Untangling the Solar Wind and Magnetospheric Drivers of the Radiation Belt Electrons

Many solar wind parameters correlate with one another, which complicates the causal-effect studies of solar wind driving of the magnetosphere. Conditional mutual information (CMI) is used to untangle and isolate the effect of individual solar wind and magnetospheric drivers of the radiation belt electrons. The solar wind density (n(sw)) negatively correlates with electron phase space density (PSD; average energy similar to 1.6 MeV) with time lag (tau) = 15 hr. The effect of n(sw) has been attributed to magnetopause shadowing or other loss mechanisms, but when the effect of solar wind velocity (V-sw) is removed, tau shifts to 7-11 hr, which is a more accurate time scale for this process. The peak correlation between V-sw and PSD shifts from tau = 30-50 to 44-56 hr, when the effect of n(sw) is removed. This suggests that the time scale for electron acceleration to 1-2 MeV is about 44-56 hr following V-sw enhancements. The effect of n(sw) is significant only at L* = 4.5-6 (L* > 6 is highly variable), whereas the effect of V-sw is significant only at L* = 3.5-6.5. The peak response of PSD to V-sw is the shortest and most significant at L* = 4.5-5.5. As time progresses, the peak response broadens and shifts to higher tau at higher and lower L*, consistent with local acceleration at L* = 4.5-5.5 followed by outward and inward diffusion. The outward radial diffusion time scale at L* = 5-6 is similar to 40 hr per R-E.

Automated summary

Many solar wind parameters correlate with each other, making it challenging to study the cause-and-effect relationship between solar wind and magnetosphere. By using conditional mutual information, the effect of individual solar wind and magnetospheric drivers on radiation belt electrons can be isolated. It is found that solar wind density negatively correlates with electron phase space density, and the effect of solar wind velocity shifts the time scale for this process. The peak correlation between solar wind velocity and electron phase space density suggests a time scale for electron acceleration.