The active region that this study focuses on was initially classified as N O A A AR 8906 on 9^*^ March 2000. This study considers the development o f this region over three rotations from its appearance around the east limb on 9^*^ March 2000 until its approach o f the west limb on 12^^ M ay 2000. Table 5.1 and Figure 5.2 provide a comparison between activity associated with this region throughout the three observed rotations.
During the March rotation AR 8906 dominated activity on the disk as the largest and most com plex active region. During both the March (L ‘) and April (2"^) rotations, a large number o f GOES events were observed to originate from the vicinity o f AR 8 9 0 6 \ These events included 6 M -class flares during March and 9 M -class flares during April.
In addition, 10 CMEs were observed to originate in association with activity in the vicinity o f A R 8906 during the March rotation and 9 CMEs were observed to originate with activity associated with this region during April.
' It should be noted that the active region was reclassified A R 8948 follow ing its rotation and consequent reappearance around the east limb in April. However, for the purposes o f consistency, this active region will be referred to as A R 8906 throughout this chapter
A co n sid erab le decrease in activity orig in atin g from A R 8906 w as observed betw een April and M ay. W hereas high levels o f C M E and flaring activity had been o bserv ed durin g both M arch and A pril rotations, the M ay rotation saw the region asso ciated w ith o n ly 1 B -class G O E S ev en t and 2 C M E s.
Table 5.1: D istrib u tio n o f G O E S events and C M E activity from A R 89 0 6 observed b etw een 9'^ M arch and 12^*^ M ay 2000. C M denotes central m eridian in this case.
Date o f CM G O ES Events CM E
Pass (2000) X M C B Onsets
13''’ M arch 0 6 67 0 10
10"’ April 0 9 39 0 9
7"’ M ay 0 0 0 1 2
I—I GOES Events tm CME Launches
m 12
r 10
lyyyy i
9101112151415161718 6 7 8 9101112L514 5 6 7 8 9 1011 March April May
Figure 5.2: D istribution o f G O E S events h aving m ag n itu d e in excess o f B 1.0 and C M E onsets o rig in atin g from A R 8 9 0 6 ' during the p eriod 9^*^ M arch to 12^'’ M ay 2000. H igh flaring (ind icated in the figure b y “G O E S E ven ts” ) and C M E activity is o b serv ed during both M arch and A pril rotations. T h e final (M ay) rotation show s th at C M E activity
Chapter 5. Formation of a Non-active Region Sigmoid 15
8 9 0 6 8 9 0 6 8 9 8 4
89 4 9
2 0 0"
m:
Figure 5.3: S X T (a, b, c) and M D I (d, e, t) full disk ob serv atio n s taken n ea r consecu tiv e
m eridian passes. 13‘^ M arch (a,d), lO^*’ A pril (b,e) and M ay (c,f). S tro n g active region
flux disp erses o v er three rotations, form ing an exten d ed filam en t channel. T h e grow th o f this filam ent ch an n el can be observ ed in F igures (e) and (f). S X T d ata has resolution 9".8 p e r pixel in each ca se and M DI d ata has resolution 2" p er pixel. S o la r north points up and east to the left. A rrow s in F igures (c) and (f) indicate the em erg in g flux region A R 8984. M ag n eto g ram d ata show s p o sitiv e m agnetic flux (lin e-o f-sig h t m agnetic field oriented tow ards the o b serv er) in w hite and n egative in black. Im ages are reverse c o lo u r such that strongly em ittin g regions a p p e ar dark.
F igure 5.3 p rovides a co m parison betw een full disk S X T and M D I data as the region cro sses central m eridian during each o f the th ree studied rotations. C om p ariso n o f F igures 5.3d, e and f illustrates the action o f differen tial rotation and flux diffusion on A R 8906 o v e r the co u rse o f three rotations. C om parison w as also m ade b etw een the re g io n ’s ex ten t in w hite light data (as ca lcu lated by S G D ) as it cro ssed the central m eridian durin g the M arch and A pril rotations. T h is show ed that the su n sp o t region had d ec reased in area
to ap proxim ately 18% o f the size o bserved as it cro ssed the central m eridian durin g the prev io u s (M arch) rotation. D ifferential rotation, co m b in ed w ith the action o f flux diffusion arising from g ra n u la r and su p erg ran u lar m otion, has caused the flux to disperse and begin form ation o f an ex ten d ed neutral line.
T h ro u g h o u t its ev o lu tio n , A R 8906 is situated en tirely w ithin the southern h em isphere. C o n seq u en tly the region follow s the hem isp h eric rule that sug g ests regions with m agnetic fields subject to right-handed tw ist, in dicated in this c o n tex t by S -shaped coronal structures, will p re d o m in a te in the southern h em isphere.
a
b Emergingc
Flux /
.0
i. A
17:53 UT
19:28 UT
2 2 : 1 5 UT
feature. A sigm oidal feature is observ ed to form betw een em erg in g flux in the north w est and the o u tly in g filam ent channel to the south east. F igure (a) illu stra te s A R 8906 p rio r to flaring. F ig u re (b) illustrates flux em erg en ce, possibly the ca u se o f a C 8 .6 class flare peaking at 19:11 U T. SX T d ata h as resolution 4".9 p er pixel. S o la r north p o in ts up and ea st to the left. Im ages are reverse c o lo u r such that strongly em ittin g regions a p p e ar dark.
T h e second (A pril) rotation show s an overall d ecrease in the level o f flaring and C M E activity from the area u n d er o bservation; now co m posed o f an active region and exten d ed neutral line (F igures 5.3b and e). T he m ost C M E active p eriod d u rin g the second rotation coincides w ith the appearance o f the strongly sigm oidal featu re illustrated in F igure 5.4. Initially o bserved follow ing a C 8.6 class flare from the region on the 10^^ A pril at 19:11 U T, this feature form s a conn ectio n b etw een em erg in g flux at the centre o f AR 8906 and the neutral line that now ex ten d s tow ards the south w est o f th e rem ain in g
Chapter 5. Formation of a Non-active Region Sigmoid 17
active region. F igure 5.5, prov id es a co m parison betw een the location o f th e sigm oidal featu re o bserved in soft X -ray d ata and the re g io n ’s photo sp h eric m agnetic structure seen by M D I. T he north w est ed g e o f the sigm oidal feature is co n sisten t w ith a region o f strongly em erg in g flux at the centre o f AR 8906. T he south ea st edge o f the feature, how ever, ex ten d s to w ard s the m ore d iffuse flux regions o f the filam en t channel.
F ig u r e 5.5: S X T (a) and M D I (b) o b serv atio n s taken on 10 A pril 2 0 0 0 p roviding
co m p ariso n betw een the location o f the sigm oidal feature and the region's ph o to sp h eric m agnetic structure. T he sigm oidal feature is o bserved in F igure (a) to c o n n e ct new ly em erg in g flux in the north w est to the g radually d iffu sin g filam en t chan n el. S X T and M D I d ata have resolution 4".9 and 2" p e r pixel respectively. S o la r north points up and east to the left.
F ig u res 5.4 and 5.5 d esc rib ed above, both show the A pril sigm oid in S X T full disk data. T h is feature appears to co n sist o f a single structure. U nfo rtu n ately , although partial fram e ob serv atio n s w ere m ade o f this region at this tim e, the size o f th e sigm oidal feature ex ceeded that o f the field o f view . T h erefo re, it w as n o t po ssib le to verify this o b servation using d ata o f h ig h e r resolution.
In th is case, the sigm oid was o bserved to gradually fade o v e r a period o f ap p ro x im a te ly 12 hours, rath er than d isap p ea r in direct association with C M E onset.
During this active CME period, a further active region labeled A R 8949 is observed to emerge in close proximity to the extended filament channel. This active region was observed to increase in size throughout the April rotation. The proximity to A R 8906 may also have contributed to the increase in activity during this period.