Unusually quick development of a 4000 nT substorm during the initial 10 min of the 29 October 2003 magnetic storm

Global geomagnetic field data, IMAGE FUV data, and many other in situ observations are presented for the initial 10 min of the magnetic storm starting 29 October 2003 at around 0610 UT. Within 1 min after sudden commencement (SC), two independent strong westward ionospheric electrojets (>2000 nT) at the inner magnetospheric region started simultaneously, one in the evening‐midnight sector and the other in the morning sector. Both activities expanded and accompanied auroral expansion. The locations (inner magnetosphere), morphologies (expansion), and intensities (>2000 nT) of both activities fall into substorm expansive phases. Having such simultaneous independent 2000 nT level expansions makes this event unique. The interplanetary magnetic field condition before the SC was not favorable in causing an AL < −2000 nT activity. A timing analysis indicates that these strong westward electrojets were most likely triggered by the interplanetary shock, with the triggering location not farther than the geosynchronous distance. They are also probably maintained by the direct energy pumping from the solar wind because cross‐tail current derived from the closely located GOES‐10 and Polar did not decrease very much during this period. A local but even stronger geomagnetic (nearly 4000 nT) and auroral activity started only 6 min after the start of SC at postmidnight where and when the above two expanding activities met each other, although the relation between the onset of 4000 nT activity and the preceding expansions is not clear. The suddenness of this third activity (3000 nT change within 2 min) is another unique feature.