Characterizing the lower ionosphere with a space - weather - aware receiver matrix

Current ionospheric models are very good at specifying regular diurnal and seasonal variations of the E and F regions of the ionosphere. Less is known about the behavior of the D region, although progress has recently been made with models such as the Data-Driven D-Region (DDDR). However, significant departures from modeled behaviors are observed even during solar minimum conditions, due to complex ionospheric weather effects arising from both solar activity above and terrestrial atmospheric perturbations below. The D-region perturbations directly affect VLF communications, and also affect daytime absorption of frequencies from LF through HF. Perturbations in the E and F regions affect HF propagation, and may impact transionospheric communications at much higher frequencies. In order to characterize ionospheric weather and its effects on operational systems, better observing networks are needed, comparable to those utilized to study and forecast mesoscale (10-1000 km) tropospheric weather. Current terrestrial observatories used in space-weather studies are widely separated geographically. They often do not record continuous observations, making it difficult to quantify the spatial and temporal behavior of space-weather phenomena. We describe a passive sensor network designed to map conditions in the D and E regions and in the F-region bottom side on a continental scale, based on continuous monitoring of propagation effects on signal strength in the VLF through HF frequency range (10 kHz to 30 MHz.) This network is inexpensive to build and to operate, providing information about ionospheric conditions along the monitored signal paths, and enabling space weather effects to be inferred. This weather information is used to update mesoscale D- and E-region models, which are in turn used by radio-propagation modeling tools to analyze signal observations. A prototype network has been deployed in the western United States, with six fixed and two transportable sensors. Data from the prototype and from an earlier HF-only experiment show that D-region variability, sporadic E, and bounds on F-region parameters can be inferred. When used in conjunction with existing ionospheric observatory data, this sensor network offers an affordable means of studying mesoscale D- and E-region weather patterns, with excellent temporal resolution (a few minutes) over entire solar cycles.