Geomagnetic disturbances of the C05 event (21 January 2005). Panel (a) shows measurements of the H component recorded at mid-latitude observatories. From top to bottom: SUA, IRT, FRN, BSL and SPT (increasing in MLong). Panel (b), from top to bottom, displays SYM-H and Dst indices (top panel), and the H component recorded at the four low-latitude observatories involved in the computation of the Dst index: HER, KAK, HON and SJG. In every upper-right corner, the subtracted magnetic field value and the difference between the maximum and the minimum value in the last 24 h preceding the disturbance (ΔH_{max 24 h}) have been indicated. Vertical solid lines, S1 and S2, mark shock discontinuities and vertical dashed lines T1 and T2 mark southward turnings of B_z. These marks are also shown in Figure 8 to facilitate further analysis.