Ratio of component amplitudes versus frequency and time for all observing sessions. In order to be able to plot both modes on the same scale, we give I_c2/I_c1 for B-mode and the inverse quantity, I_c1/I_c2 for the Q-mode. For the B-mode, I_c2/I_c1 changes with time and this evolution is similar for every mode instance. This makes it possible to estimate the time since the start of the B-mode for L77924 (top row), although we did not directly record the transition in this observation. The vertical black lines mark the transitions between the modes.