AbstractUsing multiple pairs of International Monitor for Auroral Geomagnetic Effects ground magnetometers together with simultaneous measurements from two of the Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft constellation, when they were flying over the magnetometers in magnetic conjunction and in close azimuthal separation, we are able to calculate the phase differences of Ultra Low Frequency Field‐Line Resonances, and, through that, their azimuthal mode number, wavelength and propagation characteristics. A cross‐wavelet technique is applied, that exposes the times and frequencies of common power between time series from azimuthally aligned magnetometers in space or on the ground, yielding their relative phase. Using the amplitude ratio and phase differences between ground stations with similar longitudes that are separated in latitude, a correction to the mode number calculation is demonstrated, accounting for the phase differences that arise from the L shell separation of the THEMIS probes.