Observational Evidence of the Four‐Wave Interaction Between Magnetospheric Whistler Mode Waves

Abstract

Whistler mode wave is a crucial emission which can significantly affect the electron dynamics in the magnetosphere. The nonlinear three‐wave interaction between whistler mode waves has been frequently observed, while the four‐wave interaction between these waves is seldom reported. Here, we present two multiband whistler mode wave events by Van Allen Probes, in which the relatively weak wave bands occur exactly corresponding to the strong wave bands. We find that the frequencies of the weak and strong bands satisfy the frequency matching conditions of the four‐wave interaction, and the wave normal angles of the weak bands are almost within the possible ranges calculated from the wave vector matching conditions. Additionally, the cross‐correlation coefficients between the amplitudes of the weak bands and that of strong bands approach 0.80. These results indicate that the weak bands are very likely to be produced by the four‐wave interaction between whistler mode waves. Similar to the three‐wave interaction, the four‐wave interaction can extend the frequency of whistler mode waves from close to 0.5fce to a wider range in both the lower (0.26fce) and upper (0.78fce) bands, and thus potentially play an important role in the electron dynamics.