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

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.

💡 Research Summary

This paper presents a comprehensive statistical investigation of energetically‑dominant sunward‑propagating Alfvén waves (SAWs) near 1 AU using more than two decades of measurements from the Wind spacecraft. The authors first define SAWs as intervals in which the Alfvénic fluctuations are both energetically dominant (the normalized cross‑helicity |σc| ≥ 0.6, meaning one propagation direction carries at least four times the energy of the opposite) and incompressible (plasma and magnetic compressibilities cn, cB ≤ 0.2). After discarding intervals with large data gaps or mixed magnetic polarity, 64 588 one‑hour windows satisfy these criteria, of which 5 705 (≈ 8.8 %) are classified as sunward‑propagating (σc·Bx < 0).

The dataset is further split into fast (speed ≥ 500 km s⁻¹) and slow (speed < 500 km s⁻¹) solar‑wind streams. Slow wind shows a slightly higher occurrence of SAWs, and the fraction of SAWs decreases with increasing window length (1 h, 3 h, 6 h, 8 h, 12 h), consistent with the idea that small‑scale turbulent patches can locally reverse the dominant Alfvénic polarity within a globally anti‑sunward cascade.

To explore a possible source of SAWs, the authors identify large‑scale magnetic switchbacks—intervals where the magnetic field folds back on itself for longer than one hour. They employ suprathermal electron strahl pitch‑angle distributions (290 eV channel) to detect field inversions: a sustained sunward strahl together with a sunward‑pointing magnetic field indicates an inverted topology (a switchback). The strahl classification is resampled to the 24‑second cadence of plasma and field data, then aggregated to one‑hour intervals by taking the modal topology.

Cross‑referencing the 1 hour SAW intervals with the switchback catalog yields 286 coincidences out of 1 636 SAW intervals, i.e., 17.5 % of SAWs occur within large‑scale switchbacks. This statistical link supports the hypothesis that switchbacks can generate or trap sunward‑propagating Alfvénic fluctuations, perhaps through magnetic field rotation, local reconnection exhausts, or the expansion‑driven generation mechanisms proposed for Parker Solar Probe observations.

The paper therefore establishes three key results: (1) SAWs are a minority but persistent component of the solar‑wind turbulence spectrum, more prevalent in slow wind and at smaller inertial‑range scales; (2) the occurrence of SAWs declines with increasing temporal window, matching expectations from MHD turbulence theory that predicts localized patches of opposite Alfvénic polarity; (3) a non‑negligible fraction (~ 1/6) of SAWs are associated with large‑scale magnetic switchbacks, indicating that switchbacks constitute a viable source for at least part of the sunward Alfvénic population.

These findings have important implications for solar‑wind heating and turbulence models. Since counter‑propagating Alfvén waves are essential for nonlinear cascade and plasma heating, the presence of SAWs—especially those linked to switchbacks—could modify the local cascade rate and energy transfer pathways. Future work suggested includes detailed spectral analysis of SAW‑containing switchbacks, investigation of ion‑scale kinetic signatures, and modeling of how expansion‑driven switchbacks interact with pre‑existing turbulence to produce sunward Alfvénic energy.