4  The big flare of 2001 April 10

On 10 April, 2001, AR 9415 produced one of the biggest flares of this solar cycle, of class X2.3. The soft X-ray emission began to increase around 05:00 UT. The synoptic CDS observations recorded a high-velocity event close to the flare site (see Pike and Mason, 2002).

The Cambridge AR campaign was planned to observe AR 9415 that day, but observations only started at 14:08 UT, i.e. 9 hours after the flare. The activity, however, was still at high levels with continuing new small flares occurring. The overall cusp-like Fe XIX emission lasts for most of the sequence, that ended at 23:21 UT (see Figure and YOHKOH SXT), as are post-flare TR loops. TR densities in the post-flare loop seen at the centre of the FOV were estimated directly from O IV ratios to be 1.5 x 10¹¹ cm^-3. During the CDS observations, two small GOES C2 flares occurred.