Butterfly like whistler wave observations from the magnetospheric multiscale (MMS) mission

MMS1, one of the spacecraft of the magnetospheric multiscale (MMS) mission observed a unique kind of whistler mode chorus waves on 1 March, 2016 at 23:38:25 UT. The wave is characterized by having frequency ∼(0.1-0.5)×fce , where fce is the electron cyclotron frequency. Further, it shows right hand polarization having wave normal angle below 250. These properties signify the wave mode to be lower band chorus waves. The event is very unique as it shows both the rising tone and falling tone of chorus waves arranged in such a beautiful way that it makes the perception of a butterfly in the electric and magnetic field spectrogram. The satellite was located near the dawn side of the equatorial region at the time of observation. The source of the generation is traced out to be the electron temperature anisotropy present just before the appearance of wave signatures. Auroral Electrojet (AE) index is found to be ≥400 nT which is a clear signature of sub-storm activity during that event. Further analysis is carried out and found that the event is also detected in the other three spacecrafts, i.e., MMS2, MMS3 and MMS4. The spatial and temporal scale extent of the chorus waves will be discussed. An enhancement in the differential energy flux of the pitch angle distribution of electrons is also noticed. Further, the role of this butterfly like chorus waves in the dynamics of inner magnetosphere will be discussed.