Spectral investigation of traveling ionospheric disturbances: IONOLAB-FFT

Ionosphere is an important layer of atmosphere which is under constant forcing from both below due to gravitational, geomagnetic and seismic activities, and above due to solar wind and galactic radiation. Spatio-temporal variability of ionosphere is made up of two major components that can be listed as spatio-temporal trends and secondary variabilities that are due to disturbances in the geomagnetic field, gravitational waves and coupling of seismic activities into the upper atmosphere and ionosphere. Some of these second order variabilities generate wave-like oscillations in the ionosphere which propagate at a certain frequency, duration and velocity. These oscillations cause major problems for navigation and guidance systems that utilize GNSS (Global Navigation Satellite Systems). In this study, the frequency and duration of wave-like oscillations are determined using a DFT (Discrete Fourier Transform) based algorithm over the STEC (slant total electron content) values estimated from single GPS (Global Positioning System) station. The performance of the developed method, namely IONOLAB-FFT, is first determined using synthetic oscillations with known frequencies and durations. Then, IONOLAB-FFT is applied to STEC data from various midlatitude GPS stations for detection of frequency and duration of both medium and large scale TIDs (traveling ionospheric disturbances). It is observed that IONOLAB-FFT can estimate TIDs with more than 80% accuracy for the following cases: frequencies from 0.6 mHz to 2.4 mHz and durations longer than 10 min; frequencies from 0.15 mHz to 0.6 mHz and durations longer than 50 min; frequencies higher than 0.29 mHz and durations longer than 50 min.