Abstract:
We report observations of co-existing rising and falling tone emissions of electromagnetic ion cyclotron (EMIC) waves
by THEMIS E spacecraft. The investigation of these fne structures of the EMIC waves is essential from the point of view
of understanding the connection between the proton holes and the proton hills in velocity phase-space. The wave
packets of rising and falling tones are tracked by Poynting vector analysis, where we observe that the rising tones are
propagating northward and the falling tones are propagating southward. The nonlinear wave growth theory supports
our observations. We propose a model where the proton velocity distribution function evolves through the formation
of proton holes on the negative side of the distribution function and mirrored resonant protons forming proton hills
on the positive side of the distribution function, allowing us to observe the co-existing rising and falling tone EMIC
waves.
