Chin.Astron.Astrophys. (19891 13/l 26-34 ,h Pergamon Press plc a translation of Printed in Great Britain Acta Astrophys Sin. (19881 8/4 301-307 0275-1062/89$10.00+.00

OBSERVATIONS OF THE MAGNIZTIC AND VELOCITY

FIELDS OFTWO SOLAR FLARES

LI Jing, AI Guoxiang, ZHANG Hongqi, LI Wei, CA0 Ai Beijing Astronomical Observatory, Chinese Academy of Sciences

Received 1987 September 3

Abstract After we have installed a CCD imaging system on our solar magnetic telescope, we have obtained many two- dimensional, real time, fine-detailed magnetic and velocity field pictures. In this paper we report on the two sunspot groups assoicated with two flares. Based on our analysis we point out that mutual penetration of opposite polarity regions exists generally and bright cores of flares all occur at front of mutual squeezing between such regions. This provides a strong support for the squeezing force-free field model of flares. We also found that, in regions where flares occur, the direction of the flow field is downward.

Key words: Solar flares - magnetic field - velocity field

1. INTRGDUCTION

Beginning in June 1987, the solar magnetic telescope of Beijing Astronomical Observatory has used a CCD image receiving and reducing system to effect two-dimensional, real time data collection and treatment of localized images of the Sun. Within 2-3 minutes, we could secure detailed data on the longitudinal magnetic fields and radial velocity fields in the lines Fe1 5324A (corresponding to the photosphere) and H6 4861A (the chromosphere), as well as monochromatic images of the Sun. Since then up to the present, we have made these observations on three sunspot groups containing large flares. Since the advent of magnetographs many people have usefully discussed the relation between flares and magnetic field and proposed various observational and theoretical models for the energy storage and release, such as the neutral sheet, magnetic shearing motion and flux emergence. The model of neutral sheet [ll is a theoretical model, it uses the instability of current sheet to explain the energy release in flares. The model of shearing motion [2] was observationally based. It was observed that flares in general appear close to the neutral line in the longitudinal magnetic field where the field gradient is high and that, before a flare, the chromospheric filaments run almost parallel to the neutral line and the transverse field is inclined at a small angle to the latter. After a flare, the filament become perpendicular to the neutral line, this shows that flares occur during the transformation of

Magnetic and Velocity Fields 27

a shearing force-free field to a potential field [3,41. This theory has been gradually perfected after the analysis of the large flare of August 1972. The model of flux emergence was also observationally based; it was observed that almost all flares occur in regions of magnetic flux emergence [51. The reason that emergent flux can trigger off flares is that when it meets a magnetic flux tube above, perturbation will develop which may cause elastic release of energy [61. These models explain rather well the flare morphology in the magnetic field from different angles, but because of lack of detailed observational data of the field, further work must be done before they are unified into one single theory. Also, since the solar atmosphere is a result of coupling between magnetic and velocity fields, the simultaneous observation of the two field assumes importance , and this particularly so for flares.

Observations of flare magnetic and velocity fields, especially the latter, have not been much described and few conclusions have been drawn. This is because flares are transient phenomena, having a time scale of lo3 seconds while the previous observations with spectrograph sweeps had a low time resolution, for example, the 65cm solar telescope of Okayama Astrophysical Observatory took 90 minutes to complete a magnetogram covering an area of 8’XE’ [21. Previous observations of the velocity field are even more scanty, and their time and spatial resolutions just cannot compare with such video frequency systems as the present solar telescopes. The earlier video magnetographs developed in the West during the 70s had also some difficulties in measuring fine structures of magnetic field. The vector magnetograph of Marshall Space Flight Center, USA, had a CCD receiver, but the local weather conditions were not conducive to observing fine structures and there was no measurement of the velocity field. At the Big Bear Solar Observatory, the seeing is good and longitudinal and radial velocity fields can be observed and studied, but in the treatment of the magnetic data, because only the difference between right and left polarizations is measured, what the magnetogram shows is the product of the magnetic field signal with the background light and so has large errors around and within the spots, and is unable to give the fine details in the magnetic field. The solar telescope of Beijing Observatory possesses the desirable feature of high resolution both in time and space, its final data are (DC/AC), directly proportional to the magnetic field intensity and so it is very advantageous for analysing the magnetic fields of active regions.

2. INSTRUMENT AND OBSERVATION

The solar magnetic telescope, through adjusting the position of the filter bands, is capable of working in the two wave bands of Fe1 5324A and HH 4861A and measuring 10 physical parameters in the longitudinal magnetic field, the transverse field, the radial velocity field and the brightness field.

The CCD receiver system consitutes a video frequency real time reduction system for the telescope. It has 500 w 582 pixels. The image treatment system has 512 x 512 pixels. The observed region has a size of 6’x4’, the frame rate is l/25 s, to observe 255 fields requires 40 seconds, each observed point corresponds to an angular resolution of 0.7”xO.5”.

Using this system, we have observed the changes in the magnetic and velocity fields before, during and after three flares. We shall give a detailed description of the first two events in this paper. The third

28 LI Jing et al.

Fig.1 The photospheric magnetogram of active region II87036 on 1987 July 23, 0054UT. 5.76lx3.59' area. N-polarity, dashed; S-polarity, full. The contour levels are f20, 40, SO, 160, 320, 640, 1280, 1600, 2240G.

Fig.2 The line-of-sight velocity field in H6, H87036, 1987 July 23, 0609 UT. 5.76lx3.59' area. Downward dashed, upward full. Contours are *400, 800, 1600, 2400, 3400, 4600, 6000, 7400, 9600 km/s.

Magnetic and Velocity Fields 29

PLATE I

30 LI Jinq et al.

PLATE II

Magnetic and Velocity Fields 31

Fig.3 the longitudinal ~a~netogr~ in Fe1 5324A. 1987 August 8, 0038 UT. Active region H87044. 5,15'x3.35' area. S-polarity dashed, N-polarity full. Contours are SO, 40, 80, 160, 320, 640, 1289, 1600, 1920, 22406.

Fig.4 The line-of-sight velocity field in Hp. 1987 August 8, 0135UT. Flare positions marked with numerals 1-6. Full Lines downward, dashed, upward. Contours are f400, 800, 1600, 2400, 3400, 4600, 6000

kml5 *

32 tI Jinq et al.

event occurring on August 9 will be reported in another paper. The sunspot group associated with the flare of 1987 July 23 was

loacated at E53, S19, and was assigned Huairou sunspot group number H87036. The largest flare occured at UT 0432. For this sunspot group we have obtained a fairly complete set of data. Beginning from July 21 when its position was E70, S20, until July 31, we carried out observations of the longitudinal field, and on two days (23 and 24) we also measured the H@ velocity field.

The other sunspot group is H87044, for which we observed the magnetic and velocity fields on 11 days (August 2, 4, 5, and 7-12). For 4 successive days (7-IO), observation was made of the transverse field. The flare occurred on August 8 and the position of the sunspot was W25, S28. Unlike the first group, this is a normal group, that is, its leader spots are at a lower latitude than its follower spots. The leader is made up of three spots close together while the follower is a loose bunch.

The two sunspot groups have the following in common: the flares occurred or began to occur in the follower portion. Here the spots are small and scattered, the opposite polarity regions interpenetrate, making it a favorable place for flares.

In our observation, the monochromatic image, the magnetic field and the velocity field were obtained separately. In order that the same region was observed in the different observations, we used photoelectric telescope tracking on the one hand and visual monitoring on the other, noting down the position of the spots and adjusting continually. By so doing, the error should not exceed 2” in general.

3. MOBPHOLGGY AND ANAtYSIS

Plate I. i-4 respectively show, for the sunspot group H87036, the photospheric monochromatic image and longitudinal field in Fe1 5324A and the chromospheric monochromatic image and radial velocity field in HB 4861A. These were all obtained on the same day, July 23, successively at UT 0049, 0054, 0432 and 0609. At UT 0432, a large flare erupted and before and after this, there was continuous chromospheric brightening in the same region. From Plate I.3 (6’x6’ area), we can see that in the follower group, the uppermost spot (marked “a”) was moving away from the others and the flare occurred just betweem them. (The position of the flare is marked by a circular arc in Plate I.1 and a circle in I.3). Plate I.2 (white for S polarity, black for N) shows that the spot “a” had the opposite polarity to the others and that, here, regions of opposite polarities penetrated into one another (arrow points to the position of the flare). The bright knots of the flare can be seen to be located in the front of an intrusion. A contour diagram of Plate I.2 is given in Fig. 1, where the position of the flare is indicated by a crescent. Here, the gradient of the longitudinal field is 0.08 G/km. The velocity field of Plate I.4 was taken after the flare, but since between 0432-0609 UT there were frequent small eruptions, we could still regard it as representing the velocity field of the flare. Here, white means towards the observer, black, away. The corresponding contour diagram is shown in Fig.2. The position of the flare is indicated with a circle in both. According to the analysis of chromospheric velocity field by AI Guoxiang et al. IS], the zero velocity line corresponds to the neutral magnetic line. The figure shows that the flare is located on one side of the zero velocity line, where the latter has the greatest twist. At the flare, there is a

Magnetic and Velocity Fields 33

downward velocity of about 6.0 km/s and a velocity gradient of 2.0 (m/s)/km, - theae indicate the sort of material motion during the flare.

The corresponding pictures for the sunspot group Ha7044 of August 9 are shown in Plate 11.1-4, and Plate II.5 ia 8 HP image during another flare. Open circular arcs mark the positions of flares. We note that although the flares occur amongst loose broken spots, they still avoid the umbrae, in agreement with a previous conclusion 191. In Plate 11.2, the positions of six flare cores are marked with the numerals l-6. We can see that these are all locations where one polarity intrudes into the other. These fine structure magnetograms show that interpenetration of opposite polarity regions exists generally and when opposite polarities crowd on each other 8nd when the intrusian has reached a certain level then there will be the pessibility of flare eruption. The contour diagram of Plate II.2 is shown in Fig. 3. We see that, at the flares, the opposite polarities had comparable strengths, both being about 1280 G. Another notable feature here is that in the leader epots among the spots of one polarity there wsa a ribbon of the opposite polarity. Nuch a structure is extremely unstable; possibly, 8 flare was in the making.

The chromospheric velocity field shown in Plate II.4 was obtained at 0136 UT. Inside the circle marked, we can see three isolated grey points snd two grey connected strips, their positions almost coincide with the positions of flares (white patches) shown in Plate 11.3. The greyness is due to the light of flares spilling out, washing out the signals of the velocity field. The contour disgram of PLate II.4 is shown in Fig. 4, from which we see that at flare 1, the velocity was 4.6 km/s, at flare 2, 2.4 km/s, while at the other four, na definite values can be given because af the light spilling. At all six locations, the velocity ~8s downward. This shows that, during flares, there is downward motion of the order of a few km/s.

in the chromasphere, matter is froren in the magnetic field lines, hence we believe that the chromospheric velocity field also reflects its magnetic configuration. In the upper middle part of Plate II.4 above the region of the flares marked by 8 circle we can see a series of ring structures which, considering projectian effects, can be regarded 8s magnetic loops connecting opposite polarity regions. When there is no fl8re, such structures are not obvious. They are peculiar to the flare velocity field; when there is a flare, the matter of the active region is excited; it carries the magnetic lines aa it moves, forming strong and conspicuous filamentary and ring-like structures in the chromospheric velocity field.

This group of sunspots produced another flare at 0341 UT (shown in Plate II). in the fallower group, the positions of the flare cores were almost the same as before; what is different is the small flare occuring amidst three close spots in the leader graup, at the positian where an opposite palarity strip intruded, as described shove. Observations on the next day showed that the three separate spots had coalesced into one and the strip with the opposite polarity had disappeared. Apparently, the crowding together of opposite polarities, their interpenetration, erruptions and fins1 recombinatian amount to a process of energy release.

4. DISCU88ION

Observations of theae flares and our experience in .a previous analyais of lags flares [Q] have led us to believe the crowding together and

34 LI Jing et al.

interpenetration of opposite polarity fields is the cause of flare eruptions. Observations from June through December have consistently shown that flares occurred at such locations, moreover, irrespective of the size of the spots, thus showing that the crowding together and interpenetration is a sufficient condition for flares.

The measurement and calibration of flare velocity field is a difficult problem and is subject to many factors. One is that during a flare an absorption line can become an emission line, so particular care must be exercised. In this paper, the flares are not very large and the direction of radial velocity did not show any change before and after the flares, so we think that the lines did not undergo reversal. Another factor is that if the velocity is too large then saturation may occur making measurement impossible. A third factor affecting the calibration is the asymmetry of the lines during flares, but generally speaking, this happens only during the explosive phase of the flare IlO], and particularly for large flares. Lastly, there is the broadening of the lines which tends to make the measured velocity too small. In all, the treatment of the velocity data must be done with great care.

We have also obtained some simultaneous data on the transverse field of the active regions and the photospheric velocity field, and these may be used for further analysis. We shall take advantage of the high time and space resolutions of the magnetic field telescope to carry out comprehensive observation and analysis of the magnetic and velocity fields throughout the entire flare process in order to arrive at ever more objective conclusions.

REFERENCES

( 1 _l Svestka, Z., Solar Flares, Ed. B. M. McCormac, (D. Reidcl Publishing Company) p. 301).

( 2 ] Hagyard, M. J., Smith, J. B., Tcuber, Jr. D. and West, E. A., Solar Physics, 81(1984),

f 3 ] Zirin, H. and Tanaka, K., Solar Phyricr, 92(1973), 173.

f 4 ] Kurokawa, H. and Hanaoka, Y., Solar Phyricr, 108(1987), 251.

f 5 ] Priest, E. R., Solar Magnetohydrodynamics, (1982), Ed. B. M. McCormac

[ 6 ] Li Hongwei and Pallavicini, R., Solar Phyricr, 107(1987), 271.

[ 7 ] Okayama Astrophysical Observatory, Solar Vector Magnetogram (1986).

(81 AI Guoxiang et al. Kexue Tongbao 33 (1988) 42. (9) AI Guoxiang and KONG Panxi, Chin.Astron.Astroph.vs. 7/l, 6-11 (1983)

= Acta Astron.Sin. 23/3, 211-218 (1982). (10) As (11, p.24.