Investigations of particles and plasma wave dynamics in Earth's magnetosphere are often based on particle energy, and electric and magnetic field measurements from sensors flown on in-situ spacecraft. These data provide coincident particle and wave measurements over large distances. However, space-based sensor platforms often mix spatial and temporal variations, particularly for field variations in the ultralow frequency (ULF; 1-100 mHz) range (Jacobs et al., 1964). Recent satellite constellation missions have provided coincident, spatially separated measurements. Even so, data from multiple locations, recorded at the same time, remain the strength of ground-based measurements.The ability to remote sense magnetospheric phenomena using data obtained from distributed ground sensors provides several advantages over in-situ measurements from satellites. For example, ground magnetometers are cheaper and easier to install and maintain over long periods of time. Magnetometer networks can be placed at numerous locations in both latitude and magnetic local time, allowing for large spatial coverage when mapped into the magnetosphere. The challenge is to infer plasma dynamics in space from data obtained from ground based observations.

This paper presents a comparison of three methods to estimate the latitudinal resonance width of field line resonant, ultra-low frequency waves detected at the ground. These are a spatial domain, full-width half-maximum method and frequency domain amplitude-phase and amplitude-division methods. These methods were used to estimate the resonance width of several field line resonant intervals occurring on 26 November 2001, 1 October 2012, and 19 June 2015. The 19 June 2015 interval used data from one low, two mid, and one high latitudes. It was found that the resonance width estimates were different for each method and with how the data were processed. The most suitable methods and data processing were determined from a damped driven harmonic oscillator model. The amplitude-division methods yielded the most accurate results when the ground magnetic field data were processed with a boxcar window function or a frequency domain, exponential smoothing taper. The amplitude-phase method tended to underestimate the resonance width. The full-width half-maximum method gave accurate results for a high spatial resolution linear piece-wise curve fitted to the spectral amplitude with latitude profile. An accurate estimate of the latitudinal resonance width requires a correct choice of data processing, estimate method, resonance profile with latitude, and resonance model.