Rising tone whistler waves near the electron-scale reconnection

An MMS observation on Dec. 24. 2016 found a rising tone whistler wave mode near the diffusion region of subsolar magnetopause reconnection. The spectrum of whistler wave starts from ~0.5 Fce (electron cyclotron frequency) in the frequency domain, then increases until the spectrum reach ~1.0 Fce showing very clear features of a rising tone whistler emission. The frequency change rate agrees with the theoretical rate calculated from the measured MMS data. This whistler mode is generated by an electron beam streaming away from the X-line. Therefore, the beam would be responsible for not only the generation of whistler waves but also the nonlinear second order resonance that can make the frequency rise and the wave power amplify. Another observation on Nov. 23. 2016 found a similar rising tone whistler mode. In this Nov. 23. 2016 event, the whistler wave starts from ~0.2 Fce and ends at ~0.5 Fce.

In both events, there exists only one rising tone whistler emission, unlike inside the Earth's magnetosphere, where quasi-periodic rising or falling tone whistler emissions are observed. That means rising tone whistler waves can exist either in a very small region near the diffusion region or in a very short time period, or possibly both. In these two events, the rising tone whistler waves are observed together with electron-scale reconnection; thus, the generation of rising tone whistlers might be related to the very early phase of the evolution of the reconnection.