Van Allen Probes observation and modeling of chorus excitation and propagation during weak geomagnetic activities

We report correlated data on nightside chorus waves and energetic electrons during two small storm periods: 1 November 2012 (Dst approximate to-45) and 14 January 2013 (Dst approximate to-18). The Van Allen Probes simultaneously observed strong chorus waves at locations L=5.8-6.3, with a lower frequency band 0.1-0.5f(ce) and a peak spectral density approximate to 10(-4)nT(2)/Hz. In the same period, the fluxes and anisotropy of energetic (approximate to 10-300keV) electrons were greatly enhanced in the interval of large negative interplanetary magnetic field Bz. Using a bi-Maxwellian distribution to model the observed electron distribution, we perform ray tracing simulations to show that nightside chorus waves are indeed produced by the observed electron distribution with a peak growth for a field-aligned propagation approximately between 0.3f(ce) and 0.4f(ce), at latitude <7 degrees. Moreover, chorus waves launched with initial normal angles either <90 degrees or >90 degrees propagate along the field either northward or southward and then bounce back either away from Earth for a lower frequency or toward Earth for higher frequencies. The current results indicate that nightside chorus waves can be excited even during weak geomagnetic activities in cases of continuous injection associated with negative Bz. Moreover, we examine a dayside event during a small storm C on 8 May 2014 (Dst approximate to-45) and find that the observed anisotropic energetic electron distributions potentially contribute to the generation of dayside chorus waves, but this requires more thorough studies in the future.