Electrostatic nature and compressibility of perpendicularly propagating waves (k_∥ ≈ 0). The black dashed line shows the theoretical dispersion relation of the IBWs, the red line corresponds to the LHW, and the white line denotes the FW dispersion ω = kv_f . (a) Polarization state of the perpendicular electric-field components. (b) PSD ratio of perpendicular to parallel electric-field fluctuations. (c) Ratio of electric field to magnetic field. (d) PSD ratio of proton number density.