This work was supported by National Natural Science Foundation of China grants 41631071, 41774170, 41274174, 41174125, 41131065, 41421063, 41231066 and 41304134, Chinese Academy of Sciences grants KZCX2-EW-QN510 and KZZD-EW-01-4, CAS Key Research Program of Frontier Sciences grant QYZDB-SSW-DQC015, National Key Basic Research Special Foundation of China Grant No. 2011CB811403, and Fundamental Research Funds for the Central Universities WK2080000077. We acknowledge the University of Iowa as the source of EMFISIS data (this acknowledgement does not imply endorsement of the publication by the University of Iowa or its researchers). The Van Allen Probes data are available at the websites: EMFSIS, http://emfisis.physics.uiowa.edu/Flight/, ECT, http://www.rbsp-ect.lanl.gov/datapub/.