In-situ switchback formation in the expanding solar wind

Abstract: Recent near-sun solar-wind observations from Parker Solar Probe have found a highly dynamic magnetic environment, permeated by abrupt radial-field reversals, or "switchbacks." We show that many features of the observed turbulence are reproduced by a spectrum of Alfv\'enic fluctuations advected by a radially expanding flow. Starting from simple superpositions of low-amplitude outward-propagating waves, our expanding-box compressible MHD simulations naturally develop switchbacks because (i) the normalized amplitude of waves grows due to expansion and (ii) fluctuations evolve towards spherical polarization (i.e., nearly constant field strength). These results suggest that switchbacks form in-situ in the expanding solar wind and are not indicative of impulsive processes in the chromosphere or corona.

• The paper demonstrates that switchbacks form naturally from intrinsic solar wind turbulence induced by radial expansion, eliminating the need for impulsive solar events.
• Simulations using compressible MHD methods (Snoopy and Athena++) reveal that enhanced Alfvénic fluctuations lead to increased turbulence and switchback occurrence.
• Results align with Parker Solar Probe observations, showing consistent magnetic compressibility and spectral behavior near plasma β ∼ 1.

In-situ Switchback Formation in the Expanding Solar Wind: An Overview

The study, "In-situ switchback formation in the expanding solar wind," by Squire et al. aims to understand the emergence of magnetic switchbacks in the solar wind, observed by the Parker Solar Probe (PSP). These switchbacks are characterized by abrupt, large-angle deflections in the magnetic field direction. Significantly, this paper positions itself in the ongoing discourse about whether these structures are remnants of solar surface events or form spontaneously in the solar wind. The authors propose through comprehensive numerical modeling that these switchbacks are an intrinsic feature of solar wind turbulence developed naturally due to expansion processes, dispelling the need to invoke impulsive solar events as their primary origin.

Methodology

The authors employ compressible magnetohydrodynamic (MHD) simulations utilizing an expanding-box framework to mimic the solar wind's outward motion. They initialize their simulations with broad-range, low-amplitude Alfvénic fluctuations. Two complementary numerical methods are utilized: a modified version of the Snoopy code for achieving larger expansion factors, and a modified Athena++ code better for capturing shocks. These approaches collectively allow an analysis of the development of solar wind turbulence from varying initial conditions and across different plasma β (ratio of thermal to magnetic pressure).

Numerically, the study achieves initial conditions through superpositions of outward-propagating Alfvénic waves, exploring various spectra such as narrowband Gaussian and broader spectrum distributions evolving into turbulence. This setup permits an exploration of how increasing normalized amplitudes, facilitated by radial expansion, influence turbulence characteristics including the formation of switchbacks.

Results

The simulations reveal that as Alfvénic fluctuations increase with radial expansion, switchbacks corresponding to abrupt field reversals occur. This coincides with large, outward-propagating fluctuations with constant magnetic field strength, a condition termed 'spherical polarization,' which becomes particularly apparent around β ∼ 1. The authors demonstrate that switchbacks become more prevalent as the normalized fluctuation amplitude increases. Importantly, these fluctuations maintain a near-constant magnetic field magnitude, akin to observations from PSP, underscoring the natural occurrence of switchbacks without necessitating external solar events.

Several metrics help quantify these findings, including the switchback fraction and magnetic compressibility, which show good alignment with PSP's turbulence observations. Spectra generated from simulations tend toward the observed power-law behavior in the solar wind, aligning with approximately $k_{\perp}^{-1.5}$, further supporting the in-situ formation hypothesis.

Implications and Future Research

This research supports the model where switchbacks form due to intrinsic solar wind dynamics, eliminating the essentiality of impulsive sources at the solar periphery. Such findings prompt a reconsideration of turbulence theories in the context of a radially expanding wind and offer insights for both observational and theoretical frameworks in solar physics.

Future studies might aim to extend numerical simulations to capture a broader range of expansion factors, and incorporate additional wind phenomena such as the arcuate Parker spiral or variable solar wind expansion rates. Comparisons with upcoming and more detailed PSP measurements can further validate these findings, presenting an exciting avenue for advancing our understanding of solar wind turbulence and its underlying mechanisms.