Density And Velocity Fluctuations of Alpha Particles in Magnetic Switchbacks

TL;DR

This study investigates density, abundance, and velocity fluctuations of alpha particles within magnetic switchbacks observed by the Parker Solar Probe, revealing complex behaviors and proposing a method to estimate the wave frame from particle data.

Contribution

It provides new insights into alpha particle dynamics in switchbacks and introduces a novel technique to determine the wave frame using particle measurements alone.

Findings

No consistent alpha abundance differences inside vs outside switchbacks.

Alpha velocities can increase, decrease, or remain unchanged within switchbacks.

The wave frame can be estimated from particle data via sphere fitting.

Abstract

Magnetic switchbacks, or sudden reversals in the magnetic field's radial direction, are one of the more striking observations of Parker Solar Probe (PSP) thus far in its mission. While their precise production mechanisms are still unknown, the two main theories are via interchange reconnection events and in-situ generation. In this work density and abundance variations of alpha particles are studied inside and outside individual switchbacks. We find no consistent compositional differences in the alpha particle abundance ratio, $n_{\alpha p}$, inside vs outside, nor do we observe any signature when separating the switchbacks according to $V_{\alpha p}/V_{pw}$, the ratio of alpha-proton differential speed to the wave phase speed (speed the switchback is travelling). We argue these measurements cannot be used to rule in favour of one production mechanism over the other, due to the distance between PSP and the postulated interchange reconnection events. In addition we examine the 3D velocity fluctuations of protons and alpha particles within individual switchbacks. While switchbacks are always associated with increases in proton velocity, alpha velocities may be enhanced, unchanged, or decrease. This is due to the interplay between $V_{pw}$ and $V_{\alpha p}$, with the Alfv\'enic motion of the alpha particles vanishing as the difference $|V_{pw} - V_{\alpha p}|$ decreases. We show how the Alfv\'enic motion of both the alphas and the protons through switchbacks can be understood as approximately rigid arm rotation about the location of the wave frame, and illustrate that the wave frame can therefore be estimated using particle measurements alone, via sphere fitting.