Ionospheric total electron content perturbations induced by lithosphere-atmosphere-ionosphere interaction

Thomas Dautermann, Purdue University

Abstract

Volcanic explosions or shallow earthquakes are known to trigger acoustic and gravity waves that propagate in the atmosphere at infrasonic speeds. At ionospheric heights, coupling between neutral particles and free electrons induces variations of electron density that are detectable with dual-frequency GPS measurements. Using GPS data collected at continuous stations in the Caribbean, we identified an ionospheric perturbation following a major volcanic explosion at the Soufriere Hills Volcano (Montserrat, Lesser Antilles) on July 13, 2003. Its frequency content shows peaks centered at 1 mHz and 4 mHz, consistent with previous observations and theory and indicative of a gravity and acoustic component. We retrieve a horizontal velocity of 624 m/s for the acoustic component, which, given sound speed at the altitude of electron density peak (∼735 m/s) implies upward propagation at an angle of about 33 degrees, consistent with ray tracing results. ^ We model the acoustic component as the result of a N-wave pressure source at ground level and use ray-tracing to propagate the neutral pressure wave, accounting for the dispersive characteristics of the atmosphere while conserving total acoustic energy. Plasma velocity is derived from neutral velocity using a finite difference solution of the magneto-hydrodynamic momentum equation. We use the continuity equation for charge densities to compute corresponding electron density variations, then numerically integrate these variations along satellite-to-receiver line-of-sights, while accounting for the GPS satellite displacements. We minimize the misfit between observed and model waveforms to estimate a total acoustic energy release of 1.53×10¹⁰ J. This method could be applied to any explosion of sufficient magnitude, provided that continuously recording GPS instruments are available at near to medium range from the source. Stacking over groups of neighboring GPS receivers may help improve the detection threshold and investigate events of smaller magnitude. ^ Shortly after the explosion we also observed volumetric strain variations with a frequency of ~4 mHz, just like the atmospheric TEC perturbation. The strain signal exhibits a double pulse structure similar to to observations in GPS total electron content after the eruption within the same frequency band. Spectral analysis shows an amplitude peak at 4 mHz for both datasets, with similar waveforms and signal duration. To show that the signature originated from a single isotropic source, we model the explosion within the atmosphere, exciting normal modes of a realistic Earth model with atmosphere. We use a normal mode summation technique to reproduce both the atmospheric and the strain signal. We find that both the atmospheric acoustic signal and also partially the strain signature were caused by the explosion of the SHV. The double pulses observed in data from both instruments result from the superposition of the dominant fundamental and two first harmonics atmospheric modes that trigger resonant coupling with the solid Earth around 4 mHz. ^

Degree

Ph.D.

Advisors

Eric Calais, Purdue University, Laura Pyrak-Nolte, Purdue University.

Subject Area

Geophysics|Physics, Electricity and Magnetism|Atmospheric Sciences|Physics, Fluid and Plasma

COinS