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2. Travel-Time Magnetoseismology
Like earthquakes to the Earth system, several types of impulsive phenomena, such as sudden impulses and Pi2 waves, occasionally occur in the magnetosphere. These impulsive events start with an abrupt perturbation of magnetospheric field lines in a local region, and the signal propagates to other corners of the system later by means of MHD waves.

For sudden impulses, the preliminary impulse seen on the ground mainly results from the Alfvén wave that propagate down from the corresponding field line. The peak of a preliminary impulse relates to a propagation path that can preserve the maximum wave energy. This travel path consists of a shortest route from the source location to the field line of concern, as well as the field-aligned component that leads to the ground observer, shown below.

The travel time along these paths to the ground locations at different L -values is plotted in the below figure. The differentiation in signal's arrival time is the basis of travel-time magnetoseismology.


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