MHD-test particles simulations of moderate CME and CIR-driven geomagnetic storms at solar minimum and comparison with Van Allen Probes radiation belt electron measurements

Abstract:

We have investigated two contrasting drivers of radiation belt electron response at solar minimum with MHD-test particle simulations of 1) the 14 May 2019 CME-shock driven geomagnetic storm which produced a Dst = -65 nT ring current response; and 2) the 1 September 2019 interval of IMF Bz <0, enhanced solar wind dynamic pressure and velocity which produced a more prolonged enhanced ring current with minimum Dst = - 52 nT and multiple AE index enhancements exceeding 1500 nT indicative of substorm injections. The latter event with characteristic features of a prolonged CIR-driven storm produced the hardest relativistic electron spectrum observed by Van Allen Probes in the last two years of the mission, which ended in October 2019. MHD simulation fields using the Lyon-Fedder-Mobarry global MHD code coupled to the Rice Convection Model were run with L1 solar wind input for both events, along with 3D test particle simulations in those fields of radiation belt electron response. Initial flux profiles used measurements from the Relativistic Electron Proton Telescope and MagEIS energetic particle instruments on Van Allen Probes for test particle weighting in order to produce fluxes which can be compared with measurements. The two classes of events, CIR-driven which are characteristic of solar minimum and CME-shock driven which are far less common at solar minimum, are contrasted for their geoeffectiveness in modifying outer zone relativistic electron fluxes near the end of Solar Cycle 24.