HF-type II source regions on the pressure wave bubble. Panel a: modelled wave with the values of M_A plotted on the wave’s surface roughly around the start of HF-type II radio burst. Circles titled L1–L4 represent the regions where the source of HF-type II could be located. Panel b: temporal evolution of the Alfvén mach number (M_A) in the selected regions which are represented by the colored median lines. The quartiles and decile values are given only for L2. The temporal evolution of the geometry in these regions is presented in panel c.