Occurrence characteristics of outer zone relativistic electron butterfly distribution: A survey of Van Allen Probes REPT measurements

Using Van Allen Probes Relativistic Electron Proton Telescope (REPT) pitch angle resolved electron flux data from September 2012 to March 2015, we investigate in detail the global occurrence pattern of equatorial (||3 degrees) butterfly distribution of outer zone relativistic electrons and its potential correlation with the solar wind dynamic pressure. The statistical results demonstrate that these butterfly distributions occur with the highest occurrence rate similar to 80% at similar to 20-04 magnetic local time (MLT) and L>similar to 5.5 and with the second peak (>similar to 50%) at similar to 11-15 MLT of lower L shells similar to 4.0. They can also extend to L=3.5 and to other MLT intervals but with the occurrence rates predominantly<similar to 25%. It is further shown that outer zone relativistic electron butterfly distributions are likely to peak between 58 degrees and 79 degrees for L=4.0 and 5.0 and between 37 degrees and 58 degrees for L=6.0, regardless of the level of solar wind dynamic pressure. Relativistic electron butterfly distributions at L=4.0 also exhibit a pronounced day-night asymmetry in response to the P-dyn variations. Compared to the significant L shell and MLT dependence of the global occurrence pattern, outer zone relativistic electron butterfly distributions show much less but still discernable sensitivity to P-dyn, geomagnetic activity level, and electron energy, the full understanding of which requires future attempts of detailed simulations that combine and differentiate underlying physical mechanisms of the geomagnetic field asymmetry and scattering by various magnetospheric waves.NSFC [41204120, 41474141, 41304130, 41574160]; China Postdoctoral Science Foundation [2013M542051, 2014T70732]; NSF Geospace Environment Modeling grant [AGS-1103064]SCI(E)EIARTICLEbbni@whu.edu.cn115644-56524