The Wave Normal Angle Characteristic of Whistler-Mode Waves in the Dayside Terrestrial Space Based on MMS Observations

Wave normal angle (WNA) is an important parameter in the analysis of wave-particle interactions. The WNA distribution may significantly influence the effective interaction between the waves and solar wind electrons. As a region of direct interaction with the solar wind, the WNA distribution of whistler-mode waves in the dayside terrestrial space is still unclear. This paper reports statistical work on the WNA distribution of whistler-mode waves in the dayside terrestrial region. The results show that whistler-mode waves with quasi-parallel WNA tend to increase gradually near the magnetopause, while the occurrence rate of oblique waves $\left(\theta \ge 35°\right)$ increases significantly in the magnetosheath with increasing solar wind dynamic pressure $\left(P_{sw}\right)$. Under the strong $P_{sw}$ condition, the distribution of WNAs both in quasi-parallel and oblique decreases sharply within the 0.5–0.8 $f_{ce}$ frequency range. Moreover, we find that strong $P_{sw}$ can effectively affect the amplitude of oblique whistler-mode waves. These results can improve understanding of the kinetic process associated with whistler-mode waves.