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

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. Alfvénic velocity-magnetic field correlations suggest that most of these structures, unlike most near-Sun solar wind fluctuations, exhibit sunward-directed Alfvénic 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 Alfvénic solar wind. While these structures might be Alfvénic 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.

arXiv comment11 pages, 4 figures. Submitted to The Astrophysical Journal Letters and currently under review

References in corpus (18)

• SciPy 1.0--Fundamental Algorithms for Scientific Computing in Python
• Array Programming with NumPy
• Measurement of the electric fluctuation spectrum of magnetohydrodynamic turbulence
• The multi-scale nature of the solar wind
• Solar Wind Turbulence and the Role of Ion Instabilities
• Parker Solar Probe: Four Years of Discoveries at Solar Cycle Minimum
• Contribution of cyclotron-resonant damping to kinetic dissipation of interplanetary turbulence
• Kinetic Signatures and Intermittent Turbulence in the Solar Wind Plasma
• Coherent events and spectral shape at ion kinetic scales in the fast solar wind turbulence
• Compressive Coherent Structures at Ion Scales in the Slow Solar Wind
• Coherent structures at ion scales in fast solar wind: {\it Cluster} observations
• Density Fluctuations Associated with Turbulence and Waves: First Observations by Solar Orbiter
• Observation of an MHD Alfvén vortex in the slow solar wind
• Density Fluctuations in the Solar Wind Driven by Alfvén Wave Parametric Decay
• Magnetospheric Multiscale Observation of Kinetic Signatures in the Alfvén Vortex
• New Observations of Solar Wind 1/f Turbulence Spectrum from Parker Solar Probe
• The Parametric Decay Instability of Alfven waves in Turbulent Plasmas and the Applications in the Solar Wind
• Magnetic twist profile inside magnetic clouds derived with a superposed epoch analysis