Analysis of longitudinal field measurements. Top: time series of B_l measurements. All values have a negative sign, and range between −1 and −23 G. Bottom: generalised Lomb-Scargle periodogram of the longitudinal field time series. The analysis does not yield any significant (FAP < 1%) periodicity. The window function of the entire time series has been included and is mirrored with respect to the x axis (VanderPlas 2018), to highlight aliases due to the observation cadence.