(a) Wavevector distribution of E_z, showing prominent beam-driven Langmuir (BL) wave excitation and limited backscattered Langmuir waves in k_∥, k_⊥ space. (b)–(c) ω–k dispersion analysis with analytical curves for Langmuir (L) and fundamental modes (O/F), indicating dominant energy in the BL branch and signatures of mode coupling into the fundamental. The dashed lines illustrate dispersion curves, which were derived from magnetoionic theory. Rows represent results for Box A (top), Box B (middle), and Box C (bottom).