Fig. 1: First column: EM fluctuations were observed in the vicinity of the electron diffusion region (EDR 2 in Table 1).

During this interval, the four MMS spacecraft formed a tetrahedron with a quality factor of ~0.984. The average spacing among the four spacecraft was about 26 km ~4 d_e, where 1 d_e is 7 km given an average electron density of 0.56 cm⁻³. a illustrates the spacecraft trajectory and propagating of EM fluctuations, b three components of magnetic field, c electron bulk velocity, d electric field fluctuations, e spectrum of E, f magnetic field fluctuations, g spectrum of B, h ellipticity, i electron beta \({\beta }_{e}\). The vectors are shown in local boundary normal (LMN) coordinates, refer to Wang et al.⁵⁵, and references therein, where L is along the reconnecting magnetic field direction, N is the normal direction of the current sheet, M is the out-of-plane direction of the current sheet, and N = L × M completes the right-hand coordinate system. To perform statistical analysis, we uniformly adjust the +M of all EDR events to the direction of the reconnection electric field, i.e., E_R > 0 in all EDR events in this work. The second and third columns: estimated frequency-wave number (f-k) power spectrum of EM fluctuations in the vicinity of the reconnection site. j–l show the f-k_L, f-k_M, and f-k_N of EM fluctuations in the B_L < 0 side (20:24:03-20:24:07 UT) of the reconnecting current sheet, while m–o show the f-k_L, f-k_M, and f-k_N of E_M fluctuations in the B_L > 0 side (20:24:07-20:24:11 UT). The wave number k has been normalized by \(\sqrt{{\rho }_{e}{\rho }_{i}}\), where \({\rho }_{e}\) and \({\rho }_{i}\) are the electron and ion gyration radii, respectively. The dashed lines denote the fit of the linear dispersion relation \(f=\frac{{V}_{{ph}}}{2\pi }{{\bf{k}}}\).