Correlation coefficient of long-lasting quasi-radial IMF events

Long-lasting quasi-radial interplanetary magnetic field (IMF) events represent a distinct class of solar wind conditions characterized by an unusual magnetic field orientation. Unlike typical solar wind intervals, these events deviate significantly from the nominal Parker spiral configuration, leading to altered large-scale spatial correlations of the IMF. In this study, we investigate the spatial correlation characteristics of long-lasting quasi-radial IMF events and compare them with those observed during normal Parker spiral conditions. Quasi-radial IMF events are identified using the criterion Bx / B >0.9 sustained for more than 4 hours, following a definition similar to that adopted in previous studies. Applying this criterion to Wind magnetic field observations from 2001 to 2024, we identify a total of 753 long-lasting quasi-radial IMF events. We calculate correlation coefficients between Wind and ACE magnetic field measurements without applying a time shift, thereby focusing on the intrinsic spatial correlation rather than propagation effects. The correlation analysis is performed for multiple magnetic field parameters, with particular emphasis on the magnetic field magnitude. Our results indicate that, during quasi-radial IMF conditions, the correlation coefficient of the magnetic field magnitude exceeds that of typical Parker spiral intervals when the spacecraft separation distance is greater than approximately 30 Re. However, within the intermediate separation range of roughly 30–150 Re, the correlation values are generally lower than those observed during Parker spiral conditions, suggesting a scale-dependent modification of IMF coherence under quasi-radial configurations. These findings imply that long-lasting quasi-radial IMF events exhibit distinct spatial correlation behaviors compared to nominal solar wind conditions, potentially reflecting differences in solar wind structure, turbulence properties, or magnetic field topology.