The D-region forms the bottom-most layer of the Earth’s ionosphere, with ionization present between altitudes of 60-90 km. Too high for balloons, too low for satellites, and too tenuous for radio sounding, measurements of this region remain sparse in both space and time, and the region is therefore relatively poorly understood. Nonetheless, the D-region is a critical part of the space environment, affecting long wavelength radio communications and over-the-horizon radar, among other effects. The D-region also strongly interacts with and influences the propagation of very-low-frequency (VLF) waves, which are a critical component of the inner magnetosphere, and radiation belt physics in particular.

The two primary methods for studying the D-region are i) sounding rocket measurements, and ii) subionospheric VLF remote sensing. Sounding rockets can make in-situ measurements of the D-region plasma density; of course, such measurements are highly localized in time and space, and cannot provide much insight into temporal or spatial variations. Subionospheric VLF remote sensing takes advantage of powerful Navy VLF transmitters, whose signals propagate efficiently in the waveguide formed between the conducting Earth and the D-region ionosphere. As these transmitter signals are monitored some distance from the source, changes in the observed amplitude and phase can be attributed to the ionosphere, and through model inversion, the D-region state can be inferred. However, these measurements are extremely underdetermined, and an effective method for large scale D-region monitoring has not yet been established.

In this talk, I will describe recent observations and modeling aimed at improving our observations of the D-region. But the talk will focus on a set of upcoming experiments that will make new, novel observations of the D-region. First, the upcoming VIPER sounding rocket will make in-situ plasma measurements of the D-region, but for the first time will combine these with in-situ measurements of VLF waves as they propagate through this region. Next, the CANVAS CubeSat, currently under development at CU, will make global VLF observations above the ionosphere, which will be used to infer the “transfer function” of the D-region for VLF waves. Finally, a newly-funded experiment will deploy a large array of ground-based VLF receivers (which we call the AVID array). When combined with a novel inversion technique, the AVID array will enable 2D monitoring of the D-region over the better half of North America. Combined, these different approaches to D-region studies will build new insight into the least-understood layer of the Earth’s ionosphere.