3. Characteristics of the flare kernels

On 18 January, vigorous activity appeared in the active region AR 5312, and flare evolution, characteristics and related conditions can be seen obviously in the multi-band optical data. It is shown from Tables 3 (click here) and 5 (click here) and Figs. 1 (click here) and 2 (click here) that activities of ordinary (or H) flares took place frequently and kernels of the WLF were brightening continuously.

3.1. The sources of kernels and their motions

The original bright points of kernels 1 (containing 4 smaller bright points), 4 and 5 (from an ordinary flare) were located between umbra and penumbra of sunspots or in penumbra fibrils. After the bright points became brighter and larger, they covered the umbra and penumbra of the sunspots or photosphere. The original bright points of kernels 2 (containing 4 smaller bright points), 3, 6 (containing 3 smaller bright points) and 7 were located on the plages lying respectively in penumbra of sunspots, on the photosphere or at two ends of the extended light bridge connecting sunspots B₁ and B₂. The penumbra of sunspots, photosphere or plages were covered by the brightening and enlarged bright points.

The H off-band observations (see the bottom of Fig. 1 (click here)) show that before and after the first maximum of the ordinary flares the redshift was stronger than the blue shift. However, kernel 3 showed blue shift in 3 intervals before and after the maximum of the WLF. For kernel 6, before, during and after the maximum of the WLF, its leading part (in the west limb) changed from blue to redshift, and then back to blue shift; while the following part (in the east limb) changed from redshift to blue shift and then back to redshift. The motion directions of the leading and following parts are opposite but in the same scale. This seems to be a cycling motion. Kernel 7 shows a redshift. The kernels lying in the umbra and penumbra of the sunspots morphologically showed a small displacement.

 

Activity No. Beg. Max. End X-ray Remarks

05:08 05:09 05:12D SF C9.6 NOAA (1989), YUOBS^a

06:06 06:37 06:37D SB NOAA (1989), YUOBS

06:40 06:44 06:48 SB YUOBS

07:02 07:07U 07:13D 1F NOAA (1989), YUOBS

07:20 X1.0 NOAA (1989), YUOBS

07:28 YUOBS

07:35 YUOBS

07:45 YUOBS

07:53 YUOBS

08:05E 08:19U 08:23D SF NOAA (1989), YUOBS

08:51E 09:12U 09:52 1N M9.0 NOAA (1989)

17:17 17:27 17:46 SF NOAA (1989)

17:57 18:37 19:08 SF NOAA (1989) ^a Yunnan Observatory.

 

 

Instrument Aperture A^** Resolution

Number (mm) (mm) (mm²) ('') Others filter wavelength at center: 6563 Å, band- width: 0.48 Å and adjustable range: Slit height: 20 mm; plane ten bands; average linear time needed for each scanning: around 1 min. ^* the diameter of solar image, ^** the area of frame.

 

 

Kernel White light flare (UT)

number plage Flare Size

Beg. Max. End Life Beg. SR Time End Life N

05:32-08:04 06:26 06:37 06:44

07:00 07:08 07:03 07:04-07:05 4

07:20 07:35 00:35 07:19-07:20 07:23 00:20

05:32-08:17 06:26 06:37

06:44 06:48

07:00 07:08 07:03 07:04-07:05 4

07:20 07:19-07:20 07:28

07:35 07:33-07:34

07:45 07:43-07:44 07:45 00:42

07:53 08:00 01:00

06:35-07:53 07:00 07:08 07:03 07:04-07:05 a

07:20 07:19-07:20 07:28

07:45 00:45 07:33-07:34 07:35 00:32

05:32-07:53 06:26 06:37

06:44 06:48

07:00 07:08 07:03 07:04-07:05

07:20 07:19-07:20 07:28

07:45 00:45 07:33-07:34 07:35 00:32

06:58-08:04 07:00 07:08 07:03 07:04-07:05

07:20 07:19-07:20 07:28

07:35 07:45 00:45 07:33-07:34 07:35 00:32

03:37-08:17 06:40 06:44 06:48

07:00 07:08 07:03 07:04-07:05 3

07:20 07:19-07:20 07:28

07:35 07:45 00:45 07:33-07:34 07:35 00:32

06:52-07:13 07:00 07:08 07:10 00:10 07:03 07:04-07:05 07:08 00:05 b

a: The internal structure is undistinguishable;
b: The size can not be measured precisely;
N: Number of smaller kernels;
SR Time: Spectral responsing time.  

3.2. Lifetimes of the kernels

It can be seen from Table 3 (click here) that several bursts appeared in AR 5312, and 6 H flare maxima occurred from 07:00 to 08:00UT. Before the first 4 maxima of the H flare, the white light flare and its continuum spectra were observed (no observations were made before 07:28UT due to bad weather). In Table 5 (click here), it is shown that the kernels of the WLF have different lifetimes, the lifetime of kernel 7 is 5 minutes, kernel 1 is 20 minutes, and for kernels 3, 4, 5 and 6 each is 32 minutes. The lifetime of kernel 2 is over 40 minutes. The phenomenon may indicate that the WLF is typically a long lifetime type. Table 5 (click here) also shows that the WLF appeared 2 to 3 minutes after the H flare, and its maxima appeared respectively before the first maximum (such as kernel 7) and 1 to 2 minutes before the second maximum of the H flare. However, the disappearance of the WLF is dependent on the lifetimes of the kernels. If the lifetimes of the kernels are shorter, it may disappear faster or vice versa. The kernels all appeared simultaneously, but their maxima and disappearance were in different times. The kernels disappeared before the disappearance of the H flare.

3.3. The shape and size of the kernels

The kernels have different shapes. Kernels 3, 4 and 7 are nearly circular in shape. Kernels 1, 5 and 6 are elliptical, and kernel 2 is cashew-like. The shapes of the kernels may possibly be dependent on their intrinsic structure and projection on the sight-line direction. The sizes of the kernels are also different, from the observations of the H flare at 07:03:53UT, except for kernels 3 and 7. The sizes of other kernels (by the average size of smaller kernels in each kernel) are in the range cm² (see Table 5 (click here)). This result is in agreement with those given by Dezso et al. (1980) and Hiei (1980).

3.4. Plages

One can see from Figs. 1 (click here) and 4 (click here) that before the appearance of the WLF there were plages with different area and brightness at the locations of the WLF kernels. The plages were relatively stationary, and some small flares had occurred on them. The lifetimes of the plages seem to be associated with that of the WLF. For example, the lifetime of kernel 7 is short, and that of the plage in its place is also short, only about 20 minutes. The lifetimes of other kernels are longer, and the plages in their places have longer lifetimes. A typical plage is the one at the place of kernel 6. Its lifetime is about 5 hr.

  
Figure 1: A series of H filtergrams for the flare on 18 January 1989 in the active region (AR 5312). The arrows indicate active filaments (time in UT). Bottom: H off-band observations

  
Figure 2: The overlying of photospheric sunspots and chromospheric H. A, B and C represent sunspot umbra (04:35 UT), the dotted line shows the WLF kernels and solid line with number indicates the spatial location of the incident slit and the scanning step. The bottom of the figure is sunspots on the photosphere before the WLF on 18 January

  
Figure 3: The evolution of sunspots on the photosphere and longitudinal magnetic field (provided by Hairou station of Beijing Observatory)

  
Figure 4: H line profile variations of the kernel 2 in the first time interval. The solid and dotted lines indicate the observed intensity of the flare and the intensity (I₀) of nearby undisturbed background, respectively; is wavelength and K represents the WLF kernels, and the number before K shows the observing time duration, the first number in the suffix of K represents kernel number, the second the scanning order, the third and fourth numbers represent the sequential order of line profiles (perpendicular to the slit)

3.5. Fibrils and filaments

Before the flare, chromospheric fibrils bent toward the active region and formed an orderly pattern, nearly in the direction of southeast to northwest, along which the flare and the kernels shifted. It is interesting that the direction along which the chromospheric fibrils moved was just perpendicular to the motion of penumbra fibrils on the photosphere. The phenomenon is in good agreement with the magnetic field observations at Beijing Observatory (Zhang et al. 1991).

It also can be seen in Fig. 1 (click here) that three branches of a large quiescent dark filament at the west end are activated (i.e. changes take place in the longitudinal components of the magnetic field) before the flare and bend toward the active region, combining with the chromospheric fibrils. In and around the active region, some small filaments (as indicated by the arrow in Fig. 1 (click here)) are activated one after another before the continuum emission. However, only in the place where the small filaments were activated and disappeared rapidly (that is, the annihilation of the longitudinal components of the magnetic field) can white light flares and continuum emission be observed. In other areas, only ordinary flares can be observed. For example, no continuum emission was observed in the bright area on the northern end of the light bridge connecting B₁ and B₂.

3.6. Sunspots and magnetic field

AR 5312 passed over the solar disk 3 times in total. The observations were made on its second pass. From its appearance to disappearance on the solar disk, the major sunspot had been surrounded by the same penumbra. The active region showed a complex internal structure and rapid evolution, and new flux emerged continuously. Squeezed and sheared effects also took place between the sunspots. A white light flare (S31 E30, Neidig et al. 1993) occurred on 10 January, and another (S30 W65, see Figs. 1 and 5) appeared on 18 January (Xuan et al. 1991). Before the flare on 18 Jan. there were a number of penumbra fibrils around the spatial locations of all kernels except for kernel 3, i.e. in sunspot group A, between group A and B, and on the light bridge connecting B₁ and B₂, and some fibrils extended toward the photosphere where some fibrils were in arch-like shape. The initial bright points of the kernels originated in umbra and penumbra, at the boundary between penumbra and the photosphere, in penumbra fibrils, on the photosphere or on the light bridge.

It is shown in Fig. 3 (click here) that the magnetic configuration of the active region is a -structure in the evolution process, and the twisted and closed magnetic neutral lines between the sunspot groups A and B change constantly. On 18 January, the neutral lines were compressed in the east-west direction and began opening from the closed pattern in the south. A large change in the magnetic structure occurred. Kernels 2 and 5 lie on the changing neutral lines and kernels 1, 4, 6 and 7 are near the lines. In the locations of the kernels, new emerging fluxes are enhanced, but the gradient of the longitudinal field is not very large. The magnetic field observations of Beijing Observatory show that the direction of the penumbra fibrils on the photosphere was the same as that of the transversal component of the magnetic field (Zhang et al. 1991). Therefore, the kernels were constrained by the small transverse field on the photosphere.

3.7. Spectra of the kernels

We present only H line profiles of the kernels. The analysis of the other lines can be found in Xuan et al. (1991). It can be seen from Fig. 4 (click here) that the spectral line profile centers of the kernels show different changes, and no reversal profile appears at the center of kernel 1. The line profile centers in kernel 2 from intervals 1 to 4 show a strong reversal (see Fig. 4 (click here)). The closer to the edge of the kernels, the line center reversal is weaker and has no reversal. In the meantime, the line profiles with reversal in kernel 2 have a wide line width, and for others the line width is relatively narrower. The changes in the line width of kernels 3 and 6 and line center reversal are similar to kernel 2. This may suggest that the reversal of the observed line profile centers is not only dependent upon the properties of the kernels themselves, but also has temporal and spatial characteristics. In comparison with Table 2 (click here) and from the reversal of the H line centers and line width changes, we find that kernels 2, 3 and 6 of the white light flare on 18 January 1989 correspond to the temperature distribution in the semi-empirical models F₁ and F₂ (Machado et al. 1980; Fang et al. 1993), while both line widening and center reversal do not appear in kernel 1. This is possibly due to hydrogen affected by nonthermal excitation and ionization of ion (Hénoux et al. 1993). It is shown that the white light flare belongs to a mixed type with properties of both types I and II.

It is also shown in Fig. 4 (click here) that the observed profiles of the kernels display different redshift and blue shift. For example, the redshift of the kernel 1 is rather significant. The reversal appears at the centers of the profiles in kernel 3 and the blue end is stronger. At the centers of the profiles in kernel 6, the reversal appears, but the red end is stronger. In the first time interval, kernel 2 shows an obvious blue shift, but at the line profile centers with larger reversal, the red end is stronger. In intervals 2, 3 and 4, the blue end is stronger at the profile centers with an obvious reversal, opposite to the situation in the first interval. This probably indicates that the direction and scale of material motions are different for different kernels in the same white light flare.