Radiation belt 2D and 3D simulations for CIR-driven storms during Carrington Rotation 2068 Mary Hudson a,n , Thiago Brito a , Scot Elkington b , Brian Kress a , Zhao Li a , Mike Wiltberger c a Physics and Astronomy Department, Dartmouth College, Hanover, NH 03755, USA b Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO 80303, USA c High Altitude Observatory, NCAR, Boulder, CO 80307, USA article info Article history: Received 20 September 2011 Received in revised form 29 March 2012 Accepted 30 March 2012 Available online 26 April 2012 Keywords: Solar wind Magnetosphere Modeling Radiation belts abstract As part of the International Heliospheric Year, the Whole Heliosphere Interval, Carrington Rotation 2068, from March 20 to April 16, 2008 was chosen as an internationally coordinated observing and modeling campaign. A pair of solar wind structures identified as Corotating Interaction Regions (CIR), characteristic of the declining phase of the solar cycle and solar minimum, was identified in solar wind plasma measurements from the ACE satellite. Such structures have previously been determined to be geoeffective in producing enhanced outer zone radiation belt electron fluxes, on average greater than at solar maximum. MHD fields from the Coupled Magnetosphere–Ionosphere–Thermosphere (CMIT) model driven by ACE solar wind measurements at L1 have been used to drive both 2D and 3D weighted test particle simulations of electron dynamics for the CIR subset of the month-long CMIT fields. Dropout in electron flux at geosynchronous orbit and enhancement during recovery phase, characteristic of CIR-driven storms, is seen in these moderate (Dstmin ¼56,  33 nT) events, while the two CIRs were characterized by increased solar wind velocity in the 650–750 km/s range. The first beginning March 26 produced a greater enhancement in IMF B z southward and stronger magneto- spheric convection, leading to a greater radiation belt electron response at GOES. This study provides the first comparison of 2D and 3D particle dynamics in MHD simulation fields, incorporating the additional diffusive feature of Shebansky orbit trapping of electrons in the magnetic minima on the dayside above and below the equatorial plane. Overall loss occurs during the main phase for 2D and 3D simulations, while incorporation of plasmasheet injection in 2D runs produces a moderate enhance- ment for the March 26–30 storm, less than observed at GOES, and recovery to initial flux levels as seen for the April 4–7 storm. & 2012 Elsevier Ltd. All rights reserved. 1. Introduction Distinct solar wind drivers operate during the declining phase and around solar maximum, as evident in the top panel of Fig. 1, where the weekly averaged solar wind speed is plotted from a combination of measurements from the IMP 8, Wind and ACE spacecraft, data available from NSSDC OMNIWeb (Li et al., 2006, 2011). A comparison of the weekly averaged solar wind speed with 30-day averaged flux of 2–6 MeV electrons measured by the low altitude polar orbiting SAMPEX satellite (bottom panel) shows the geoeffectiveness of intervals of recurring high speed streams during the declining phase from sunspot maximum (sunspot number plotted in the top panel). High speed solar wind streams originating in persistent coronal holes and modulated by the solar rotation period are characteristic of the declining phase of solar activity. Two such intervals are seen in Fig. 1, in 1993–1994 and 2003–2004, when coronal hole structures extended down to the ecliptic plane, serving as the source of fast solar wind (Tsurutani et al., 2006), while confined to the polar regions of the sun near solar maximum when the magnetic field is mainly dipolar (Hundhausen, 1977). The episodic peaks in flux from 1998 to 2001 are associated with high speed interplanetary shocks triggered by coronal mass ejections (CMEs). The recently prolonged minimum in sunspot number, solar wind speed and radiation belt electron flux is the most unusual of the space age (Gibson et al., 2009). CIRs were still present during 2008, but solar wind speed and magnetic field strength were at record lows, making them less geoeffective than earlier in the declining phase (Gibson et al., 2011). Tsurutani et al. (2006) have presented a schematic in Fig. 2 of the two classes of storms characteristic of solar maximum and solar minimum drivers, defining storm phases, with the top panel showing the stronger Dst minimum associated with CME-initiated Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/jastp Journal of Atmospheric and Solar-Terrestrial Physics 1364-6826/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jastp.2012.03.017 n Corresponding author. Tel.: þ1 603 646 2976. E-mail address: mary.hudson@dartmouth.edu (M. Hudson). Journal of Atmospheric and Solar-Terrestrial Physics 83 (2012) 51–62