Ion-Scale Solitary Structures in the Solar Wind Observed by Solar Orbiter and Parker Solar Probe

TL;DR

This study identifies and analyzes ion-scale magnetic solitary structures in the solar wind using high-resolution data from Solar Orbiter and Parker Solar Probe, revealing their properties, distribution, and potential origins.

Contribution

The paper introduces a machine learning method to detect nearly a thousand ion-scale structures, providing new statistical insights into their occurrence and physical characteristics in the near-Sun solar wind.

Findings

Structures are more common closer to the Sun, peaking around 30-40 solar radii.

Most structures are found in low-beta environments with associated density and magnetic fluctuations.

They exhibit sunward Alfvénic polarization and are primarily oblique to the magnetic field.

Abstract

We investigate a class of ion-scale magnetic solitary structures in the solar wind, characterized by distinct magnetic field enhancements and bipolar rotations over spatial scales of several proton inertial lengths. These structures are revisited using high-resolution data from the Solar Orbiter and Parker Solar Probe missions. Using a machine learning-based method, we identified nearly a thousand such structures, providing new insights into their evolution and physical properties. Statistical analysis shows that these structures are more abundant closer to the Sun, with occurrence rates peaking around (30 - 40, R_sun) and decreasing farther out. High-cadence measurements reveal that these structures are predominantly found in low-beta (beta <= 1) environments, with consistent fluctuations in density, velocity, and magnetic field. Magnetic field enhancements are often accompanied by plasma density drops, which, under near pressure balance, limit field increases. This leads to small fractional field enhancements near the Sun (approximately 0.01 at 20 R_sun), making detection challenging. Magnetic field variance analysis indicates that these structures are primarily oblique to the local magnetic field. Alfv\'enic velocity-magnetic field correlations suggest that most of these structures, unlike most near-Sun solar wind fluctuations, exhibit sunward-directed Alfv\'enic polarization in the plasma frame. We compare these findings with previous studies, discussing possible generation mechanisms and their implications for the turbulent cascade in the near-Sun Alfv\'enic solar wind. While these structures might be Alfv\'enic solitons, vortices, or flux ropes, we refrain from a definitive classification pending further evidence. Further high-resolution observations and simulations are needed to fully understand their origins and impacts.