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

TL;DR

This study investigates magnetic deflections near the Alfven surface in the solar wind, revealing how the Alfven Mach number influences deflection angles and the potential role of the conversion layer in forming magnetic switchbacks.

Contribution

It introduces the concept of the conversion layer near the Alfven surface and analyzes how magnetic deflections evolve with the Alfven Mach number in the solar wind.

Findings

Magnetic deflection angles > 90° are unlikely in sub-Alfvenic regimes.

Deflection angles increase with the Alfven Mach number, suggesting a steepening mechanism.

The conversion layer near Ma=1 is critical for magnetic energy conversion and switchback formation.

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.