The results of examining the field-alignment of the observed waves with periods between 1–5 min. The top-left panel shows the altitude (in units of Earth radii R_E) at which the propagation direction matched the field orientation for the observed waves (red) and the combination of all three Monte Carlo tests (the Monte Carlo results are combined for clarity as they were all consistent with one another). The middle-left panel shows the observed azimuth of propagation for field-aligned (green) and non-field-aligned (magenta) waves, while the bottom-left panel compares observed wavelengths for the same groups. Each histogram is independently normalised, and the vertical axis gives the relative probability in arbitrary units. The right hand panels show occurrence rates as a function of time and month for the observed field-aligned waves (top-right), observed non-field-aligned waves (middle-right) and Monte Carlo results using 180 km HWM14 winds (bottom-right).