Chin.Astron.Astrophys.(l990)14/1,16-24 3 Pergamon Press plc A translation of Printed in Great Britain Acta Astron.Sin.(1989)30/2,188-193 0275-1062/90$10.00+.00

OBSERVATION OF MAGNETIC AND VELOCITY FIELDS

OF ENHANCED WORK ON THE SUN

LI Jing AI Guo-xiang ZHANG Hong-qi HAN Feng Beijing Astronomical Observatory, Academia Sinica

Received 1988 January 25

ABSTRACT The solar magnetic field telescope provides quasi-simultaneous, two-level, two-dimensional magnetic and velocity fields and monochromatic image of the sun. Using this system we observed systematically fine structures in the magnetic and velocity fields of enhanced networks on the solar disk. We found good coincidence of the network boundaries and supergranules. Strong vertical flow was found in the boundaries of magnetic field networks. We also witnessed the combining of small networks into larger ones, the collapse of the small networks followed by the formation of the larger networks.

1, INTRODUCTION

Priest (1982) gave six types of magnetic structures on the sun: 1) polar region field, 2) large scale unipolar regions, 3) active regions, 4) transient regions, 5) permanent active regions and 6) network fields. With innovations in instrumentation, finer magnetic field observations led Martin et al. (1984) to propose the concept of “inner network field” and the discovery of “cancellation” of opposite polarity dipoles and “merging” of same polarities as agencies promoting the formation of magnetic network. There has been a great deal of work done on the relevant observation and analysis and some conclusions have been obtained [21: 1) at the photospheric layer, magnetic flow tubes with diameters smaller than 400km are distributed along the edges of supergranules. 2) Strong horieontal components of network field are found in the lower choromosphere or regions of minimum temperature. 3) Spicules are distibuted on the edges of supergranules. 4) The network field in the chromospheric layer has nearly a uniform width. However, in the reports, little related to the velocity field and quantitative results are almost absent. In view of this, we shall give in this paper maps and quantitative data of the magnetic and velocity fields of networks obtained with a magnetic field telescope and an analysis of these data.

As early as 1979, Prata et al. [3] put forth the concept of “enhanced network”, which probably corresponds to Priest’s Type 5 structure. The authors pointed out that the magnetic network

Enhanced Network

Figs. 1.1-1.4 Fe1 X5324 magnetogr~s *f an enhanced network on June 15-M. S-polarity in black, N-polarity in white

(reversed in 1.3)

LI Jing et al.

Fig. 2 1.1 Monochromatic H/3 image showing supergranulation. Fig. 2 1.2 Photospheric magnetogram. N in white, S in black. Fig. 2 :.3 H@ velocitygram. white away, black towards obsem rer.

Enhanced Network 21

Fig, 2.4 (a) Contour map of the magnetogram of Fig. 2.2. Levels: 20, 40, 80, 160, 320, 640 G.

(bf Contour map of the velocitygram of Fig. 2.3. Levels: 20, 80, 160, 320, 640, 960 m/s.

boundaries corresponded with supergranules, but velocity observations were lacking then. In this paper we shall discuss in depth the structure of network fields.

2. O~~VATION AND DATA TRRATMENT

The magnetic field telescope observes local regions of the solar disk of size 6 ‘x 4 ‘. There are 500x 582 pixels in the CCD, each pixel covers an area of 0.7”~0.5”. When drawing the maps, normal points averaged over 3~ 4 pixels were used so that resolution of the maps is about 2” * 2”.

An important aspect in the comparative analysis of the magnetic and velocity fields is positioning. About 5 minutes separated the magnetic from the velocity observations of the same region and if ther is a shift in the position then it will be labour lost. Hence steps were taken to avoid setting errors. First we set the telescope on photoelectric tracking with an accuracy of about 3”, reaching l”-2” under good weather conditions. Then we selected

22 LI Jing et al.

characteristic features, particularly the dark filaments, and used hand guiding. By these means the two pictures basically matched in position.

For quantitative analysis, a key point is calibration. For the Fe1 5324 A magnetic field, we use the theoretical calibration in Ref.[4], 1G = lO’ac/dc. For the 5324A velocity map, theoretical calculations and measurement of the solar rotation curve tell us that its calibration coefficient is almost in 1:l ratio with that of the magnetic field, that is, lm/s = lO”ac/dc. For W, 4861A, from the formula given in Ref.[4], the value should be 4 times that for 5234A, that is, lac/dc = 4alO’m/s.

3. MORPHOLOGY AND ANALYSIS

During the period 1987 June 15 - August 25, enhanced network field was observed on 8 days and nearly 20 frames of photospheric longitudinal .magnetic field and several frames of simultaneous chromospheric W, velocity field were obtained. We now divide these data into two sets and describe and analyse them separately.

The first set relates to the magnetic field observations of an enhanced network field that lasted four days. Figures 1.1-1.4 are the photographs of the photospheric field on June 15, 16, 17 and 18. The one for June 17 had polarity reversed due to a slip in the observing mode, but this will not affect our scrutiny of magnetic structures. This region had no sunspots and was located far from active regions, but at the chromosphere level we could see dark filaments over the neutral line. It was a large area and the structures were relatively stable, showing little changes from day to day. So it was quite easy to find it the next day in our continuous monitoring.

Clear network organization can be seen in the four pictures. There were several smaller network structures in the leading portion, their size is about 50”, comparable to supergranules (these are marked with circles in Fig.l.l). These structures together formed a larger network with a size over 100”. On the second day clear changes in this enhanced network region could be seen: some smaller structures had collapsed and the larger network was now defined by some medium sized (60”-70”) networks, these are circled in Fig. 1.2. The changes on the third day were relatively small, only the picture of smaller networks gathering into a larger one became even clearer (Fig.1.3). These facts show that the rise and fall in an active region is smaller denser network structures evolving into larger ones followed by gradual fading out. In terms of magnetic flux, active regions are where the flux is concentrated, under the action of the flow field, magnetic dissipation dilutes the flux, gradually down to the order of magnitude of the flux in quiet regions.

The second set of data we shall discuss refer to the observations of the magnetic field and the velocity field in the enhanced network of August 24. We also obtained the velocity field of the

Enhanced Network 23

same region on the next day, the features were basically the same as before. From observations about one month before we deduced that this region was evolved from a sunspot active region in the previous rotation (Huairou No. 67036 = Boulder 4627). It was very active at the time and we have described it in a previous paper [6]. Figures 2.1-2.3 are the monochromatic HI, image of the network field on the 24th, the Fe1 X5324 photospheric longitudinal magnetic field and the I@ chromospheric radial velocity field, respectively. In Fig. 2.2, white is N-polarity, black is S-polarity. In Fig. 2.3, white means away fron, black towards, the observer.

Supergranule structures can clearly be seen in Fig.2.1. In these photographs, circles of the same size mark the same structures. We can see that along the boundary of supergranule the velocity is downward. Fig. 2.2 shows very clear network structures, their correspondence with the velocity is even clearer, they are also in one-to-one correspondence with supergranules, where the magnetic field is concentrated, the flow is strong. Digitized contour maps of Figures 2.2 and 2.3 are shown in Figures 2.4(s) and 2.4(b), the last having been obtained Fig. 2.3 after applying first and second order corrections for curvature. The corresponding structures in the two maps are marked by the same numbers or a circle. On network boundaries, the magnetic field reaches 160-320G, while the corresponding velocity reaches 0.484.64km/s. Also, in the interior of networks, although the magnetic field is small, there still exists velocities of 0.16-0.32km/s and the direction is opposite to that on the network boundary. See the location marked “1” and “*‘I in the maps. They correspond to the interior of supergaranules in Fig. 2.1. In regions outside networks, for example near the neutral line in Fig. 2.4(a), the velocity is also high, reaching 0.64km/s.

The situation shown in Figs. 2.1-2.2 can be well accounted for by the convection theory [7, 81. When a lump of hot adiabatic plasma rises to the top of the convection layer, the fall in the ambient density and temperature causes the hot material to spread, flowing rapidly in the horizontal direction and downward. And this is what we observe at supergranule boundaries. Since the magnetic field in frozen in, this flow field drags along the magnetic lines, forming magnetic field concentrations on the boundaries of rising gas blobs; and this is what we see in Fig. 2.4(a). The formation of network structures in the chromosphere is due to the same cause, hence here too we see increased magnetic field accompanied by a strong velocity field [9].

Another noteworthy feature in Fig. 2.2 is that the magnetic field is not evenly distributed along the supergranule boundary. Rather, it shows up as isolated elements with a size of 3”-4”, giving the network a tree-like appearance.

Observations show that network structures are on many scales. From a microscopic point of view we can divide these into three groups. One group is between 5”-ZO”, comparable to the size of sunspot magnetic field, that is, magnetic flux density of this size is conducive to the formation of sunspots. This paper does not deal

24 LI Jing et al.

with this group of structures. The second group is between 25”-90” and such features are seen in Figs. ‘1.1, 1.2, 2.2 and 2.3. This scale corresponds to the scale of supergranules [lo]. The third group is larger than 90”. These structures are initiallydefined by separate smaller network structures, the latter slowly evolving into their individual boundary , as shown in Figs. 1.3 and 1.4. They already approach the large scale magnetic field on the sun and begin to reflect some properties of the atmospheric circulation.

4. DISCUSSION

The above analysis shows that network structures on the sun are a consequence of convection in the solar atmosphere, while the formation of enhanced network regions is dominated by the supergranular flow. When the magnetic field of an active region through energy release and magnetic energy dissipation reduces the magnetic flux, supergranular flow will push the magnetic lines to its boundary,. there to form strong network field and giving rise to strong magnetic field even where the velocity field is strong. It is noteworthy that such phenomenon is rarely seen in quiet regions and is peculiar to decaying active regions. The formation of enhanced networks is a result of coupling between a strong magnetic field with a strong velocity field.

By “network” we mean magnetic network in the first instance, any corresponding structures in the monochromatic image and the velocity field are found later. The networks have the regular magnetic field and velocity_field pattern of supergranules and match well with the boundaries of supergranules. Such enhanced network field amounts to a huge dipole field, and there are dark filaments above its neutral line, as in an active region. Therefore, the study of such network structures has a two-fold meaning: we can use it to examine the decay of active regions as well as to study their formation in quiet regions and their evolution. Our continuous observation over four days of an enhanced network field revealed that, characteristically, some smaller network structures forming a larger pattern gradually break up and give rise to the formation of a large network structure.

REFERENCES

[l] Priest E-R., Solar Magnetohydrodynamics, Reidel, 1982. [2] Sturrock P.A. (ed.) Pysics of the Sun, Vo1.2, Ch.2, Reidel, [3] Prata S.W., Solar Pbys. 33 (1973) 119. 141 AI Guo-xiang et al. Chin.Astron.Astrophys. 6 (1982) 129-136 =

Acta Astron.Sin. 23 (1982) 39-48. [5] Allen C.W., Astrophysical Quantities, (Chinese translation by

YANG Jian, 1976) 17. [6] Gibson E.G., The Quiet Sun (Chinese translation by

LIN Yuan-zhang et al., 1981) 165. [7] Bray K.J. et al., The Solar Granulation, Reidel, 1984, p.186. [El Ref.[l], p.68. [9] Ref.[l], p.85.