Excitation and Dispersion of Whistler Waves Inside the Contracting Magnetic Hole

Magnetic holes (MHs) are the magnetic structures characterized by the significant decreases of magnitude field, and they have been widely observed in various space plasmas. The high-resolution observations by NASA's Magnetospheric Multiscale Mission and the Dispersion RelAtion From Timing (DRAFT) method enable us to study the wave-particle interactions in MHs in great detail. In this paper, we report whistler waves in a contracting MH and reveal their dispersion relation with DRAFT method, which is well-consistent with the theoretical prediction. During the excitation of the whistler waves, we also find the formation of pancake distribution (corresponding to electron temperature anisotropy T_⊥/T_// > 1). By calculating the growth rate of the whistler waves with an analytical model, we find such waves are generated by the temperature anisotropy (T_⊥/T_// > 1) at the center of MH, which is formed during the contraction. These results suggest that wave-particle interactions may be closely related to the large-scale evolution of the MH structures.