WIND DRIVEN ELECTRONS. “This result was later augmented by the understanding that those high-speed streams that were coincident with southward interplanetary magnetic field were more efficient in enhancing the electron fluxes. As southward interplanetary field and high solar wind speed are the main drivers of magnetospheric storm activity, it is not surprising that the average electron flux levels trace geomagnetic activity.”
https://link.springer.com/article/10.12942/lrsp-2007-1