Mesospheric Properties from ELF/VLF Wave Generation Experiments and VLF Scattering Experiments at HAARP

In 2024 and 2025, the University of Florida (UF) performed a variety of extremely-low-frequency/very-lowfrequency (ELF/VLF) wave generation experiments and VLF scattering experiments using the High-frequency Active Auroral Research Program's (HAARP) Ionospheric Research Instrument (IRI) in Gakona, Alaska.The ELF/VLF wave generation experiments focused on elucidating the wave generation process in the mesosphere/lower ionosphere.We were able to identify meaningfully different ELF/VLF source altitudes as a function of modulation frequency between 1 and 20 kHz, for instance, indicating the ability to detect the heating and cooling timescales throughout the mesospheric source region.Additional heating using a second heater beam produced interesting changes in the magnitude of the ELF/VLF signals generated and in the altitude of the ELF/VLF source region as a function of modulation frequency.We were able to selectively heat the ELF/VLF source region and, separately, the ionospheric reflection point in the D-region ionosphere.Results are presented from the perspective of analyzing how ELF/VLF radio wave propagation depends on the ability of mesosphere/lower ionosphere to absorb energy.