Collaborative Research: Ground-based Very Low Frequency (VLF) and High Frequency (HF) Measurements in Support of the VIPER Sounding Rocket Experiment

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Radio emissions in the Very Low Frequency (VLF) range are known to propagate long distances bouncing between the surface of the Earth and the partially ionized layer of the upper atmosphere near 90 km in altitude called the D region. The D-region impacts long range radio communications between extremely low frequencies (ELF), used for long-range submarine communications, and high frequencies (HF), used by many critical communications systems. VLF emissions can also escape the ionospheric layer and propagate to much higher altitudes with denser plasma. VLF emissions of both natural and man-made origin can also cause wave-particle scattering in the magnetosphere resulting in particle precipitation in the ionosphere. VLF radio wave propagation and signal strength reduction has been challenging to model and observe, mostly due to the lack of continuous measurements in the D region and significant uncertainties in the knowledge of both plasma and neutral densities at these altitudes. In this project, an enhancement will be made to the existing VLF Trans-Ionospheric Propagation Experiment Rocket (VIPER) experiment, to launch from Wallops Island in July 2020. VIPER will provide important information about radio waves detected along its trajectory, enabling a determination of the VLF attenuation through the ionospheric D region. Specifically, this project will support the installation of ground-based VLF and HF receivers near the transmitter site to provide ground-truth measurements to compare to the rocket measurements. These ground measurements will constrain the ionospheric state and will enable an accurate assessment of the attenuation of the VLF wave as a function of altitude. This project will also support the education of a student at CU Boulder, the training of a postdoc at CU Boulder and a postdoc at UC Berkeley, enhancing their continuing educations and providing a unique opportunity to learn about the valuable work at Wallops Flight Facility.This collaborative effort with provide direct multi-point measurements of VLF wave amplitude and phase, referenced directly from the source transmitter. VIPER will measure the height-resolved amplitude and phase of the NAA (24.0 kHz) transmitter signal broadcast from Cutler, Maine, enabling a determination of the VLF attenuation through the collisional D-region ionosphere. VIPER will also measure the plasma density and HF signal amplitudes during the ionospheric traverse. Together with the rocket measurements, we will have VLF measurements at the transmitter; downrange on the ground; and downrange as an altitude profile from 60 to 150 km. These measurements will provide unprecedented insight into the attenuation of VLF waves by the collisional D-region plasma. In turn, this attenuation will yield insight into the effect of ground-based transmitter signals on the D-region ionosphere and their injection into the magnetosphere. The project also has the potential to make measurements of lightning sferics (on the ground) and whistlers (above the ionosphere), providing a broadband (DC to MHz) view of this attenuation. The lightning observations are a secondary goal and are not critical to the success of the VIPER project.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.