The Structure and Origin of Switchbacks: Parker Solar Probe Observations

Abstract

Switchbacks are rapid magnetic field reversals that last from seconds to hours. Current Parker Solar Probe (PSP) observations pose many open questions in regard to the nature of switchbacks. For example, are they stable as they propagate through the inner heliosphere, and how are they formed? In this work, we aim to investigate the structure and origin of switchbacks. In order to study the stability of switchbacks, we suppose the small-scale current sheets therein are generated by magnetic braiding, and they should work to stabilize the switchbacks. With more than 1000 switchbacks identified with PSP observations in seven encounters, we find many more current sheets inside than outside switchbacks, indicating that these microstructures should work to stabilize the S-shape structures of switchbacks. Additionally, we study the helium variations to trace the switchbacks to their origins. We find both helium-rich and helium-poor populations in switchbacks, implying that the switchbacks could originate from both closed and open magnetic field regions in the Sun. Moreover, we observe that the alpha-proton differential speeds also show complex variations as compared to the local Alfvén speed. The joint distributions of both parameters show that low helium abundance together with low differential speed is the dominant state in switchbacks. The presence of small-scale current sheets in switchbacks along with the helium features are in line with the hypothesis that switchbacks could originate from the Sun via interchange reconnection process. However, other formation mechanisms are not excluded.

The Temperature, Electron, and Pressure Characteristics of Switchbacks: Parker Solar Probe Observations

“…In general, the alpha particles are preferentially heated via Alfvén-cyclotron dissipation when the alpha-toproton differential flow normalized by local Alfvén speed is close to 0, whereas the protons are more heated when the differential flow increases (Kasper et al 2008), and the heating efficiency is also related to β ∥p (Kasper et al 2013). As shown in Huang et al (2023a), the alpha-to-proton differential speeds in different regions of switchbacks are predominantly low, implying the switchbacks should be favorable for the heating of alpha particles. Moreover, Figure 2 shows the β ∥p varies in a wide range in switchbacks; thus the heating efficiency could vary in different regions.…”

“…In this work, we use the 1748 switchbacks identified by our automatic algorithm from E1 to E8 for the following statistical analysis, with the searching method and switchback event lists detailed in Huang et al (2023a). Additionally, we note that there are many different definitions of switchbacks based on the comparison of magnetic field rotations regarding the background, and different criteria of the rotation angles are applied (e.g., Bale et al 2019;Kasper et al 2019;de Wit et al 2020;Horbury et al 2020;Woolley et al 2020;Bandyopadhyay et al 2021;Fargette et al 2021;Wu et al 2021;Fargette et al 2022).…”

“…Following the method of Kasper et al (2019), we separate an individual switchback into five parts: leading/ trailing quiet period (LQP/TQP), leading/trailing transition region (LTR/TTR), and spike. As described in Huang et al (2023a), we identify the different parts of an individual switchback in negative (positive) magnetic sectors with the following criterion: the spike interval satisfies −( + )B R /|B| < 0.25, the quiet region complies with −( + )B R /|B| > 0.85, and the transition region includes all data between these thresholds. Therefore, the spike is generally characterized by a fully magnetic field reversal, as shown by the blue-shaded region, where B R /|B| changes polarity while the dominant E-PADs stay in the same direction.…”

“…Between the quiet periods and the spike are located the transition regions where the magnetic field rotates from the quiet period to the spike or vice versa, and they usually contain large-amplitude fluctuations. In general, we select comparable intervals for the five parts of switchbacks, but it varies for each event as described in Huang et al (2023a). Furthermore, QP (TR) means the combined region of LQP (LTR) and TQP (TTR) in the following.…”

“…In this work, we investigate the thermal characteristics, electron pitch-angle distributions, and pressure variations of both inside and outside of theswitchbacks. Following the method of Kasper et al (2019) and Huang et al (2023a), we identify thousands of switchbacks with PSP observations during encounters 1-8 (E1-E8), except for E3 due to data gaps. Based on the analysis of the proton temperature anisotropies and the alpha-to-proton temperature ratios, we investigate whether switchbacks contribute to solar wind heating.…”