2. Gradual formation of the large activity complex and its sudden end
2.1. Numbers and areas of northern-hemisphere active regions adjacent to Carrington longitude L = 321° during 39 consecutive CMPs
To estimate levels of activity, we used the Russian bulletin Solneshnye Dannye for 1981-1983, and studied the Central Meridian Passages (CMPs) of the Carrington heliographic longitude L = , the center of gravity of NOAA 3804, for the three year period 1981 - 1983. From the Daily Charts of the Sun and The Daily Charts of the Magnetic Fields of Sunspots, we determined for each rotation the number of active regions visible in the northern hemisphere during the CMP of the mentioned L, and lying in the interval DL = - and - . For each active region, we took the heliographic longitude and latitude of its center of gravity and determined the maximum area reached during its disk passage, expressed in millionths of the solar hemisphere. The first CMP considered occurred on January 15, 1981 (Julian Date 619.92), and the last one occurred on November 18, 1983 (Julian Date 1656.77).
Constructing a grid of heliographic longitudes occupied by these active regions at the CMP of L = during the given time interval, we see from Fig. 1 a very high concentration of active regions in the longitudinal interval DL = - before the September 1982 rotation. After that rotation, no further new activity developed in this interval during the period studied. If we determine for this same longitudinal interval the total area of all active regions for all of the above CMPs of L = during this period, we obtain the result illustrated in Fig. 2. There we see a progressive growth of this total area until Julian Date 1165.64, or July 15, 1982. The effect is even more pronounced in the accompanying plot in Fig. 2 of the mean area of these active regions. All active regions disappeared from this longitude interval the third rotation after the CMP of July 15, 1982, at least until the end of 1983.
On the same daily maps of solar activity on the days of CMP of L = , we see a progressive concentration of sunspot activity into one region with its peaks in NOAA 3776 and its successor NOAA 3804, followed by its dispersion in longitudes as it rapidly diminishes and finally disappears altogether. We note also a continuous shift of the center of gravity of activity westwards.
2.2. Large-scale development on the and magnetic synoptic charts
If we compare the Sacramento Peak Observatory synoptic charts for DL between - and - for successive CRs Nos 1721 - 1738, with the Stanford Solar Observatory's magnetic synoptic charts over the same DL range for this period, we observe the following developments. On both sets of charts before July 1982 (CR 1724), there is a similar gradual evolution of the magnetic field to a more complex structure in the longitudinal interval - . We then see a rapid weakening and decrease in area of the magnetic field over this entire region (Fig. 3). Further insight into the nature of this active-region complex is gained if we consider the large longitudinal filament that marks one of its boundaries, and is discussed in our companion paper (Jordan et al, 1997). This filament resembles one of the "pivot lines" described by Mouradian et al. (1987, 1990). These lines are filaments found to trace the boundaries of regions of continuously developing activity that rotate rigidly around associated "pivot points," in contrast to filaments that rotate under the influence of the differential rotation.
In the longitudinal interval - , Fig. 3 reveals a large positive unipolar region in the northern hemisphere in CR 1722. This region not only expands poleward by about in latitude in just two rotations, but also crosses the solar equator in successive rotation 1723, and by rotation 1725 has formed a completely connected parabola-like positive polarity feature joining both polar regions. This global feature reaches the polar latitudes through the combined effects of magnetic field expansion and differential rotation, and is formed by the successive joining of positive polarity fields of individual active regions (see Figs. 7a, b in Bumba & Hejna 1987). Like the "pivot line" filament described above, the feature rotates like a rigid body for at least 5 rotations, in contrast to the smaller unipolar regions from which it was formed, and whose dynamics are influenced by the action of differential rotation.
A similar negative unipolar region is seen to develop in the northern hemisphere just westward from the extended positive-polarity unipolar region already described. By rotation 1732, a large negative-polarity northern-hemisphere coronal hole has appeared in the northern hemisphere. By rotation 1735, it has clearly crossed the equator and has filled much of the region formerly occupied by the large activity complex (Fig. 3). This coronal hole seems to be the last evolutionary stage of the former activity complex, although it has shifted about westward.
It should also be noted that the global magnetic field exchanged its polarity with respect to the solar equator and also to the large filament that traced its main polarity boundary, as shown in the lower-latitude regions on the Stanford maps for rotations 1722 and 1735 (Fig. 3). This reveals a complete global reconstruction of solar magnetic fields in both hemispheres during the period studied.
2.3. Relationship of the activity complex to the global magnetic field during the phase of maximum activity
The peak activity level of the long-duration complex was reached during June and July of 1982, as seen in Fig. 2. During the June rotation, CR 1723, active region NOAA 3776 contained the largest sunspot group of cycle 21 observed after July 1978 and yielded a large number of flares that were observed in both the visible and in the X-ray range (Bumba & Klvana, 1997).
We can see a striking global configuration of the Sun's background magnetic field on the magnetic synoptic maps from the Wilcox Solar Observatory during rotations 1722 and 1723, when the active regions passed through the central meridian (Bumba & Klvana, 1997, Figs. 1,2). The activity zone of the whole Sun was covered by a global bipolar magnetic pattern, with the leading positive-field region occupying almost half of the area and the following negative-polarity part occupying the other. NOAA 3776 originated on the polarity boundary of this bipolar region, in its northeastern part. The local polarity boundary between the leading and following fields of NOAA 3776 coincides exactly with the northeastern part of the global boundary of the global magnetic pattern, and has the same signs as the global pattern of leading and following fields. Thus NOAA 3776 and its successor NOAA 3804 were "twice bipolar" (locally and globally), and we can expect the distribution of their magnetic lines of force to be dipole-like to a high degree. Their position on the boundary of a global magnetic field must be the result of long-lasting activity development in the area, as illustrated in Fig. 2.
The maps of the longitudinal magnetic component obtained by the Ondejov magnetograph during three days of the July 1982 CMP of this activity complex (Fig. 4) show a large region of positive polarity extending in the east-west direction. This region is surrounded by large islands of negative polarity, also oriented mainly east-west and lying southwards of the positive-field region. The numerous negative-polarity islands around the main region of positive polarity indicate that there exist many rapidly developing regions with high magnetic field gradients capable of enhancing strong chromospheric and coronal activity. We also see in Fig. 4 the gradual weakening and decrease in area of this longitudinal field, especially in its eastern part where the islands of negative polarity successively diminish, thus reducing the field gradients. The curvature of the boundary increases toward the east.
While the main local magnetic field boundary in Fig. 4 is almost parallel to the equator for most of its length, there is a secondary magnetic field boundary bordering the eastern edge of the positive polarity region and running essentially north-south. It is highly probable that the erupting loops observed during the July 9 event (Jordan et al, 1997) developed above the main local magnetic-field boundary that runs perpendicular to the axis of the large north-south quiescent prominence/filament mentioned earlier. This feature lies above the eastern secondary magnetic boundary which coincides with the large-scale magnetic field boundary in that region.
On June 18, the sunspot group was strongly concentrated into one cluster around a large complicated spot, while on July 15 the spots were extended into a long chain almost parallel to the equator, although close to the east limb they exhibited a more longitudinal orientation. There was a rapid increase in the group's area and in the spot magnetic field intensities during the first four or five days after July 15, followed by a decrease in the group's area and its gradual disintegration, along with a corresponding decrease of the spot-field intensities.
© European Southern Observatory (ESO) 1998
Online publication: December 16, 1997