Nature 436, 825–828 (2005)

In the print and PDF versions of this Letter, the colour scale bars in Fig. 2a, c and d should have been respectively labelled as follows: ‘Proton number flux (cm-2 s-1 sr-1 keV-1)’; ‘B (nT2 Hz-1)’; and ‘DOP’. This labelling is correctly shown in the online HTML full text version.