The 17 March 2015 St. Patrick’s Day Storm is the largest geomagnetic storm to date of Solar Cycle 24, with a Dst of -223 nT. The magnetopause moved inside geosynchronous orbit under high solar wind dynamic pressure and strong southward IMF Bz causing loss, however a subsequent drop in pressure allowed for rapid rebuilding of the radiation belts. Local heating is evident during main phase in this and the 17 March 2013 storm, which reached Dst of -132 nT. Both storms show similar effects on outer zone electrons: first a rapid dropout due to inward motion of the magnetopause followed by rapid increase in flux above the pre-storm level early in the recovery phase and a slow increase over the next 12 days. These phases can be seen in temporal evolution of the electron phase space density measured by the Energetic Particle, Composition and Thermal Plasma Suite (ECT) instrument on Van Allen Probes.
Using the Lyon-Fedder-Mobarry global MHD model driven by upstream solar wind measurements with the Magneotsphere-Ionosphere Coupler, we have simulated both St. Patrick’s Day 2013 and 2015 events, analyzing LFM electric and magnetic fields to calculate radial diffusion coefficients. These coefficients have been implemented in a radial diffusion code, using the measured electron phase space density following the local heating as the initial radial profile and outer boundary condition for subsequent temporal evolution over the next 12 days, beginning 18 March. Agreement with electron phase space density at 1000 MeV/G measured by the MagEIS component of the ECT instrument suite on Van Allen Probes (30 keV – 4 MeV) was much improved using radial diffusion coefficients from the MHD simulations relative to coefficients parametrized by a global geomagnetic activity index.