Spectral evolution of INDIVIDUAL SIR events. Spectral evolution of each SIR event is given from the downstream to upstream of the reverse shock, with the event number in the upper right corner. In each plot, a vertical black dashed line indicates the stream interface while two vertical dotted lines show a range where the channel switch happened during this event. The time interval marked with red colour denotes the downstream, followed by three time intervals with blue, green, and cyan colours inside the upstream region. Four spectra are shown with the same colours as their corresponding time intervals.