Persistent Coronal Streamers and the Identification of Sunspot Clusters

Jing Li, Barry LaBonte , Loren Acton and Greg L. Slater

Astrophysical Journal, 565, 1289 (2002)

Abstract
 We use limb synoptic plots to study long-lived features of the lower solar corona. The most persistent features are the polar sinusoids, which are generated by streamers associated with active regions. We find that the lifetimes of these structures (up to about 10 solar rotations) are much longer than the lifetimes of individual sunspots (typically less than one solar rotation). The long lifetimes of the polar sinusoids are due to clusters of spatially related but non-contemporaneous spots. The continuous emergence of sunspots and magnetic flux from a spot cluster in the photosphere provides the long life span of the coronal streamers. Two thirds of $\approx$180 sunspots recorded in the southern hemisphere in a 1 year period near the 1996-97 solar minimum were members of non-contemporaneous clusters. The clusters suggest large-scale, long-lived structure of the sub-photospheric magnetic field from which sunspots emerge.

Figure 1||Figure 2| Figure 3||Figure 4| Figure 5||Figure 6| Figure 7||Figure 8| Figure 9||Figure 10

Fig.1 Coronal limb synoptic plot at 1.000-1.015 solar radii from SXT/Yohkoh SSC files for interval April 1996 to July 1997. Date is on the x-axis and polar angle (PA) is on the y-axis, where PA=0 degree corresponds to the north pole, while PA=90 degree is the east limb. To display the entire southern (180 degree) and northern (0 degree and 360 degree) polar holes, the plotted polar angle range is extended by 90 degree.

Fig.2 Single X-ray image taken August 27 1996 05:47:14 UT. The bright active region is NOAA 7986. The coronal streamers appear as diffuse emission above the solar limb. A bright polar streamer near southern polar hole marked with "Coronal Polar Streamer" is associated with NOAA 7986. It forms a coronal sinusoid on the limb synoptic map. The dark region on the disk is the "Elephant's Trunk" Unipolar Magnetic Region.

Fig.3 Limb synoptic plot made for the interval June 28 to October 6 1996. The overplotted curves are the simulations of streamer sky-plane motion carried by solar rotation. The four curves are calculated from Eqs. (1)-(4) with four published solar differential rotations. This figure indicates that the four empirical rotation rates (see Table 1) all successfully match the data. The x-axis is date and y-axis is polar angle.

Fig.4 Sinusoidal curves over-plotted on a limb synoptic map for the interval April 1996 to February 1997, which curves are calculated from Eqs. (1)-(4). The curves with higher amplitude represent the projected motions of active regions as they are carried by solar rotation. The curves with lower amplitude represent the latitudinal boundaries of magnetic structures defined by underlying active regions. The latitude -60 degree is used for plotting the lower amplitude curve. Both curves follow the same rotation period. In this figure, the curves represent the sunspot groups related with CS1.

Fig.5 Same as Figure 4 but for CS2.

Fig.6 Same as Figure 4 but for CS3.

Fig.7 All the sunspot groups appearing in the southern hemisphere between April 1996 and July 1997 are presented in this figure. The symbols represent the sunspot groups identified from the coronal sinusoids CS1, CS2 and CS3. The three spot clusters related with 3 coronal sinusoids are marked with symbols: "x" for CS1; "diamond" for CS2; and "+" for CS3. They are connected with solid lines to indicate the time sequence. Their Carrington longitudes have been corrected for differential rotation with respect to the first sunspot group within each spot cluster. Other sunspots are marked with circles.

Fig.8 Observing epoch versus sunspot latitude. The symbols have the same meaning as in Fig. 7.

Fig.9 Observing epoch versus direct (solid lines) and corrected (dotted lines) Carrington longitude. The symbols have the same meaning as those in Fig. 7. When the sunspot latitude is less than Carrington effective latitude, +/- 14.926 degree, the corrected Carrington longitude is smaller than the direct one (e.g. CS1). Otherwise, the corrected Carrington longitudes are greater than the direct ones (e.g. CS2 and CS3).

Fig.10 Comparison between sunspots (symbols) and simulated sunspot Carrington longitudes as function of time (solid lines). The Carrington longitude versus time (solid straight lines) are found by comparing theoretical Carrington longitude from Eq. (5) with the linear least-square fit of related sunspots. The initial sunspot dates, Carrington longitudes and latitudes are listed in Table 4, which are used to calculate the straight lines from Eq. (5). If a sunspot latitude is lower than +/- 14.926 degree (Carrington effective latitude), the slope of a line will be positive (see graph of CS1); otherwise, the slope is negative (see graphs of CS2 and CS3).