear-driven turbulence in the solar wind (Ruffolo et al 2020). As of today, the mechanisms leading to the formation of switchbacks is still unclear and it is unknown whether different populations of switchbacks might be generated by distinct mechanisms.…”