Radial Evolution of Near-Sun Magnetic Switchbacks Alfvenicity, Occurrence Rate, and Size

Abstract

Magnetic switchbacks, characterized by reversals of magnetic field direction, are widely observed in the inner heliosphere by Parker Solar Probe (PSP). With PSP reaching perihelia near 10Rs, observations from the first 24 encounters enable studies of near-Sun switchback evolution at r > 10Rs. We construct a switchback catalog within 10 < r < 55Rs by identifying magnetic field reversals with stable field magnitude and strahl-electron polarity. Statistical analysis shows that switchback Alfvenicity decreases with increasing radial distance, consistent with solar wind evolution beyond the Alfven critical point. Meanwhile, switchback occurrence rate and spatial size increase with distance, suggesting continued generation and expansion during solar wind propagation. At a given radial distance, the fraction of solar wind containing switchbacks is positively correlated with background solar wind radial velocity (VR) and Alfven Mach number (MA), while the local occurrence rate is mainly controlled by MA. These results suggest that switchback patches preferentially form in faster and higher-MA solar wind. The spatial size of switchbacks shows no clear dependence on MA or VR, implying that their size evolution is probably not determined by source conditions. Solar activity influences switchback evolution through changes in background solar wind properties, with a larger fraction of higher-MA switchbacks during solar minimum. We further identify anisotropy relative to the background magnetic field direction: the local occurrence rate and spatial size are approximately 1.5 times as large in the perpendicular direction as in the parallel direction, indicating distinct magnetic topology of switchback patches