A M E A S U R E M E N T OF T H E M A G N E T I C F I E L D D I R E C T I O N AT

T H E S I T E OF M A J O R F L A R E S

H. LUNDSTEDT*

Institute for Plasma Research, Stanford University, Stanford, Calif., U.S.A.

(Received 18 September, 1981; in revised form 10 May, 1982)

Abstract. Lundstedt et al. (1981) showed that the direction of the photospheric magnetic field at the site of a flare is a good predictor of the solar wind velocity observed at Earth four days later. We describe here how the field direction was obtained, and discuss possible errors involved in the determination of the angle. The discussion also includes a characterization of the solar active regions.

1. Introduction

Recent results (Lundstedt et al., 1981) indicate that the acceleration of the solar wind by flares is strongly dependent upon the direction of the photospheric magnetic field at the flare site. If the field has a southward component, high speed solar wind plasma will usually be observed near Earth four days after the flare occurred. If the field has a northward component such high speed solar wind is less frequently observed. This was

observed from August 1978 to November 1979.

The purpose of this paper is therefore to describe the determination of the field direction. We will estimate the errors involved in the measurements. A description of the active regions involved is also given.

2. The Mt. Wilson Classification of Active Regions

All the active regions observed on the Sun with the Mr. Wilson magnetograph between August 1959 and December 1978 have beef1 given magnetic classification in a system similar to the Mr. Wilson sunspot-classification scheme. The classification system is described in Table I. The system is discussed in detail by Smith and Howard (1968). In the following we compare flares in the interval 1968-1978 with the Mt. Wilson

classification. We calculated during this interval the central meridian passage (crop) time of the flare

sites and compared it with the crop time of the already classified active regions. In that way we were able to associate a classified active region with a flare in 88 ~o of the cases. The exceptions were due to cases when no magnetogram, and therefore no classified

active region, existed at the time of the flare. Many times the same active region gave rise to several major flares. Out of 75 active regions 21 (28~o) of them gave rise to 2 major flares and 11 (15%) of them gave rise to 3 or more flares.

The difficulties, especially during the interval of sunspot maximum, in separating the multibipolar regions from the simple bipolar regions led us to not treat them separately.

* Now at Institute for Astronomy, Lund University, Lund, Sweden.

Solar Physics 81 (1982) 293-301. 0038-0938/82/0812-0293501.35. Copyright �9 1982 by D. Reidel Publishing Co., Dordrecht, Holland, and Boston, U.S.A.

294 H. L U N D S T E D T

TABLE I

Magnetic region classifications

Designation Definition

B BB AB BA BC BS

BY BCS BYS BBC BBS

Simple bipolar region. Two adjacent simple bipolar regions. Simple bipolar region preceded by a region of a single polarity. Simple bipolar region followed by a region of a single polarity. Simple bipolar region with one polarity partially encircling the opposite polarity. Simple bipolar region with an area of opposite polarity embedded in one or both of the

main bipolar components of the region. Bipolar region with a peninsula of one polarity extended into the opposite polarity. Bipolar region with the characteristics of both a BC and a BS region. Bipolar region with the characteristics of both a BY and a BS region. A BB region with a C characteristic. A BB region with a S characteristic.

B region with a Y characteristic.

25

20 u3 z o

W rY m 1 5 >

u_ o l o o~ w m g D z

5

1 JAN. 1 9 6 8 - 2 0 DEC. 1978

BB BBC BBS BBY

Fig. 1. The number of active regions is compared to the class of active region. During the interval 1 January, 1968 through 20 December, 1978 we had 23 active regions of class BC-BBC. The second most

frequent class was BYS, with 16 active regions of that class.

F igure 1 shows the d is t r ibut ion o f the ac t ive region c lasses a s soc ia t ed with our set

o f flares. T h e m o s t c o m m o n class is c lear ly s h o w n to be one wi th one polar i ty encirc l ing

the oppos i t e polar i ty ( B C - B B C ) . This s i tua t ion of ten occu r s w h e n a n e w region

deve lops in a m u c h o lder region. F igure 3a shows an example o f this c lass o f act ive

region. A B C - B C region o c c u r r e d app rox ima te ly three t imes m o r e of ten than did a

M A G N E T I C F I E L D D I R E C T I O N 295

Fig. 2.

20

U3

,<

"-" 15 1:3 r v

I I-- n- O

<

i

O 03

8 t.tl m

Z

1JAN. 1968 -20 DEC. 1978

[ ] [ ]

B NORTHWARD B SOUTHWARD

B BC BS BY BCS BB BBC BBS BBY

BYS AB BA

The number of southward and of northward flares is compared to the class of active region. Both types of flares show the same distribution of active region classification.

simple bipolar region (Figure 3c). The second most frequent class of active region was a BYS region (Figure 3b). A BYS region occurred approximately twice as often as did a simple bipolar region.

Figure 2 shows the distribution of the southward and northward flares with classes of active regions. As can be seen there is no statistically significant difference. The southward flares are therefore not coming from more complex active regions.

3. Determination of the Field Direction

Full disk solar magnetograms from Mt. Wilson Observatory, in the form of isogauss contour drawings published in Solar Geophysical Data, were used in defining the direc- tion of the photospheric field at each flare site. The Mt. Wilson instrument measures the line of sight component of the magnetic field. Contours are plotted in the range + 5 G to + 80 G (+ 0.5 mT to + 8 roT). During the interval 1967-1973 all the Mt. Wilson observations were made with a square aperture 17.5 arc sec ( ~ 12500 km at the disk center) (Howard, 1974a, b, 1976). An increase in resolution was carried out later and since 5 July, 1975 all full-disk magnetograms have been taken with the 12.5 arc sec aperture ( ~ 9000 km at disk center).

As a rule solar magnetic field observations and Hct flare observations were made at different times, and often on different days. The flare position is obtained from the Ha

296 H. LUNDSTEDT

0 ~

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(el

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2

(d)

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~ �84

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~ ~ 7 30~

- , " , f

60~

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. . . . MINUS l~ = 12000 krn

(f) 40~

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Fig. 3. Mt. Wilson magnetograms to illustrate the different kinds of active regions and how the direction (0) of the photospheric magnetic field at flare site (indicated by an F) was defined. The positive (outward) polarity of the region is encircled: (a) This magnetogram was observed on 28 October, 1968. The flare occurred on 29 October, 1968 and 0 was 295. The active region was classified as a BBC region. (b) This magnetogram was observed on 19 September, 1978. The flare occurred on 23 September and 0 was 295. The active region was classified as a BYS region. (c)This magnetogram was observed on 26 June, 1977. The flare occurred on 26June, 1977 and 0 was 150. The active region was classified as aB. (d)This magnetogram was observed on 15 September, 1977. The flare occurred on 16 September, 1977 and 0 was 110. The active region was classified as anAB. (e) This magnetogram was observed on 26 September; 1968. The flare occurred the same day, but 0 was not defined because the magnetic structure was too small. (f) This magnetogram was observed on 20 June, 1968. The flare occurred the same day, but 0 was not

defined because the active region was only fragments.

MAGNETIC FIELD DIRECTION 297

(a)

30~

. . . . ~ ~.' ,-~

~, ~- .~,~...:~.~

AUGUST 23, 1979

0 ~

AUGUST 25, 1975

(b)

(c)

::/

i

0 ~

~ I ,g/'~' .~:,i;,- I%~L

r . ~ . o !~'~ I ~ ' r ~ �9

SEPTEIVlBE~ i 8 I 1978

30~

0 ~

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~,~, :, ,,,, L~.'~v , - ~,,

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SEPTEMBER 19, 1978

50OE 0 ~ 6 0 ~ W

AUGUST 22, 1979 AUGUST 25, 1979 AUGUST 30, 1979

Fig. 4. Mt. Wilson magnetograms to illustrate different cases of emerging flux and how the shape of the bipolar region changes. The positive polarity of the region is encircled: (a) Two magnetograms show a small bipolar region (indicated by arrows) developing during 51 hr. (b)Two ma~etograms show how a BYS region is born through emergence of new flax. The second magnetogram was observed 24 hr later. (c) Three magnetograms show how a reversed complex active region develops during 9 days. The direction of the primary neutral line (indicated by a solid line) remains almost constant. We also see several cases of

emerging flux (indicated by arrows) during the 9 day's.

298 H. LUNDSTEDT

observations. To get the position of the flare region at the time of the magnetic field measurements, we therefore have to change the longitude of the flare by the amount = 13.2At deg day- 1 where A t = tnare - tm.f. is the difference in days between the time of

the flare and the time of the magnetogram. The value 13.2 degrees per day is the Carrington average synodic rotation rate for sunspots. We used the nearest available Mt. Wilson magnetogram, and the mean of(At) was found to be 22 hr, i.e. less than one day. This time difference puts a lower limit on what size of the magnetic structure we can study. The shape of the structure could change due to growth or decay of magnetic flux. Since the growth rate is roughly 10 times faster than the decay rate (Howard, private communication), and we are only interested in changes within a few days, we only have to study the growth of an active region. This will be discussed later.

Transparent Stonyhurst disks were used to find the sites of the flares on the magneto- grams. Flares almost invariably occur along a magnetic neutral line, i.e. where the vertical magnetic field is zero. We therefore determine the direction of this neutral line, which divides the two polarities of magnetic field, at the flare site. The field direction at the flare site is then defined to be perpendicular to the neutral line, and is measured with 0 = 0 ~ for exactly eastward direction, increasing toward the north (Figure 3). Since the neutral line can be sometimes tangled and curved, we also have to determine an upper limit on what structure size we can study. This will also be discussed later.

We show in Figure 3 different cases that elucidate our method of determining a 0 value. Figure 3 a illustrates as already mentioned the most frequent class of active region. This BBC region gave rise to seven major flares during the interval 27 October to 2 November, 1968. We show the magnetogram observed on 28 October, and how we determined the direction of the photospheric field for the flare (indicated by an F) that occurred on 29 October. The flare took place south of the area of positive or outward polarity magnetic field. We therefore have a flare with a southward directed photospheric magnetic field, 0 = 295 ~ The neutral line is straight over 100000 km (8~

Figure 3b shows a BYS class of active region. This region has a peninsula of positive polarity extending into the opposite following polarity. On 12 September, 1978 a small region of opposite polarity emerged inside the following region. The island of opposite polarity grew and the region looked very much like the region in Figure 3a. On 19 Sep- tember the island and the preceding region merged together and the peninsula was produced. The BYS region produced three major flares during the interval 23 September through 30 September. We show the field direction for the flare that occurred on 23 September. The neutral line is again smooth over approximately 8 ~ Since the flare occurred below the outward field we have a southward flare. The direction was measured to be 295 ~ as for the earlier flare.

Figure 3c illustrates a simple bipolar region. The magnetogram and the flare were observed on the same day, 26 June, 1977. The day before the flare occurred the active region showed a BY shape. The active region produced one major flare. The direction of the photospheric field was determined to be 150 ~ or slightly northward. The neutral line is not straight over a distance greater than 8 ~

Figure 3d illustrates an interesting A B region, i.e. a simple bipolar region preceded

MAGNETIC FIELD DIRECTION 299

by a region of a single polarity. The region is also reversed. Only in exceptional cases do the active regions have their large scale bipolar field oriented north-south, or

alternatively east-west but reversed from the proper direction for that hemisphere. Tang (1980) found only 2.4 ~o reverse polarity regions in the total population of active regions with a lifetime of 9 days or greater. The reversed active regions also take place at very low latitudes (Figures 4d and 5c, Smith and Howard, 1968). The flare indicated on the magnetogram occurred on 16 September, 1977. The magnetogram was observed on 15 September. The flare took place above the outward field and is therefore a northward flare. The direction was measured to be 110 ~ using a neutral line that was straight over a distance greater than 8 ~ On 18 September the positive polarity island merged together with the following polarity of the bipolar region and two other major flares occurred.

Sometimes the magnetic structure in the magnetogram is too small to accurately define a direction. Figure 3e illustrates such a situation. The magnetogram was observed on 26 September, 1968. On the same day a major flare took place, but the bipolar structure was too small (a few degrees). No direction was defined for that flare.

Figure 3f illustrates another example when no value could be defined, which is when no clear bipolar structure exists at the time of the flare. Only the fragments of an old active region are left in Figure 3f.

4. Errors in the Measurements

In this section we try to establish the errors affecting the value of 0 obtained by the method described in Section 3. First of all the position of a flare is given as the center of gravity of an area, and thus the position of the flare is uncertain within a circle of approximately 2-3 degrees diameter. In order to be used in our work the neutral line must therefore be straight over that scale.

Secondly an error is introduced when the magnetic field and the flares are observed at different times. The time difference is generally less than one day. No major change in the shape of the bipolar region, and thereby a change in the 0 value, must take place within a couple of days. This will put a lower limit on the size of the bipolar region structure in which we can define 0. In Figure 4 we show several cases of emerging magnetic flux.

The growth rate is roughly a factor of 10 faster than the decay rate. Stenflo (1976) proposed the following empirical law for the decay rate of magnetic structures:

,~d~ ~ _ 10 7 Wb s ' (1) dt

It is interesting to notice that the value for the decay rate is about three orders of magnitude faster than would be expected from diffusion. Larger flux concentrations seem to break up into smaller flux elements through some kind of plasma instabilities. Equation (1) gives us a rough value for the growth rate (10 8 Wb s- l ) .

In Figure 4a we show how a small bipolar region develops. The second magnetogram

300 H. LUNDSTEDT

is taken 51 hr later. During that time we observe an approximate net flux increase of 12 • 1012 V~b, or all average growth rate of 0.7 x 108Wb s -1.

In Figure 4b we show a flux island merge together with the preceding polarity region. The second magnetogram was observed 24 hr later. During that time we measure an approximate net flux increase of 7 x 1012Wb, or an average growth rate of 0.8 x l0 s Wn S - 1 ,

The overall change in shape of a bipolar region during a 9 day time interval is shown in Figure 4c. The direction of the main neutral line (indicated by the solid line) remains almost the same during the whole interval, even thought we notice several small scale changes (indicated by arrows). As seen through these few examples, we can have a change in shape over distances of about 5 ~ during less than two days. This puts a lower limit on the size of magnetic structure in which we can define 0. On the other hand, the direction of the primary neutral line remains almost constant over distances of 8 ~ during many days. If we therefore restrict ourselves to defining a 0 for structures larger than about 6 ~ we reduce the error caused by the difference between the flare time and the time of the magnetogram.

5. Summary

The most common class of active region that gave rise to our set of flares was found to be a B C - B B C in the Mt. Wilson classification system. The second most frequent class of active regions was a BYS. Often they are related to each other. A BYS active region was often produced out of a B C - B B C region by emerging flux.

Southward flares did not come from more complex regions than did northward flares. As a rule magnetic field observations and flare observations are made at different

times. We therefore studied how the shape of the active regions changed during a couple of days due to newly emerged flux. We found that new flux could change the shape of structures over distances of about 5 ~ within a couple of days. The new flux often produced peninsulas. The general shape over distances greater than about 6 ~ remained constant. The direction of the primary neutral line, and therewith 0, in Figure 4c remained the same during more than 9 days. The upper limit of neutral line length in which to measure 0 was chosen to be about 8 ~ Above that, the neutral line often becomes very curved.

Acknowledgements

I thank John Wilcox and Philip Scherrer for guidance, encouragement and hospitality during my visit at Stanford. I am particularly grateful to R. Howard for the use of Mt. Wilson magnetograms and for helpful discussions. This work was supported in part by the Office of Naval Research under contract N00014-76-C-0207, by the National Aeronautics and Space Administration under grant NGR05-020-559 and contract NAS5-24420, by the Division of Atmospheric Sciences, Solar Terrestrial Research program of the National Science Foundation under grant ATM80-20421, and the Max C. Fleischmann Foundation.

MAGNETIC FIELD DIRECTION 301

References

Howard, R.: 1974a, Solar Phys. 38, 59. Howard, R.: 1974b, Solar Phys. 38, 283. Howard, R.: 1976, Solar Phys. 47, 575. Lundstedt, H., Wilcox, J. M., and Scherrer, P. H.: 1981, Science 212, 1501. Smith, S. F. and Howard, R.: 1968, in K. O. Kiepenheuer (ed.), 'Structure and Development of Solar Active

Regions', 1AU Syrup. 35, 33. Stenflo, J. O.: 1976, in V. Bumba and J. Kleczek (eds.), 'Basic Mechanisms of Solar Activity', IA U Syrup.

'71, 69. Tang, F.: 1980, 'Reversed-Polarity Regions', Big Bear Solar Observatory Report 199.