The work at Umeå University was supported by the Swedish Research Council (VR), the Kempe Foundation, and Stiftelsen Lars Hiertas Minne Grant No. FO2010-1015. Numerical simulations were performed at High Performance Computer Center North (HPC2N), Umeå, Sweden, within SNAC Project No. 001-10-159. The work at Princeton University was supported by the U.S. Air Force Office of Scientific Research.