Midlatitude Ionospheric Irregularity Spectral Density as Determined by Ground‐Based GPS Receiver Networks

We present a new technique to experimentally measure the spatial spectrum of ionospheric disturbances in the spatial scale regime of 40-200 km. This technique produces a two-dimensional (2-D) spectrum for each time snapshot over two dense Global Positioning System (GPS) receiver networks (GPS Earth Observation Network in Japan and Plate Boundary Observatory in the Western United States). Because this technique created the spectrum from an instantaneous time snapshot, no assumptions are needed about the speed of ionospheric irregularities. We examine spectra from 3 days: one with an intense geomagnetic storm, one with significant lightning activity, and one quiet day. Radial slices along the 2-D spectra provide one-dimensional spectra that can be fit to a power law to quantify the steepness of the falloff in the spatial scale sizes. Continuous data of this type in a stationary location allow monitoring the variability in the 2-D spectrum over the course of a day and comparing between days, as shown here, or even over a year or many years. We find that the spectra are highly variable over the course of a day and between the two selected regions of Japan and the Western United States. When strong traveling ionospheric disturbances (TIDs) are present, the 2-D spectra provide information about the direction of propagation of the TIDs. We compare the TID propagation direction with horizontal wind directions from the Horizontal Wind Model. TID direction is correlated with the horizontal wind direction on all days, strongly indicating that the primary source of the TIDs measured by this technique is tropospheric. Plain Language Summary The ionosphere is a part of the upper atmosphere that is ionized (made into a plasma) by incoming solar radiation. Turbulence and waves can be created in this plasma by various processes from above (solar activity) and below (e.g., thunderstorms and earthquakes). The spatial spectrum of these perturbations ranges from thousands of kilometers to meters. Characteristics of the spectrum impact the distortion of radio signals passing through the ionosphere, between ground and satellite. This paper shows a new way of measuring the spatial spectrum by using hundreds of closely spaced ground-based GPS receivers in Japan and in California. We create a map of ionospheric variation and waves at each time and then measure variations with scale sizes of 50-200 km with a new mathematical technique. We look at the difference in the spatial spectrum between a calm day, a day with significant solar activity, and a day with significant thunderstorm activity. We find that the spectrum changes a lot over the course of a day and from day to day. We also can see that waves in the ionosphere often move opposite to the direction of the ionospheric wind, indicating that they are originating from below the ionosphere.