Lower‐Hybrid‐Driven Whistler Generation During Active Ionospheric Heating at HAARP

Abstract

Active high‐frequency (HF) heating experiments provide a controlled framework to investigate nonlinear wave coupling and plasma structuring in the ionosphere. In this study, we present a coordinated experiment utilizing the High Frequency Active Auroral Research Program (HAARP), combining HF heating of the F region with simultaneous extremely low/very‐low frequency (ELF/VLF) probing. Broadband ELF/VLF measurements obtained near the facility are analyzed together with SuperDARN radar observations sampling the heated volume. During periods of HF heating, the ELF/VLF spectra exhibit systematic enhancements and structured features that are absent or significantly weaker during heater‐off intervals. These spectral changes occur in close temporal correspondence with increased radar backscatter, indicating the development of heater‐induced plasma structuring. Guided by established HF heating theory and nonlinear lower‐hybrid/lower‐hybrid‐oblique‐resonance (LH/LOR)–whistler coupling mechanisms, we interpret these observations as evidence that HF heating created a plasma state favorable for (a) enhanced coupling to the LH/LOR branch near the lower‐hybrid resonance (LHR) scale and (b) subsequent scattering, mode conversion, and parametric interactions that populate the received VLF spectrum beyond the direct stepped transmission.