Chin. Astron. Astrophys. 5 (1982) 186-192

Act.Astron.Sin. 23 (1982) 95-101 -

Pergamon Press. Printed in Great Britain

0275-1062/82/030186-07$07.50/O

FLARES OF SPOTLESS REGIONS

LUO Bao-rong Yunnan Observatory, Academia Sinica

Received 1981 November 13

ABSTRACT An analysis of 20 flares of spotless regions observed at Yunnan Observatory during the peak years of Cycle 21 shows 1) the fraction of flares produced in spotless regions is about 3X, 2) their Carrington longitudes show a tendency to drift eastward, 3) the majority of spotless flares are low- energy flares, 4) the background conditions for producing spotless flares are the same as for flares in general, namely, there must be local magnetic structures of opposite polarities. The spotless flares occur on the sides or in the vicinity of the local neutral line.

The quiescent dark filaments floating on the neutral line are activated a few hours and one or two days before the flare, the filament nearest to the flare position first enlarges, accompanied by brightening of plages. A few minutes before the flare, or during the flare, this filament rapidly weakens, even vanishes. Meanwhile, visible fibrils become less inclined to the main filament showing pressure force is transformed into shear force.

1. INTRODUCTION

While the great majority of flares occur in regions above sunspot groups with strong magnetic fields, a small fraction do occur in regions with no corresponding sunspots. This phenomenon attracted attention as early as the 30's but a more or less systematic study of spotless flares began only in the early 70's with the work of Dodson and Hedeman. In a paper entitled "Large Flares in Very Small or Spotless Activity Centres" /l/, these authors collected 83 flares (35 "spotless") of Class 2 or higher in regions free of spots or with areas less than 110 units observed by Zurich, Greenwich, Mount Wilson and McMath Hulbert Observatories between July 1957 and June 1969. Later, jointly with Mohler, /2/, they a nalysed the spotless flares as cases of "flares with problems". This work gave a description of certain properties of the flares of spotless or small spot regions but did not clarify the mechanism underlying such flares. In this paper, I analyse the spotless flare data observed at the Yunnan Obser- vatory during the peak years of Solar Activity Cycle 21, and discuss their properties and the conditions underlying their production.

2. SPOTLESS FLARES OF THE PEAR YEAPS OF CYCLE 21

From the "Table of Flares" of the "Solar Activity Monthly" edited by Yunnan Observatory, I collected data on 20 Ho flares (Class 2 1, no corresponding spots) observed there during the peak years of cycle 21 (May 1979-February 1981). They belong to 17 spotless regions and 5 of them are of Class 2. See TABLE 1 and Fig. 1 (Plates I, II).

To distinguish flares with and without associated sunspots, I omitted from my collec- tion all flares that had corresponding spots (however small) at the time of the flare. This should not be taken to mean that none of the selected flares had anything to do with sunspots: of the 20, 10 may be connected in time or in space with spots (these are marked with asterisks in TABLE 1). By a temporal connection I mean the occurrenceof spots in the flare region some short time before or after the time of the flare, this was the case with four flares # 12, 13, 14, 17. By a spatial connection I mean spot groups were found within 15" of the flare, though not at the flare. This was the case with 6 flares ## 2, 6, 7, 8, 11, 15. The remain- ing 10 flares are such that there were no spots within 15", nor any in the region within 1 or 2 rotations before or after the time of the flare.

- No

. -

1 2 3 4 5 6 7 8 9 10

11

12

13

14

15

16

17

18

19

20

T I

Pos

itio

n

1 M

ax. Area I

I

- --

-

- Cl.

end

&

_ M

,D.

<“)

Si

.-

SF

N

o .

Da

te

1 19

79 5

1

2 5

21

3 5

27

4 8

2

5 a

2

6 12

19

7 1

8

8 3

28

9 5

17

10

5 17

11

8 3

12

9 17

13

11

29

14

11

30

15

1 26

16

1 31

17

2 5

18

2 7

19

2 9

20

2 14

Da

te

1979

5

1 1P

1979

5

21

1B

1979

5

27

1N

1979

8

2 IF

1979

8

2 1F

1979

12

29

1B

1980

1

8 1N

1980

3

28

1N

1980

5

17

1N

1980

5

17

2B

1980

a

3 1N

1980

9

17

1N

1980

11

29

IN

1980

11

30

1B

1981

1

26

IF

1981

1

31

2N

1981

2

5 28

1981

2

7 2N

1981

2

9 2N

1981

2

14

1N

Time(L

egin max

--

0625

06

28

0443

05

00

0747

07

58

0845

09

10

1025

0140

01

43

0359

04

04

0634

06

40

0100

01

15

1007

10

15

0445

04

47

0616

06

43

0723

07

28

0106

01

47

0606

06

10

0056

01

07

0126

01

37

0438

04

4Q

0328

03

4i

) 1

0855

0635

s3

1 E

25

2i3

0510

s2

2 E

53

341

0810

s2

0 E

48

002

0935

N

25

E73

06

8

1055

N

25

E73

06

8

0147

S

16

ES

2 26

2

0430

N

25

00

211

0654

N

16

E35

20

1

0133

N

25

E46

25

2

1052

N

25

E46

25

2

0457

N

27

W34

01

8

0647

N

lO

W67

17

6

0737

N

16

w39

26

4

0116

S

O8

El0

20

5

0626

S

l8

E30

15

1

0133

s1

3 W

O8

129

0202

s1

9 w

31

081

0452

N

16

E19

01

1

0438

s1

5 E

05

003

0905

s1

5 w

70

003

173

126

80

161

241

241

643

193

209

161

321

161

321

482

401

401

129

105

100

138

163

463

124

238

106

165

95

164

221

204

IF

1B

1N

1F

IF

1B

1N

IN

1N

1B

1N

1N

1N

1B

1F

2N

ZB

2N

2N

1N

* left: TABLE 1

Fla

res

in

Spo

tles

s R

egio

ns

* * *

below: TABLE 2 Distribution in Longitude of

Y

Spotless Flares of the Peak

*

Years of Cycle 2

1

* * * * -

10

20

30

40

50

60

70

80

90

100

110

120

130

140

150

160

170

180

190

200

210

220

230

240

250

260

270

280

290

300

310

320

330

340

350

360

--

273

341

2

68

68

262

211

201

18

176

151

129

205

256

256

264

81

11

3 3

^. ^

_. .

_ I

_ _

188 LUO

3. GENERAL CHARACTERISTICS OF FLARES OF SPOTLESS REGIONS

1. The proportion of Spotless Flares. According to the Yunnan Observatory data, the period May'1979- 15 February 1981 saw 520 flares of Class 1, 87 flares of Class 2, 14 flares of Class 3 and 2 flares of Class 4. Of these, 15 of Class 1, and 5 of Class 2 were spotless; the proportion of the spotless is therefore 151520 = 0.029 for Class 1, 5187 = 0.057 for Class 2, and 201623 = 0.032 overalLThis result is about the same as in ,fl/. (their ratio of 0.07 includes "small-spot" flares; for "spotless" flares only, the ratio is 35/925 = 0.038).

2. Distribution of the Carrington Longitudes of Spotless Flares. TABLE 2 shows that the Carrington longitude of a spotless activity region has a tendency to drift eastwards. The drift period is about one-and-half years. The longitudes of nos. 4, 5, 11 may possibly belong to the tail-end of one drift period.

3. Accompanied Events of Spotless Flares. Spotless flares are mainly low-energy flares. Of the 20 spotless flares listed, none were accompanied by proton events or sudden ionospheric disturbances, or even radio bursts. Only a few (nos. 3, 8, 14, 17, 19) were accompanied by class-c X-ray bursts.

4. Magnetic Configurations of Spotless Flares. Although at the time of a spotless flare, there are no spots in the photosphere below, the background conditions for its production are the same as for a spot flare, namely, there must exist Opposite magnetic polarities in the atmosphere. All the 20 flares of TABLE 1 occured by the side, or in the vicinity of the neutral line in the large-scale longitudinal field. A dark quiescent filament became active on this line prior to the flare and the flare took place on either side of the filament or in the plage region on the dark filament extended from the neutral line. (see Fig. 1).

While pointing out a connection between a spotless flare and a pre-existing dark filament, Dodson wrote that this connection has not been confirmed. In the 20 cases studied here, the connection between the two seems to be a close one. However, the mere existence of a quies- cent dark filament is not a sign that a spotless flare will occur, rather, it seems more meaningful to say that the activation of a quiescent dark filament in a spotless plageregion can be taken as such a sign.

4. ACTIVATION OF QUIESCENT DARK FILAMENTS BEFORE SPOTLESS FLARES

Due to instability of the magnetic field, quiescent dark filaments floating on the neutral line become activated. Conversely, activation of the dark filaments is evidence that the local field has become activated, complicated and strengthened. When the field becomes sufficiently disturbed, energy will be released in the form of flares.

I shall now take the spotless active region of 1980 May 17 as a case study of the process of activation of dark filaments. See Fig. 1. This active region (N25 E41) successively produced two flares on May 17, one began at 0100 UT, reached maximum at 0115 UT and ended at 0133 UT and was Class 1N; the other began at 1007 UT and ended at 1052 UT and was Class 2B. These two flares developed along a tortuous neutral line, running roughly E-W. North of the line, the polarity was S; South of it, the polarity was N (Figs. 1 and 2). On May 13, one could see a dark filament extending along this neutral line on the eastern limb of the sun, its SE portion (B,) was thick and its NW portion thin and long, The other end of Bt extended

8 16 24 32 10 w 56 64 5&

so 88 96 5.14 5.15 5.16 5.~ date

Fig.2 Variation in the area of dark filament before and after the spotless flares of 1980 May 17.

Spotless Flares

5

5.

Fig. 1. Changes in the dark filaments

i . I6 0x45 U.1

_^__.__. I_-.~_.._ _._.I _d 6. 5 - 16 2iii I I

--- . li OIOO UT

in the spotless flare region of 1980 Nay 17.

7

190

------I

B, 8

8. 5 . 17 0110 UT

II. 5 . Ii 0900 U’I

4 /

“\ - sl ‘I

+Y 4

9

Y. 5. Ii 0115 U’I’

1 ih iI@\ 10. 5 . I7 0133 U’I

13. 5 . 1: 1055 UT

5. 18 0020 u.1

Fig.l.(Cont.)

Spotless Flares 191

eastwards but the picture was not clear. On May 14, B1 fast vanished with only a barely perceptible dark dot remaining while Bz became short and thick. On the lSth and 16tb, both Bt and Bz became strong and large once again, also the eastern end of BI showed a thin and curved dark filament (A) crossing a not-too-bright plage region; later, B1 and B2 both rapidly weakened (B, nearly vanished), while A became strong and large and the plages rapidly bright- ened. on the 17th at 0110 UT, two ribbon-type flares of Class 1N erupted on the two sities of filament A. Subsequently, B1 and Bz continued to weaken and A also showed sign of weakening; however, at 0900 UT, A once again picked up strength and one hour later, at 1007 UT, there erupted the double-band flare of class 2B. At the same time, the filament A vanished at the position of the flare from the Ho-chromosphere. After this Class 2B flare, on the 18th, the filaments A and B almost completely vanished.

The activation of dark filaments in the spotless active regions of 1979 May 1 and 1981 January 31 had the same general features as described above. I shall now summarize this process. In the spotless active regions, the quiescent dark filament system floating on the neutral line separating regions of opposite polarities, after disturbances in the

field, undergoes remarkablechanges inshape, size and contrast. These changes take place from a few hours to one or two days before the flare eruptions. The general sequence is as follows. Before the flare, the filament A which is nearest to the flare position rapidly enlarges while filament B on the extension of A and further out rapidly weakens. (in some regions, this process is repeated). When A reaches a maximum, plages brighten, followed by the eruption of flare(s). A few minutes before the onset of the flare, or during its development, first A, then B rapidly declines, even vanishes. At this time, often a new dark filament (C) is born at the end of A away from B. In the development of some of the double-ribbon flares in spotless regions, (e.g. the one on 1981 February 9 at 0328 UT, of

class 2N), chain-like structure is seen.

5. DISCUSSIONS

1. The above analysis shows that the background conditions for a spotless flare are the same as for a spot flare, namely, there must exist in the region magnetic fields of opposite polarities. The difference is that a sunspot magnetic flux tube has a field strength of between several hundred and several thousand Gauss, whereas the local field in a spotless region is only several or a few tens of G. However, if the local field is excited by certain factors and becomes disturbed, then the field intensity in certain parts may be increased to

several hundred G. If the field increases by 100 G., then the energy density will be increased by AE=B2/8n = 4 x 10' erg/cm3, and over a volume of 102'cm3 of a flare region, the total energy will be increased by AW = 4 x 10' x 10" = lO'*erg. This is already the energy

L_

2. 1980. 1 . 8 0350UT 3. 1980 . 1. 8 0404 UT

Fig.3 Showing the variation in the angle between a fibril

and the main polarities of sufficient strength.

LUO

of a medium-sized flare. Admitedly, this estimate is very rough but it does show that it is by no means unthinkable for spotless regions to produce flares, even large flares. The important thing is that there must be a field of opposite polarities and this field must be disturbed sufficiently to have its intensity increased to several tens or hundreds G. The activation of quiescent dark filaments floating on the neutral line is a reflection of the disturbance in the local field and, as such, can be taken as a sign of impending flare eruptions in the spotless region.

2. Many researchers like Dodson /l/ and Svestka /3/ seem to regard spotless flares as a product in the late stage of evolution of a sunspot activity region (when the sunspots have declined to below 100 units or even vanished). In the 20 cases studied here, there are certainly such examples, but also we have 10 cases which are definitely independent of sun- spots either in time (the rotation history) or in space. Also, as shown in the foregoing paragraph, the necessary condition for producing flares is not the existence of sunspots, rather, it is the existence of magnetic fields of opposite polarities of sufficient strength.

3. The tortes actrng m me acrrve region can be seen from the variation in the angle between a fibril and the main filament before the flare of 1980 January 8, 0359 UT. Fig. 3 shows that on Jan 6, this angle is about ho', and on Jan 8, it is only about 30". (Data lack- ing on Jan 7). Thus, a pressure gradually transformed into a shear. It was a sign that the active region passed from a state of energy storage to one of sudden energy release.

REFERENCES

[ 1 ] Dodson, H. W., Hedeman E. R., Solar Phys., 13 (1970), 401417. [ 9 ] Dodson, II. W., Hcdeman, E. R., Nlohler 0. C., Intenational Solar-Terrestrial Predictions Procee-

dings and Workshop Program. Preprint No. 12.

[ 3 ] Svesth, Z., Puliishingco. Solar Flares. 19%

Chin. Astron. Astrophys. 5 (1982) 192-198 Pergamon Press. Printed in Great Britain

Act.Astron.Sin. 23 (1982) 102-209 0275-1062/82/030192-07$07.50/O -

ACCELERATION OF THE SOLAR WIND BY WHISTLER WAVES FROM CORONAL HOLES

ZHANG Zhen-da, H~ANG You-ran, Department of Astronomy, Nanjing Observatory, LI Xiao-qing, Purple Mountain Observatory, Academia Sinica

Received 1981 February 23.

ABSTRACT We investigate the possibility of an additional acceleration of the high speed solar wind by whistler waves propagating outward from a coronal hole. We consider a stationary, spherically symmetric model and assume a radial wind flow as well as a radial magnetic field. The energy equation consists of (a) energy transfer of the electron beam which excites the whistler waves, and (b) energy transfer of the whistler waves described by conservation of wave action density. The momentum conservation equation includes the momentum transfer of two gases (a thermal gas and an electron beam). The variation of the temperature is described by a polytropic law. The variation of solar wind velocity with the radial distance is calculated for different values of energy density of the whistler waves. It is shown that the acceleration of high speed solar wind in