The top panel shows Doppler shift (signals from individual transmitters are artificially offset by about 4 Hz) for the analysed period at 4.65 MHz (left side, a–c) and at 7.04 MHz (right side, d–f), while the panels in the middle and at the bottom give information on RMS values of Doppler shift in different frequency (period) bands and azimuths of the wave propagation, respectively.