Analysis framework for systematically studying ionospheric response to impulsive events from below

Abstract

AbstractImpulsive phenomena in the Earth's atmosphere produce acoustic and gravity waves which perturb the ionosphere. Such perturbations are often measured using total electron content fluctuations (TEC), derived from ground‐based Global Positioning System data. Using TEC data from the Japanese GEONET ground network after the Tōhoku earthquake on 11 March 2011, we demonstrate capabilities of a new framework of methodologies for analyzing ionospheric perturbations. The framework consists of several new techniques: calculating velocity along a single direction to reduce error due to anisotropic propagation, producing normalized bidirectional band‐pass spectra that preserve relative timing between various frequencies and allowing a more systematic determination of broadband pulses, and utilizing a wavelet‐based technique that considers instantaneous wave phase changes, rather than best fit time differences, to evaluate wave characteristics (speed, direction, and wavelength) within spectral ranges of interest. Using these techniques together decreases subjectivity and reduces errors in attributing fluctuations to given sources. In validating this framework using the Tōhoku case, we consistently identify three kinds of waves: a broad‐band pulse (speed: >2000 m/s, max range: >1400 km) arriving in the ionosphere 10–15 min after the quake, acoustic waves following the pulse (period: 3–5 min, speed: 700–1000 m/s, max range: <500 km), and gravity waves (period: 10–15 min, speed: 150–500 m/s, max range: >1400 km) propagating away from the epicenter, consistent with theory and demonstrated in previous studies. This framework also can be applied to other impulsive events in the atmosphere that are more difficult to detect and attribute to sources.