On Using Traveling Ionospheric Disturbances to Detect Underground Nuclear Tests

The ionospheric delay in GPS signals observed by the ground stations can be converted to the total electron content (TEC), which is the total number of electrons in a column with unit cross section areas along the GPS signal's path between the satellite and the receiver on the ground. Spatial and temporal fluctuations in TEC are indicative of the dynamics of the ionosphere, and as such, may support detection and mapping of the traveling ionospheric disturbances (TIDs) excited by acoustic-gravity waves from point sources, as well as by geomagnetic storms, tsunamis, tropical storms, and other phenomena. The velocity of TID propagation provides an important constraint for inferring and locating its source. The first-order velocity can be calculated using the detected time of a TID arrival at ionosphere pierce point (IPP), and the slant distance from a point source event (e. g., an underground nuclear explosion, UNE) to the IPP. IPP is a point, where the ray connecting the receiver to the GPS satellite intersects the infinitesimally thin layer that represents the ionosphere, assumed at roughly 300 km altitude, which corresponds to the middle of the ionospheric F-layer. Additional improvements to TID velocity can be made by adjusting the velocity estimates for the average drag effects of atmospheric winds, when using information from multiple GPS receivers that allows for sufficient redundancy. The TEC derived from the slant signal path, referred to as slant TEC (STEC), was observed and analyzed in this study to reveal TIDs that were used to locate the North Korean underground nuclear explosion (UNE) of May 2009. The UNE was also detected by seismic stations and possibly a local infrasound network of the International Monitoring System (IMS) of the Comprehensive Nuclear Test Ban Treaty Organization (CTBTO), but no radionuclide evidence was found. We also detected TIDs that may be related to the first North Korean UNE of 2006. This paper describes the method, used to detect the UNEs, as well as provide a verification of the results against the seismic method. It is concluded, based on these preliminary results, that the global GPS infrastructure enables mapping spatial and temporal variations of TEC that may, in the future, augment other methods of detecting and locating clandestine UNEs.