Power spectral density (PSD) of the Elsässer variables z ^±, velocity fluctuations δ v , and magnetic fluctuations in Alfvén units δ b . The left and right panels correspond to the first and second sub-Alfvénic interval observed in E10, respectively. The black lines in each panel represent the theoretical spectra predicted by the NI MHD turbulence model, and f _t denotes the transition frequency at which the z ⁻ spectrum flattens in the inertial range.