Constraining VLF Trans-Ionospheric Propagation Experiment Rocket (VIPER) Ionospheric Conditions with Ground-based VLF Transmitter Observations

Very Low Frequency (VLF) electromagnetic waves propagate efficiently in the Earth-Ionosphere (EI) waveguide. While little energy escapes the EI waveguide into the magnetosphere during daytime, satellite measurements have shown geophysically significant VLF power leakage through the ionosphere at night.This leakage from artificial VLF transmitters can drive the scattering and loss of relativistic radiation belt electrons at low L-shells, thus impacting space weather phenomena. The leakage of VLF waves through the ionosphere has been modeled a number of different ways leading to ranges of absorption that vary by factors of 2 - 100 (3 - 20 dB), depending on the model. The primary source of uncertainty in these models comes from a lack of knowledge about the vertical profile of the D-region ionospheric electron density and, separately but just as important, the neutral density at D-region altitudes. Past sounding rocket experiments have attempted to address these shortcomings, but failed to fully characterize a vertical profile of EM field measurements and their relationship to the local and global structure of the ionosphere. Designed to address these gaps in knowledge, the VLF Trans-Ionospheric Propagation Experiment Rocket (VIPER) launched in late May 2021. Among other things, VIPER measured height-resolved amplitude and phase of the 24.0 kHz VLF signal originating from the NAA transmitter in Cutler, Maine. In conjunction with the VIPER experiment, two ground-based magnetic loop air-core VLF receivers were installed: one in Machias, Maine, just 15 km from the NAA transmitter, and one at the NASA Wallops Flight Facility in Virginia, near the VIPER launch site. The Machias receiver gives true amplitude and phase information about the transmitted NAA signal. The Wallops receiver then provides baseline VLF data at the range of VIPER to constrain any changes in the signal measured by VIPER to the altitude of measurement. From data gathered by these two receivers, we present an estimation of the D-region ionosphere electron density profile and variance for ±24 hours from the VIPER launch. We also provide an estimation of the D-region ionospheric conditions using these data in conjunction with an electromagnetic full-wave model. These data are compared with the VIPER rocket in-situ electron density and field measurements.