Time lags and POS speeds from t − d maps. Top panel: time lags as a function of distance along the loop for the 131 Å, 171 Å, 193 Å, and 211 Å channels. The individual values are represented by coloured symbols, the associated errors are shown by the vertical bars, and the solid lines denote the corresponding spline fits to the data. Bottom panel: projected POS speeds derived from the spline fits to the time lag values.