Energetically-dominant Sunward-Propagating Alfvén Waves Near 1 au and Their Relation to Large-scale Magnetic Switchbacks

TL;DR

This work systematically identifies energetically-dominant sunward-propagating Alfvén waves (SAWs) near 1 au using two decades of Wind data, applying thresholds on cross helicity and incompressibility to isolate SAWs. It then classifies SAWs by propagation direction and cross helicity, and employs suprathermal electron strahl pitch angle distributions to detect large-scale magnetic switchbacks, finding that a substantial subset of SAWs occur within inverted-field switchback topologies. The analysis reveals a distribution where counterstreaming strahl is most common, with inverted switchbacks comprising about 17.5% of 1-hour SAW intervals and more prevalent in fast wind, supporting switchbacks as a candidate source for a portion of the SAW population. The study provides a robust methodology and datasets for linking SAWs to switchbacks and other sources, and outlines future work to extend the analysis to the inner heliosphere with Parker Solar Probe and Solar Orbiter to elucidate the radial evolution of SAWs and their drivers.

Abstract

In this letter, we investigate the population of energetically-dominant sunward-propagating Alfvén waves (SAWs) using more than 20 years of data provided by the Wind spacecraft near 1 au. We refer to SAWs as energetically-dominant sunward-propagating Alfvén waves within inertial range scales. Key parameters such as normalized cross helicity, plasma incompressibility, and magnetic incompressibility are used to determine the SAWs. Incorporating the polarity of the heliospheric magnetic field, AW modes are identified, which enables the determination of the propagation direction. Occurrence rates of SAWs vary from 1% to 14% depending on the time scale and solar wind stream type considered. Particularly, the relationship between large-scale magnetic field switchbacks (SBs) and SAWs (for a 1-hour long time scale) is investigated. A methodology utilizing pitch angle distributions of suprathermal electron strahl is employed to identify inverted magnetic field topology. The intervals containing SAWs are cross-referenced and examined with intervals identified as SBs. For a sample of 1636 1-hour SAW intervals, 17.5% are associated with magnetic field switchbacks occurring at scales larger than one hour. The analysis lends support to the idea of switchbacks as one of the candidate sources for a portion of the SAW population.

Figures (5)

• Figure 1: Selection criteria for Alfvénic intervals. Left Panel: Distribution of normalized cross helicity for all 1-hour intervals. The absolute value of $\sigma_c$ is considered as the sign merely indicates which Alfvén mode is dominant. The threshold of inclusion, $|\sigma_c| \geq 0.6$, is denoted by the dashed line. Right Panel: Distribution of plasma and magnetic compressibilities for all 1-hour intervals. The green square denotes included intervals satisfying $c_n, c_B \leq 0.2$. Intervals must satisfy both$|\sigma_c| \geq 0.6$ and $c_n, c_B \leq 0.2$ to be considered for analysis.
• Figure 2: Sample of 1-hour intervals spanning 2022-2025, colored by propagation direction. Of the 18,264 intervals in the sample, 6,934 are filtered out (gray). The 998 sunward AWs, denoted in yellow, are relatively scarce compared to the 10,332 anti-sunward AWs (blue). All intervals shown meet the compressibility requirement $c_n, c_B \leq 0.2$.
• Figure 3: Occurrence of SAW as a function of window size. Slow solar wind streams (diamonds) contain more SAWs when compared with fast wind (squares). White markers are interpolated window sizes using cubic Hermite spline fitting.
• Figure 4: SAW parameters during an observed switchback in the inverted magnetic topology. The original 1-hour interval is centered within the red dashed lines, with six-hour neighbor windows on either side. Panel 1) radial component of the magnetic field (blue) and proton density (orange); Panel 2) magnitude of the magnetic field (blue) and plasma bulk velocity (light brown); Panel 3) magnetic radial angle; Panel 4) electron strahl PAD; Panel 5) electron strahl PAD normalized to the background flux. The gray lines in Panel 3 represent the Parker Angle near 1 au (45$^{\circ}$ and 135$^{\circ}$), along with 90$^{\circ}$.
• Figure 5: Distributions of magnetic topology split by solar wind stream for 1-hour intervals of energetically-dominant SAW. Counterstreaming strahl is indicated in blue. Uninverted and inverted topologies are represented by green and red sectors, respectively.