Direct Observation of Rising-Tone Chorus Triggered by Enhanced Solar Wind Pressure

Chorus waves play a significant role in both electron precipitation and acceleration in Earth's radiation belts. Their generation includes the initial linear process and subsequent nonlinear process. After the amplitude of whistler-mode waves reaches a sufficient value in the linear phase, the nonlinear process takes effect and results in the frequency chirping of chorus waves. Here we present the first report on the generation and intensification of rising-tone chorus driven by two sudden increases of solar wind dynamic pressure observed by Van Allen Probes on 20 December 2015. First, whistler-mode waves are excited by the anisotropic thermal electrons (similar to 10s keV), which is quite consistent with the linear theoretical expectation. Then, rising-tone chorus waves are nonlinearly generated from the existing whistler-mode waves, triggered by the first increase of solar wind dynamic pressure. Subsequently, the second increase of solar wind pressure further intensifies the rising-tone chorus. Based on the nonlinear theoretical model and Ts04 magnetic field model, we demonstrate that the decreasing inhomogeneity of the background magnetic field due to the increasing solar wind dynamic pressure is the major cause to trigger rising tones, which essentially reduces the wave amplitude threshold of the nonlinear process. Our study emphasizes the significance of solar wind dynamic pressure in the Earth's radiation belts from a micro perspective.