Context : “ EIT waves ” are freely-propagating global pulses in the low corona which are strongly associated with the initial evolution of coronal mass ejections ( CMEs ) .
They are thought to be large–amplitude , fast–mode magnetohydrodynamic waves initially driven by the rapid expansion of a CME in the low corona .

Aims : An “ EIT wave ” was observed on 6 July 2012 to impact an adjacent trans–equatorial loop system which then exhibited a decaying oscillation as it returned to rest .
Observations of the loop oscillations were used to estimate the magnetic field strength of the loop system by studying the decaying oscillation of the loop , measuring the propagation of ubiquitous transverse waves in the loop and extrapolating the magnetic field from observed magnetograms .

Methods : Observations from the Atmospheric Imaging Assembly onboard the Solar Dynamics Observatory ( SDO /AIA ) and the Coronal Multi-channel Polarimeter ( CoMP ) were used to study the event .
An Empirical Mode Decomposition analysis was used to characterise the oscillation of the loop system in CoMP Doppler velocity and line width and in AIA intensity .

Results : The loop system was shown to oscillate in the 2nd harmonic mode rather than at the fundamental frequency , with the seismological analysis returning an estimated magnetic field strength of \approx 5.5 \pm 1.5 G .
This compares to the magnetic field strength estimates of \approx 1–9 G and \approx 3–9 G found using the measurements of transverse wave propagation and magnetic field extrapolation respectively .

Conclusions :