Flares and separatrices between magnetic loops

Chin. Astron. Artrophys. (1995)19/4,459479 A translation of Acta Astron. Sin. (1995)36/2,1Sl-187 Copyright @ 1995 Elsevier Science Ltd Printed in Great Britain. All rights resewed 0275-1062/95$24.00+.00

027%1062(95)00063-l

Flares and separatrices between magnetic 1oopst WANG Jing-xiu

SHI Zhong-xian Beijing Astronomical

Observatory, Chinese Academy of Sciences,

Beijing 100080

WANG Hai-min Big Bear Sotar

Observatory, California InstiMe

of Technology, Pasadena,

Abstract Time sequences of vector magnetograms Solar Observatories have provided us the opportunity

of Hauirou to identify

CA 91125

and Big Bear the individual

magnetic loops and athe separatrices between them. Based on the continuous observatin of vector magnetic field of NOAA 7469 from 4 to 12 April 1993, for the first time, the authors have identified the magnetic loop systems and relevant separatrices for such an active region. The observational signature ofthe cross-section of separatrices on the photosphere is as follows: (1) High degree of magnetic

shear at or close to the separatrices;

(2) Steep gradient of line-of-sight magnetic field (- 0.1 G/km) crossing neutral line. (3) Flux cancellation from both sides of the separatrices. At this point transverse field partly changes its alignment. During the observed period, flare activity of the separatrices. Key words:

solar active

region-vector

took place repeatedly

magnetic

the the

in the vicinity

field-flare

1. INTRODUCTION The role of the topology release of magnetic Since the 22nd accumulated a rich has reached a stage regions is possible. t Supported Received

of magnetic

field in determining

the evolution

of the field and the

energy has been increasingly appreciated by solar astrophysicists. solar cycle, Huairou Solar Observing Station, Beijing Observatory has set of data of vector magnetic field. Systematic observation of vector field where identification of topological features of the magnetic field of active The joint observation of the vector field by Huairou Station and Big Bear

by National Natural Science Foundation 1994-01-22; revised version 1994-0524

470

SHI Zong-xian et al.

Solar Observatory

has proved the reliability

of the measurement

of the transverse

field using

video-frequency magnetographs. Analysis of the data of the joint observation has convinced us, for the first time, that the analysis of the main loop systems and the variations in their interacting interface will provide process of flares itself. In an early

study

a new observational

of 6 sunspots,

Professor

Zirinl’l

basis for investigating pointed

out that

the physical

the two parts

of

opposite polarities of a 6 spot come from different sunspot pairs, and that at the separatrix, the field is highly sheared. Machado et al.121, basing on SMM X-ray imaging observation and vector magnetograms of Marshall Space Flight Center, were the first to propose that the interaction between two or more magnetic loops is the basic configuration for producing flares. But the magnetograms available to those authors did not have sufficient temporal and spatial resolutions for sure identification of loop systems and also lacked continuous observation over any prolonged period. examination of SHI and WANG13y41 using data of Huairou Station, made a systematic X-Class flares of the 22nd cycle before 1992 and found that 96% of the flares are connected with sunspot groups containing 5 spots during the lifetime of the active region, and found the magnetic flux ratio between the two polarities of the relevant 6 spot to be as large as 6.6 : 1. These findings further corroborated Zirin’s conclusion that the neutral line that goes across a 6 spot is located on a topological boundary. From theory, DCmoulin and Mandrini15-“1 using a simplified quadrupole model, simulated the field topology of active regions and demonstrated the extreme importance of magnetic topology in the energetic process of flares. In view of this, this paper attempts a further analysis of the magnetic field topology of active regions by using the vector magnetograms of Huairou Station and Big Bear Solar Observatory, in particular, we shall attempt to identify from the data, individual loop systems and the separatrices between them, and analyse the relation in space between the separatrices and flares, thereby hopefully gaining some new understanding on the flare process.

2. VECTOR

MAGNETOGRAMS

OF

ACTIVE

REGIONS

The vector magnetograms of Huairou Station are digitized magnetic maps. The longitudinal field is expressed by isogauss contours, the solid and dashed lines representing posFrom outside inwards, the contour values are itive and negative polarities, respectively. on the longituf20,40,80,160,320,640,960,1280, *. . G. The transverse field is superposed dinal field map, expressed by orientated line segments of different lengths (proportional to the filed). The transverse field has a 180° ambiguity. The magnetic maps of Big Bear Solar Observatory are essentially the same as the maps of Huairou Station. In the grey-scale maps, white means positive, black, negative polarity. The transverse field is expressed by superposed, equal line segments. Fig. 1 is the vector magnetogram of NOAA 7469. Fig. 2 is the vector magnetogram of Big Bear Observatory. NOAA 7470 enters the filed of view on April 8. North on top, West on right. All the features mentioned in the text, that is, the identified foot-points in the longitudinal field map, are labelled by a capital letter, e.g., D, H, M, etc.. All surely identified

471

Magnetic Separatrices

separatrices

are denoted

by joining

the two letters

of the foot-points

DZ, MH, etc.. From Figs. 1 and 2, we can see the course of evolution grey-scale

picture,

the active region

we can see fine field structure and its surrounding

3. IDENTIFICATION

network

on the two sides, e.g.,

of NOAA 7469. Particularly,

and evolution

of the interaction

in the between

field.

OF MAGNETIC

LOOPS

AND

SEPARATRICES

Magnetic loops and topological three-dimensional space. With

boundaries between them are structures and surfaces in the vector magnetograms we are only examining these features in the section of the photosphere (the Z - 0 section). See Ref. [9], Fig. 2. Hence, sometimes boundary surfaces (separatrices) become coincident with boundary lines (separators). Whether or not a given pair of positive and negative magnetic foot-points belong to

the same magnetic loop can be decided of the transverse field. If the transverse

by the continuity or otherwise of the orientation field between the two foot-points keeps the same

connections, then we can regard the two foot-points and the transverse field in between to represent the same magnetic loop. If the loop satisfies the condition offorce-free field, then its properties can be approximately described by one force-free parameter cr (“approximately”, because strictly speaking, IY varies from point to point within the loop). In this case, at the point where it crosses a boundary line in the longitudinal field (&I = 0), the transverse field does not change suddenly its direction, but rather continues with the original general direction , as at the locations marked 1 and 3 in the 6d2319(UT) map of Fig. 1. Around a boundary between two different loops, such continuity of transverse is not preserved and, rather, strong shear often appears. See the spaces between D and Z, and between M and H. So, these pairs of foot-points belong to different loops. An obliquely placed “S” is used to indicate strong shear. Continuous video recording shows the gradual diminishing of the positive foot-point D through cancellation with B and the decay of magnetic flux. By 8d0357(UT), only a vestige remains. Similarly, cancellation took place between C and H, the negative H gradually diminishing, while the orientation logical connectivity).

of the transverse

also changed

(implying

a changing

topo-

Additional evidence comes from changes in the longitudinal field gradient. For the two foot-points of the same loop, the distance between the opposite polarities of the longitudinal field increases in time, while those of different loops do the opposite. The field gradient evolves in opposite directions in the two cases. When the gradient reaches 0.1 G/km, magnetic cancellation will take place, that is, “slow” reconnection will start (the time scale of “slow” reconnection is one to two orders of magnitude longer than “fast” reconnection relevant to flares). The same sort of thing should happen at the boundaries between two active regions or between an active region and the network structure of the quiet region, but because of the weakness of the transverse field, shearing is often difficult to make out. In such cases, video recording or more frequency magnetic observations are necessary. A simultaneous decay of the longitudinal field gradient and both polarities would be an effective indicator. For example, on the boundary between active regions 7469 and 7470, at 8d1630(UT), features

472

SHI Zong-xian

Fig. 1 Evolution

et al.

of NOAA 7469 in the 4-day period 1993 April 6-10

Magnetic Separatrices

473

474

SHI Zong-xian

et al.

4

a

Magnetic Separatrices

475

Fig. 4 Showing the close relationship between the positions of flares (middle and bottom panels) and separatrices

DZ, CH and MH (top) on April 7

476

SHI Zong-xian et al.

Fig. 5 Showing the close relationship between the positions of flares (right) and separatrices CH, BJ and MH and the boundary between AR7469

and AR7470

(left) on April 8.

Fig. 6 Showing the close relationship between the positions of flares (right) and the boundary between AR 7469 and AR 7470 (left) on April 9

Magnetic Separatrices

Table

1

Relationship between Flares listed in S.G.D. and Separatrices

NO 1

Date 4.6

2

Time

--

184836

Lat

Opt/x-ray

CMD

--

07

12

DB,DZ DB,DZ

12

04

lB/M1.5

0507

12

01

SN

5

0613

11

00

SF/Cl.4

6 : I

0804

10

WOE

4.7

DZ,surge

SF

235650

4

Remarks

SFlC4.4

E08

s12

231814

3

477

DZ PG DZ

SF/Cl.3

PG DZ CH,BJ,MH

0214

13

10

SF

8

154452

11

19

SF

9

155408

06

18

SFlB9.9

7470,QR

10

193022

II

21

SF/Cl.5

CH,BJ,MH CH,BJ,MH

4.8

11

193656

11

21

SFjC6.5

‘12

221650

06

23

SF

7470,QR

13

223726

08

22

SF

7470,QR 7470,QR

162419

10

44

SF

15

201752

05

35

SF/Cl.2

7470,QR

16

231306

!6

40

lF

Bright

17

232352

I6

40

SF

CH,MH

14

4.9

surge,CH,MH

174323

06

$8

lN/C6.8

7470,QR

19

232327

07

54

SF/Cl.4

Bright

surge,CXI,MH

20

232400

08

51

1N/C9.1

Bright

surge,CH,MH

003656

12

53

lNlC6.2

Bright

surge,CH,MH

18

21

4.10

4.11

-

-

Figs. 3-6 illustrate the close relationship four days, April 6-9.

E, I, J and G all belong active regionsIi71.

4. LOCATIONS

to the network

between

structure

OF FLARES

the Ho flares and the separatrices

and they reconnected

RELATIVE

TO

on the

with the field of the

TOPOLOGICAL

BOUNDARIES

We use photographs of the TV screen to compare the positions of the flares and the boundaries. The images may not be very clear because of the limited resolution of the TV camera. Generally they give only the foot-points of the magnetograms and the Ha flares, the latter identified by sudden brightening in the Ha video recording. During the four days, April 6-9, there were 47 main Ha brightenings, all taking place around the topological boundaries between the magnetic loops. 11 occurred around DB or DZ, 15 around CH or MH. 10 between active regions 7469 and 7470. Of the rest, 5 were related to EA or IA, 6 to BJ, and 2 to PG. Table 1 lists the 21 flares published in S.G.D.I*‘l, together with the related boundaries and accompanying phenomena.

SHI Zong-xian et al.

478

5. DISCUSSION

5.1 In the vector magnetic field, whenever the transverse field crosses the polarity boundary of the longitudinal field (B 11 = 0) more or less perpendicularly, the two footpoints on either side of the boundary belong to the same magnetic loop. That is, there is no place where B = 0. By contrast, the magnetic boundary between different loops is a B = 0 curve (separator) in the three-dimensional spacelg~121. High resolution vector field observations

have shown

that

in places

of strong

shear,

we have not only

Blr

= 0,

but BI (the transverse field) is also nearly zero, hence such places are also locations of topological boundaries. When two magnetic loops interact, there is magnetic flux crossing the B = 0 surface, changing the old topology into a new one. And when this happens, we could say that, essentially, and speed of magnetic energy

magnetic reconnection has begun. Depending on the size release, various classes of flare will occur, accompanied by

mass ejection. Observations have also confirmed that frequency flares appear around the topological boundaries. Thus, the importance of interaction between different topological units on flare eruptions can be readily appreciated. 5.2 The phenomenon of magnetic cancellation is always connected with topological boundaries, and not with single magnetic loops. When magnetic reconnection takes place in the photosphere-the lower layer of the solar atmosphere, it is a sort of slow reconnection, its time scale is l-2 orders of magnitude longer than the fast reconnection. After heating the corona for some time, the slow reconnection eventually develops into a fast reconnection, although we are not clear about the conditions for the conversion. And fast reconnection means the start of flare110~‘71. 5.3 As long as the field configurations on either side of the boundary remain basically the same, magnetic reconnection will continue to take place, hence persistent flare activity on either side. As given in Table 1, flares were repeatedly observed around the boundaries DZ and DB, and CH and MH. Sometimes, a small happening on the boundary will give rise to a large event, for example, a small magnetic cancellation in NOAA 5572 caused an unwinding of the dark filamentsl’31. 5.4 The single loop model of the available theory of flares will have difficulty in explaining the results found here. As a matter of fact, in all the vector field observations so far, there has never been a confirmed

case of flares relating

This may be due to insufficient

resolution

to a single,

of magnetic

isolated

field telescopes

magnetic

10opl’~~‘~l.

and there should

be

further study on this question. 5.5 Vector magnetic field observation has now provided the possibility of identifying observationally active magnetic loops and topological boundaries, thereby raising flare observation and analysis to a new level. Perhaps it has provided a new clue to physical prediction .of flares. In 1992, Shi and Wang Us1 put forward the concept of “magnetic weather”, comparing the topological boundaries to the fronts in terrestrial weather, where frequent activities occur. 5.6 To conclude, continuous observation of vector magnetic field provides a new means of identifying magnetic loops and topological boundaries. The signature of a boundary is given in the Abstract. All the main flare activity happens in the vicinity of the boundaries. ACKNOWLEDGMENT

We have received

great

help from Beijing

Observatory,

Huairou

Magnetic

Station.

Professor

Zirin has provided

479

Separatrices

us with all the video tapes for analysing

the active

region.

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