Travel-time Magnetoseismology: Successes, Challenges, and Future Directions

Abstract:

This paper summarizes the research on travel-time magnetoseismology starting from the inception of the method approximately a decade ago. Like terrestrial seismology that infers information about earthquake hypocenters and the Earth's interior by timing seismic arrivals, travel-time magnetoseismology can identify the start of the magnetic impulse and deduce the structure of magnetospheric density by measuring impulse arrivals at different locations simultaneously. Observations of sudden impulses have shown that the arrival time of the preliminary impulse is latitude-dependent in a way consistent with the MHD propagation along the so-called Tamao path. The inverse calculation that incorporates the preliminary impulse arrival times at multiple ground locations has deduced a plasmapause location in good agreement with spacecraft observations. The concept of travel-time magnetoseismology has also been applied to the study of substorm onsets, shedding light not only on the signal propagation from the magnetotail to the ionosphere but also on the start time and location in the magnetotail. An important implication from the travel-time analysis of substorm onsets is that, contrary to the common assumption, the onset location in the ionosphere does not necessarily map along the magnetic field to the onset location in the magnetotail. In most likely scenarios, the impulse signal reaches the ionosphere first at low auroral latitude, explaining why auroral intensification usually starts there. Exercising travel-time magnetoseismology in the magnetotail poses significant challenges to modeling the signal propagation in a complex geometry that can vary substantially from one case to another. The concept of the Tamao path, albeit in good agreement with numerical simulations in a dipole geometry, needs validation or revision for magnetotail problems. The paper concludes by suggesting areas where further research is likely to make clear progress in travel-time magnetoseismology.