Properties of exohiss waves during different level of geomagnetic activity

Exohiss are broadband, structureless whistler-mode waves outside the plasmapause. In our work, using the data sets detected by the EMFISIS suite aboard Van Allen Probe A, from 1st January 2013 to 30th June 2017, the exohiss waves are categorized among two types based on the direction of Poynting flux: unidirectional exohiss, and bidirectional exohiss waves. It seems that most exohiss waves are bidirectional, which are mainly distributed on the dayside. Compared to the hiss waves within the plasmasphere, the amplitude of bidirectional exohiss waves on the prenoon side increase very little with the enhancement of geomagnetic activity. Through the analysis of suprathermal electron flux associated with geomagnetic activity, this suggests that the waves may undergo very strong Landau damping during intense geomagnetic activity. On the other hand, the unidirectional exohiss waves are mainly distributed on the afternoon side, and the largest unidirectional exohiss waves are observed during the highest levels of substorm activity.