This letter introduces the idea of timing the arrival of magnetic impulsive signals at different locations to infer the density distribution of magnetospheric plasma. Tamao's model of magnetohydrodynamic (MHD) wave propagation is used as the underlying principle of this method. The methodology is similar to timing the arrival of seismic signals to understand the Earth's interior. A test of this idea on a sudden impulse event shows that the results are in good agreement with an empirical plasma density model as well as with the density estimated independently by the field line resonance method.