Dispersion relations at sub-ion scales during the interval t = 1.51τ_A − 1.60τ_A. (a)–(c) PSD traces of the magnetic-field, electric-field, and electron-velocity fluctuation for perpendicular propagating fluctuations (k_∥ ≈ 0). The black dashed line shows the theoretical dispersion relation of the IBWs, the red line corresponds to the lower-hybrid wave (LHW), and the green line denotes the fast wave (FW) dispersion ω = kv_f. (d)–(f) PSD traces of the magnetic-field, electric-field, and electron-velocity fluctuation for parallel propagating fluctuations (k_⊥ ≈ 0). In panels (d)–(f), the black dashed line represents the theoretical dispersion relation of whistler waves (WW).