An information-theory examination of the “upstream-turbulence effect” in solar-wind/magnetosphere coupling

Introduction Correlations between upstream solar-wind turbulence—measured by the amplitude of magnetic-field fluctuations (ΔB/B)—and increases in geomagnetic activity have been reported by many research groups. However, questions remain as to whether this relationship reflects a true cause-and-effect interaction. This study investigates whether solar-wind turbulence exerts a causal influence on the magnetosphere. Methods Transfer entropy (TE) was used as a quantitative measure of causality to evaluate information transfer from ΔB/B to geomagnetic indices AE and Dst. The magnitude and lag time (τ) of information transfer were compared with those derived from solar wind velocity (V sw ) and from reconnection-driven R quick and Newell solar wind coupling functions. Results The analysis shows that ΔB/B exerts a real but weak causal influence on the magnetosphere, operating over relatively long time scales. Information transfer from ΔB/B to AE and Dst is approximately an order of magnitude smaller and peaks at larger lag times than transfers from R quick and Newell solar wind coupling functions. In contrast, the lag times for information transfer from ΔB/B and V sw to AE and Dst are comparable. The information transfer from ΔB/B to AE is approximately 45% of that from V sw , while the transfer from ΔB/B to Dst is about an order of magnitude smaller than that from V sw . Discussion The similarity in response times between ΔB/B and V sw suggests that turbulence interacts with the magnetosphere primarily through viscous-like processes rather than reconnection-driven mechanisms. Although both ΔB/B and V sw influence the magnetosphere through viscous interactions, ΔB/B exhibits lower geoeffectiveness. These findings indicate that solar wind turbulence affects different magnetospheric regions and phenomena to varying degrees and confirm that its influence, while real, is relatively weak.