Reanalysis of radiation belt electron phase space density using various boundary conditions and loss models M. Daae a,b,⇑ , Y.Y. Shprits c,b , B. Ni b , J. Koller d , D. Kondrashov c,b , Y. Chen d a Department of Physics, Norwegian University of Science and Technology, 7491 Trondheim, Norway b Department of Atmospheric and Oceanic Sciences, University of California Los Angeles, 405 Hilgard Ave, Los Angeles, CA 90095-1565, USA c Institute of Geophysics and Planetary Physics, University of California Los Angeles, 7127 Math Sciences Bldg, Los Angeles, CA 90095-1565, USA d Los Alamos National Laboratory, Space Science and Applications, ISR-1, P.O. Box 1663, MS D466 Los Alamos, NM 87545, USA Received 18 January 2011; received in revised form 28 June 2011; accepted 5 July 2011 Available online 13 July 2011 Abstract Data assimilation is becoming an increasingly important tool for understanding the near Earth hazardous radiation environments. Reanalysis of the radiation belts can be used to identify the electron acceleration mechanism and distinguish local acceleration from radial diffusion. However, for any practical applications we need to determine how reliable is reanalysis, and how significant is the depen- dence of the results on the assumptions of the code and choice of boundary conditions. We present the sensitivity of reanalysis of the radiation belt electron phase space density (PSD) to the assumed location of the outer boundary, using the VERB code and a Kalman filter. We analyze the sensitivity of reanalysis to changes in the electron-loss throughout the domain, and the sensitivity to the assumed boundary condition and its effect on the innovation vector. All the simulations presented in this study for all assumed loss models and boundary conditions, show that peaks in the phase space density of relativistic electrons build up between 4.5 and 6 R E during relativistic electron flux enhancements in the outer radiation belt. This clearly shows that peaks build up in the heart of the electron radiation belt independent of the assumptions in the model, and that local acceleration is operating there. The work here is also an important step toward performing reanalysis using observations from current and future missions. Ó 2011 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Data assimilation; Magnetosphere; Outer radiation belt 1. Introduction The Earth’s radiation belts are a two-zone torus-shaped structure with the inner belt usually located below L 6 2 and the outer belt between approximately 3 6 L 6 10 (Van Allen and Frank, 1959). The outer radiation belt is highly dynamic due to the constant battle between loss- and source-processes of electrons (e.g. Reeves et al., 2003; Shprits et al., 2006a, 2008a,b; Millan and Thorne, 2007). In addition, the outer radiation belt can be a hazardous environment for any electronic equipment since it consists of electrons in the energy range from 100 keV to several tens of MeV. Thus, knowledge of the outer belt dynamics is of high value as the outer belt spatially overlaps many communication and scientific satellite orbits. Radiation belt electrons can be accelerated to relativistic energies by radial diffusion (e.g. Fa ¨lthammar, 1965; Schulz and Lanzerotti, 1974; Hudson et al., 2000; Shprits and Thorne, 2004) or by local acceleration (Summers et al., 1998; Horne and Thorne, 1998; Horne, 2005). A number of modeling studies have also indicated that both acceleration mecha- nisms may operate in the outer radiation belt (e.g. Miyoshi et al., 2003; Brautigam and Albert, 2000; Shprits et al., 2006b; Fok et al., 2008; Subbotin and Shprits, 2009). The acceleration mechanism can be identified by analyzing the radial profile of the phase space density (PSD) (e.g. Chen et al., 2006; Green and Kivelson, 2004; Iles et al., 2006; Turner et al., 2010). Presence of peaks building up in the 0273-1177/$36.00 Ó 2011 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2011.07.001 ⇑ Corresponding author at: Department of Physics, Norwegian University of Science and Technology, 7491 Trondheim, Norway. E-mail address: marianne.daae@ntnu.no (M. Daae). www.elsevier.com/locate/asr Available online at www.sciencedirect.com Advances in Space Research 48 (2011) 1327–1334