Conversion Layer Controls the Evolution of Magnetic Deflections Near the Alfven Surface

TL;DR

This study analyzes Parker Solar Probe data to link magnetic deflection angles $\theta_{def}$ with the local Alfvénic Mach number $M_a$ near the Alfvén surface ($M_a=1$). By examining sub- and super-Alfvénic intervals, velocity fluctuations, and radial energy fluxes $S_R$ and $K_R$, it identifies a conversion layer defined by $|\log_{10}(M_a)|\lesssim 0.2$ where velocity deflections approach the Alfvén speed and energy transfer shifts from Poynting- to kinetic-dominated. The results suggest that large deflections form via gradual steepening of Alfvenic fluctuations and may be enhanced by KH-like instabilities near the conversion layer, contributing to magnetic switchback formation in the solar wind. These findings highlight a local, Ma-controlled mechanism for magnetic energy conversion and the evolution of deflections as they propagate through the Alfvén surface region. The work has implications for solar wind heating, acceleration, and the in-situ generation of switchbacks.

Abstract

We examine the statistics of Alfvenic deflections in both sub-Alfvenic and super-Alfvenic solar wind with particular focus on a common parameter that underlies the definition of switchbacks: the magnetic deflection angle. Our findings are in general agreement with earlier studies that suggest magnetic deflection angles > 90 degrees are very unlikely to occur in sub-Alfvenic regimes. We find that their upper limit exhibits an identifiable trend with the Alfven Mach number Ma, suggesting that gradual steepening of Alfvenic deflections with increasing Ma is a plausible mechanism controlling deflection angles in the young solar wind. Further analysis reveals that large velocity fluctuations tend to be important in the largest sub-Alfvenic magnetic deflections with increasing contributions from the parallel component very close to Ma = 1, while virtually no magnetic deflections in the super-Alfvenic regime exhibit such large velocity perturbations. We also determine the local ratio of radial Poynting flux SR to kinetic energy flux KR and find that large sub-Alfvenic deflection angles tend to be dominated by SR, while super-Alfvenic deflections are eventually dominated by the KR associated with the radial solar wind flow. Our results show that within the vicinity of the Alfven surface (where Ma = 1), there is a critical region of parameter space within which velocity deflections approach the Alfven velocity and KR/SR is close to unity. We refer to this region (where | log10(Ma)| < 0.2) as the conversion layer. The conversion layer may play a significant role in the evolution of magnetic defections by providing the medium for converting magnetic energy to particle energy and likely driving the formation of magnetic switchbacks in super-Alfvenic solar wind.

Figures (4)

• Figure 1: Interval Examples: (a) Sub-Alfvénic Interval from encounter 23 and (b) super-Alfvénic interval from encounter 16. Each example shows, from top to bottom, the heliocentric distance of PSP in units of solar radii, the Alfvénic Mach number with a red dotted line at $M_a = 1$, and the deflection angle with a red dotted line at $\theta_{def} = 90$ degrees.
• Figure 2: Distribution of Magnetic deflection angles (a) as a function of $\log_{10}(M_a)$ with black lines indicating $M_a =1$ and $\theta_{def} = 90$ degrees and (b) as a function of Heliocentric distance with a horizontal black line indicating $\theta_{def} = 90$ degrees.
• Figure 3: Distribution of velocity deflections with respect to $\log_{10}(M_a)$ and deflection angle normalized to (a) the mean bulk velocity and (b) the mean Alfvén speed. Also included are separate velocity deflection contributions (c) parallel and (d) perpendicular to the mean magnetic field, each normalized to the mean bulk velocity. (e) Ratio of radial ion kinetic energy flux to radial Poynting flux with respect to $\log_{10}(M_a)$ and deflection angle. The solid black lines in all plots indicate the values of $\log_{10}(M_a) = 0$ (or $M_a = 1$) and $\theta_{def} = 90$ degrees. The vertical dotted lines in all plots indicate values of $|\log_{10}(M_a)| = 0.2$ as a visual aid.
• Figure 4: Diagram illustrating the characteristics of the sub-Alfvénic and super-Alfvénic regimes as well as the so-called 'conversion layer' where proximity to the Alfvén surface introduces nonlinear interactions between magnetic and velocity deflections.