Los sujetos procesales

The third rotation o f A R 8906 began on May 2000. B y this time, the com plex field

regions associated with both AR 8906 and AR 8949 had dispersed almost entirely. MDI observations o f the region shown in Figure 5 .3 f illustrate an extended neutral line with only a small, positive parasitic polarity region situated close to this neutral line. The parasitic polarity region may have indicated the remnants o f A R 8906. Weak flux emergence within this parasitic region on 5^*^ M ay 2000 led to its reclassification as an active region (AR 8984).

During the third rotation o f A R 8906, two CMEs were observed in association with activity in this region. Table 5.2 lists the main characteristics o f these events. The first event, a GOES X-ray class B6.8 two-ribbon flare, peaked at 06:23 UT on 8^*^ May. This event took place as the, now diffuse, filament channel passed close to the central meridian. In this case, an increase in soft X-ray intensity was observed to originate from the southern half o f the region. This contrasts with earlier activity which was concentrated in the northern h alf o f the region close to the active centre o f A R 8906 and the location o f most emerging flux during previous rotations. It is at this point, follow ing CME onset, that the large scale sigmoidal feature illustrated in Figure 5.1 is observed.

Figure 5.6a illustrates that, prior to eruption, soft X-ray features overlying the extended filament channel consist o f many diffuse loops arranged in an overall S-shape. H owever, SXT data illustrated in Figure 5.6b suggests that, follow ing CME onset, a change in m orphology takes place and the region gains its sigm oidal appearance. Comparison with EIT 195 Â data illustrated in Figures 5.6c and d, shows that the curved southern tail o f the sigmoidal feature is consistent with the appearance o f a post-flare

Chapter 5. Formation o f a N on-active Region Sigmoid_____________________________ j_19

arcade seen in EIT data. These loops are observed to fade over the hours follow ing eruption along with the post-CME sigmoidal feature.

T ab le 5.2: Comparison between two CME onset events observed from A R 8906 during Rotation 3 (May 2000). Only Event (A) was associated with localised flaring.

E vent (A) 8*** M ay 2000 E vent (B) 10*“ M ay 2000

Peak Time (UT) 06:23 22:16

Flare Class B6.8 N one

CME First Observed in C2 (UT) 06:50 19:26

Filament Eruption Partial Full

Faint CME ejecta was first observed in the south w est quadrant o f the L A S C 0/C 2 field o f view at 06:50 UT on 8^^ May. A height time extrapolation finds CME onset to coincide with initial mass motion observed in EIT 195 Â data but precede the GOES X- ray peak.

H a observations made by the HIDA Flare Monitor Telescope (FMT) during the eruption, show the presence o f a discontinuous filament structure beneath the sigmoidal feature. Figures 5.6e and f illustrate the disappearance o f the curved southern h alf o f the filament during the event, together with the formation o f bright footpoints. These footpoints correspond to the observation o f bright flare arcades in both EIT 195 Â and SXT data. Extensive m ovement is observed in the northern half o f the filament, although no com plete disappearance and/or eruption is observed.

Follow ing eruption on 8^*^ May, the sigmoidal feature gradually fades in SXT data until 10‘^ May. Figure 5.7a illustrates the soft X-ray morphology o f the region at this time. N o increases in soft X-ray intensity are observed between the decay o f the sigm oidal feature on 8^^ M ay and the 10^^ May. At this stage, the region under observation has rotated towards the limb and sigmoidal m orphology is no longer clearly distinguishable.

Com m encing on 10^^ May 2000, and initially observed at 16:15 UT, Figures 5.7c, d and e illustrate a gradual rise in soft X-ray em ission from the arcade. The arcade is

é

0 4 :3 6 UT

0 9 :4 5 UT

•j

0 4 :3 6 UT

0 9 :4 8 UT

Chapter 5. Formation o f a N on-active Region Sigmoid_____________________________

12^

observed to expand and brighten. Maximum em ission is observed from the region at 22:16 UT, directly follow ing Yohkoh night.

Two loop-like CMEs were observed in the south w est and north east quadrants o f the L A S C 0/C 2 coronagraph shortly after the rise in soft X-ray em ission began. A height­ time extrapolation o f each and comparison with other activity on the disk suggests that the CME initially observed in the south west quadrant o f C2 at 19:26 UT was most likely to be associated with the observed soft X-ray m orphology changes in A R 8096. The estimated onset time is consistent with the gradual rise phase o f the soft X-ray arcade illustrated in Figures 5.7c to e, but precedes the final configuration observed at 01:46 UT.