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High-resolution sunspot observations
Adv. Space Res. Vol. 11, No.5, pp. (5)225—(5)231, 1991 Printed in Great Britain. All rights reserved.
0273—1177/91 $0.00 + .50 Copyright © 1991 COSPAR
HIGH-RESOLUTION SUNSPOT OBSERVATIONS H. Zirin and H. Wang Big Bear Solar Observatory, California Institute of Technology, Pasadena, CA 91125, U.S.A.
ABS11~.ACF A high-resolution six-hour sequence for a stable round sunspot 22 deg from disk center reveals the following properties: 1)
The inflow of penumbral material to the umbra is limited to the inner half of the penumbra.
2)
The outer half of the penumbra shows the Evershed effect outward flow, extending beyond the penumbra. Both proper motions and Doppler shifts of these elements are observed. The Evershed flow peaks in discrete elements, which are clearly associated with regions of stronger and more horizontal magnetic field in the outer penumbra.
3)
From the apparent reversal of field sign in the penumbra, we estimate that field lines emerge at angle of at least 22°to the horizontal throughout the penumbra. The MMF (moving magnetic features) are generally associated with bright K-line elements. While some are
4)
bipolar and most show the penumbral polarity associated with horizontal field, there are many exceptions. 5)
The “orphan penumbra” shows motions similar to the normal penumbra. INTRODUCTION
We obtained an outstanding series of observations with the 65cm vacuum reflector of the sunspot BB1680, NOAA 5612 on 2 Aug 1989. The position was 30”E 378”S. For over six hours the seeing remained near the limiting resolution (approximately 0.25) of the 65 cm telescope. Observations in the continuum were obtained for the first two hours with a Schott VG-9 filter around 5200A, and for the last four with a bA ifiter centered on the K line. These observations were recorded from a CCD-video camera to Super-VHS videotape. At the same time, we obtained videomagnetogram and Dopplergrams in Cal 6103. The data give new information on a number of phenomena associated with normal stable spots. The video and digital images were reregistered so that highly accelerated movies could be studied. The Dopplergrams were calibrated by solar rotation. When a few hours are viewed in a few seconds, the eye easily detects systematic motions down to 100 nVs or less. The motions can be confirmed by cork movies” which correlation-track individual elements. Higher speeds are not visible this way and we have not searched for them systematically. We summarize the conclusions from the observations in several wavelengths. From the Evershed effect there was early evidence of outward penumbral flow, but the first measurements of proper motion were made by Muller (1973) /1/ who found a steady flow of about 0.5 km/sec into the umbra, a result confirmed by Zirin and Wang (1989) /2/. Muller commented on the contradiction of this effect with the Evershed flow. The outward flow of magnetic elements (MMF) was first detected by Sheeley (1969) /3/. An outward flow manifested in granulation was found by Title er al. (1986)/4/and also confirmed by Zirin and Wang. The highly-accelerated movies give the impression of massive, systematicflows. ALL the material in the inner half of the penumbra moves inward, no outward motion is found; ALL the material in the outer half, bright and dark, moves outward. The flow continues well beyond the penumbra. The division of the penumbra into an inflowing inner part and outfiowing outer part resolves the contradiction between the Evershed flow and Muller’s measurement. CONTINUUMAND K-LINE
(5)226
H. Zirin and H. Wang
Figure 2 shows the K-line image, made with a bA wide filter. This is a mixture of K-line and continuum, but shows bright points nicely.
The K-line and continuum movies show the clear distinction between an inward flow of 0.5 Km/sec in the inner half of the penumbra and an outward flow accelerating from 0.5 Km/sec at the middle to 1 Km/sec at the edge of the penumbra. The inflow appears dominated by the bright penumbral fibrils, the outflow by the dark. No fibrils extend completely through the penumbra. The continuum images show the flow extending out through the granulation; the K-line shows bright points flowing out at varying speed. While the motion in these observations is particularly well-marked, we have observed it in all other similar spots with stable penumbra. Thin and Wang (1989) 12/ showed two examples of penumbra unconnected to any umbra. At the left side of the frame (Figure 1) is an example of this phenomenon, which we will henceforth call orphan penumbra. It has of course been observed many limes before, but not, to our knowledge, specifically described. Orphan penumbra is sinipiy penumbra without an attached umbra, and its origin is as yet not determined. In this case the orphan penumbra reveals the same motions as the normal penumbra, the part furthest from the central spot moving outward and the closest moving inward. The orphan penumbra shows the same polarity as normal penumbra, even reversing sign at the far edges. In this case it is present for almost the entire disk passage and shows no relation to any vamshed spots. It behaves like a second “wave” of penumbra. Another locus of strong flow is the light bridge in the umbra. This shows continuous flow from one end to the other. MAGNETIC AND DOPPLER OBSERVATIONS At the same time as the direct video observations were being made, we took an excellent series of magnetograms and Dopplergrams with the videomagnetograph (VMG) on the 65 cm telescope. These are shown in Figures 3 and 4. The magnetograms show the best sequence of moving magnetic features (MMF) observed so far. These are seen to flow out from the penumbra about 1.0 to 1.5 Km/sec at the edge of the penumbra. They do not appear in the penumbra, but outside it. There has been considerable discussion of the nature of the MMF. It has been suggested by Harvey and Harvey (1973) /5/ that these are loops or embolisms in the outfiowing magnetic fields. If that were the case each MMF would be bipolar, the sign ofthe inner pole being the same or opposite to the spot, depending on whether the loop is directed downward or upward. The downward type are particularly evident at the right in Figure 2. While there are some bipolar MMF’s in our data, most of them are unipolar. The MMF points on the K-line images are much clearer and smaller than on the magnetograms because they represent a 1/30 sec exposure compared to an integration of 128 frames (4 sec). One of the prominent features of high-resolution magnetograms is the apparent field reversal on the limbward side of the penumbra. This arises from the longitudinal projection of horizontal field lines. In this case the spot is close enough to the disk center that there is no penumbral reversal, but the MMF’s show a clear effect at upper left. This is the hmbward direction, allowing for a substantial P-correction. Ifall penumbral field lines were horizontal, then the apparent polarity of the penumbra, as seen in the Stokes V parameter, would be positive in the half-ring of the penumbra pointed away from us and negative in the other. But the penumbral field lines are not horizontal, but intersect the surface in progressively flatter angles as we go outward. A simple geometrical calculation shows that the sign of the apparent penumbralfield reverses where the angle of thefield lines to the horizontal equals the angular distance from Sun center. In this case the field lines are tilted at least 22°from the horizontal throughout the penumbra. Further, as we go around the penumbra, the locus of the reversal moves outward to where successively flatter field lines produce zero or negative line-of-sight projections. In this way the sign reversal maps the tilt of the penumbral field lines, assuming the field lines to be radiaL Measurements of the Doppler field were also assembled in a reregistered film. These give us our best view so far of the Evershed flow. In the Doppler frame (Figure 4) we see a strong peak of the receding Evershed flow (dark) at the limbward edge of the spot. Similarly, the near boundary is white, reflecting the blue-shifted flow on the near side. Note, however, that the inner penumbra does not show the opposite pattern; in fact, it shows no wave-length shift at all. This may be because the light level is low, or because the apparent inflow in the inner penumbra is some kind ofmoving excitation, rather than a true motion. The Evershed flow, as can be seen on the illustration, is not a uniform flow, but a series of moving elements which match almost perfectly the regions of strongest reversed sign magnetic field in the outer penumbra on the far side. This means that they are closer to the horizontal than the 22°central distance. Penumbral structures that are not reversed in apparent sign do not show the strong Evershed flow, and this makes it less obvious on the near side. Even there the best fit is with reversed features, leading one to suspect that they are downflowing. The proper motion velocity of the Evershed elements is 0.5 1.0. kim’sec, roughly the same as the Doppler velocity. The Evershed elements may also be related to the larger MMF’s. There are a few anomalous motions in the penumbra, as in the upper left of Figure 4; these all correspond to field inclusions. The occurrence of “flags” discovered by Bumba (1960) /6/ can be explained by the combination of the stationary background with the moving Evershed elements. -
High-Resolution Sunspot Observations
(5)227
Some may wonder if the close comiection between the Evershed flows and the magnetic structure is due to our use of a Zeeman-sensitive line. It is generally held in the solar physics that one must use a line of g=0 for Doppler measurements. While this may be desirable for absolute confidence, there is no published evidence of any anomalies associated with the use of a Zeeman-sensitive line. Certainly our results in thousands of Dopplergrams have revealed no effect. Qose examination of the frames shows the match exists only on one side of the penumbra, and not, for example, in the orphan penumbra or the various other features. Further, we have many observations of the Evershed effect; its amplitude intensifies strongly toward the limb, while the magnetic signal decreases. There is a prominent neutral line at the right of the image; this is marked in the Doppler flow by a fairly sharp velocity gradient, with the negative polarity receding at 0.2 Km/sec and the positive approaching. Most neutral lines are found to have a convergence, and this is well-displayed in the movie, which shows continuous convergence in proper motion, substantial amounts offlux flowing into the neutral line. In this case almost all the flow is on the penumbral side, with strong flow from the naked penumbra into the neutral line, and apparent cancellation with the negative flux. Points of strong down flow appear here from time to time, lasting typically for about an hour. Whether ornot they are connected with the disappearance of flux here we cannot as yet say. CONCLUSIONS The dynamic events that occur around a stable sunspot are indeed surprising, and must play an important role in the physics of the spot. Clearly the strong motion is confined to regions where the field has some horizontal component. This component is not large; from the reversal of apparent longitudinal sign we find the tilt of field lines to the horizontal to be 22°or more out to the edge of the penumbra. So the motion may be upward along a Wilson effect dish, because it is unlikely that it crosses field lines. However the physical significance is beyond the scope of the paper. We found a strong radial flow in the penumbra of this round spot, with the inner penumbra moving inward and the outer penumbra, outward. While the outward flow was confirmed by both proper motion and Doppler shifts, the mwarrl flow did not show a Doppler shift and might be only an excitation effect. The second, or orphan penumbra, shows motions similar to the normal penumbra. This work was supported by the NSF through ATM 8816007 and NASA NAGW-1972.
REFERENCES 1. Muller, R. 1973, Solar Phys. 29 55. 2. Zirin H. and Wang, H. 1989. Solar Phys. 245, 119. 3. Sheeley, N. R. 1969. Solar Phys. 9, 347. 4. Title, A., Tarbell, T., Simon, G. and the SOUP Team, 1986. In Adv. Space Res. 6,253. 5. Harvey, J. W. and Harvey, K.L 1973. SolarPhys. 2861. 6. Bumba, V. 1960. Izv. Kr. Astr. Obs. 23,253.
(5)228
H Zirin and H. Wang
Fig. 1. A picture of BB16800 in the green, showing our normal image of umbra, penumbra and granulation.
High-Resolution Sunspot Observations
Fig. 2. The same in a bA window at the K line, showing outfiowing bright points.
(5)229
(5)23(J
1 I. Zirin and H. \Vang
a
Fig. 3. The same in longitudinal magnetic field. This frame and Fig. 4 are contracted horizontally by 4:3 by our imageprocessor. We have kept the ratio here to maintain the field. We have marked the matching features here and in Fig. 4.
High-Resolution Sunspot Observations
(~~)231
a
Fig. 4. Doppler image.
0273—1177/91 $0.00 + .50 Copyright © 1991 COSPAR
HIGH-RESOLUTION SUNSPOT OBSERVATIONS H. Zirin and H. Wang Big Bear Solar Observatory, California Institute of Technology, Pasadena, CA 91125, U.S.A.
ABS11~.ACF A high-resolution six-hour sequence for a stable round sunspot 22 deg from disk center reveals the following properties: 1)
The inflow of penumbral material to the umbra is limited to the inner half of the penumbra.
2)
The outer half of the penumbra shows the Evershed effect outward flow, extending beyond the penumbra. Both proper motions and Doppler shifts of these elements are observed. The Evershed flow peaks in discrete elements, which are clearly associated with regions of stronger and more horizontal magnetic field in the outer penumbra.
3)
From the apparent reversal of field sign in the penumbra, we estimate that field lines emerge at angle of at least 22°to the horizontal throughout the penumbra. The MMF (moving magnetic features) are generally associated with bright K-line elements. While some are
4)
bipolar and most show the penumbral polarity associated with horizontal field, there are many exceptions. 5)
The “orphan penumbra” shows motions similar to the normal penumbra. INTRODUCTION
We obtained an outstanding series of observations with the 65cm vacuum reflector of the sunspot BB1680, NOAA 5612 on 2 Aug 1989. The position was 30”E 378”S. For over six hours the seeing remained near the limiting resolution (approximately 0.25) of the 65 cm telescope. Observations in the continuum were obtained for the first two hours with a Schott VG-9 filter around 5200A, and for the last four with a bA ifiter centered on the K line. These observations were recorded from a CCD-video camera to Super-VHS videotape. At the same time, we obtained videomagnetogram and Dopplergrams in Cal 6103. The data give new information on a number of phenomena associated with normal stable spots. The video and digital images were reregistered so that highly accelerated movies could be studied. The Dopplergrams were calibrated by solar rotation. When a few hours are viewed in a few seconds, the eye easily detects systematic motions down to 100 nVs or less. The motions can be confirmed by cork movies” which correlation-track individual elements. Higher speeds are not visible this way and we have not searched for them systematically. We summarize the conclusions from the observations in several wavelengths. From the Evershed effect there was early evidence of outward penumbral flow, but the first measurements of proper motion were made by Muller (1973) /1/ who found a steady flow of about 0.5 km/sec into the umbra, a result confirmed by Zirin and Wang (1989) /2/. Muller commented on the contradiction of this effect with the Evershed flow. The outward flow of magnetic elements (MMF) was first detected by Sheeley (1969) /3/. An outward flow manifested in granulation was found by Title er al. (1986)/4/and also confirmed by Zirin and Wang. The highly-accelerated movies give the impression of massive, systematicflows. ALL the material in the inner half of the penumbra moves inward, no outward motion is found; ALL the material in the outer half, bright and dark, moves outward. The flow continues well beyond the penumbra. The division of the penumbra into an inflowing inner part and outfiowing outer part resolves the contradiction between the Evershed flow and Muller’s measurement. CONTINUUMAND K-LINE
(5)226
H. Zirin and H. Wang
Figure 2 shows the K-line image, made with a bA wide filter. This is a mixture of K-line and continuum, but shows bright points nicely.
The K-line and continuum movies show the clear distinction between an inward flow of 0.5 Km/sec in the inner half of the penumbra and an outward flow accelerating from 0.5 Km/sec at the middle to 1 Km/sec at the edge of the penumbra. The inflow appears dominated by the bright penumbral fibrils, the outflow by the dark. No fibrils extend completely through the penumbra. The continuum images show the flow extending out through the granulation; the K-line shows bright points flowing out at varying speed. While the motion in these observations is particularly well-marked, we have observed it in all other similar spots with stable penumbra. Thin and Wang (1989) 12/ showed two examples of penumbra unconnected to any umbra. At the left side of the frame (Figure 1) is an example of this phenomenon, which we will henceforth call orphan penumbra. It has of course been observed many limes before, but not, to our knowledge, specifically described. Orphan penumbra is sinipiy penumbra without an attached umbra, and its origin is as yet not determined. In this case the orphan penumbra reveals the same motions as the normal penumbra, the part furthest from the central spot moving outward and the closest moving inward. The orphan penumbra shows the same polarity as normal penumbra, even reversing sign at the far edges. In this case it is present for almost the entire disk passage and shows no relation to any vamshed spots. It behaves like a second “wave” of penumbra. Another locus of strong flow is the light bridge in the umbra. This shows continuous flow from one end to the other. MAGNETIC AND DOPPLER OBSERVATIONS At the same time as the direct video observations were being made, we took an excellent series of magnetograms and Dopplergrams with the videomagnetograph (VMG) on the 65 cm telescope. These are shown in Figures 3 and 4. The magnetograms show the best sequence of moving magnetic features (MMF) observed so far. These are seen to flow out from the penumbra about 1.0 to 1.5 Km/sec at the edge of the penumbra. They do not appear in the penumbra, but outside it. There has been considerable discussion of the nature of the MMF. It has been suggested by Harvey and Harvey (1973) /5/ that these are loops or embolisms in the outfiowing magnetic fields. If that were the case each MMF would be bipolar, the sign ofthe inner pole being the same or opposite to the spot, depending on whether the loop is directed downward or upward. The downward type are particularly evident at the right in Figure 2. While there are some bipolar MMF’s in our data, most of them are unipolar. The MMF points on the K-line images are much clearer and smaller than on the magnetograms because they represent a 1/30 sec exposure compared to an integration of 128 frames (4 sec). One of the prominent features of high-resolution magnetograms is the apparent field reversal on the limbward side of the penumbra. This arises from the longitudinal projection of horizontal field lines. In this case the spot is close enough to the disk center that there is no penumbral reversal, but the MMF’s show a clear effect at upper left. This is the hmbward direction, allowing for a substantial P-correction. Ifall penumbral field lines were horizontal, then the apparent polarity of the penumbra, as seen in the Stokes V parameter, would be positive in the half-ring of the penumbra pointed away from us and negative in the other. But the penumbral field lines are not horizontal, but intersect the surface in progressively flatter angles as we go outward. A simple geometrical calculation shows that the sign of the apparent penumbralfield reverses where the angle of thefield lines to the horizontal equals the angular distance from Sun center. In this case the field lines are tilted at least 22°from the horizontal throughout the penumbra. Further, as we go around the penumbra, the locus of the reversal moves outward to where successively flatter field lines produce zero or negative line-of-sight projections. In this way the sign reversal maps the tilt of the penumbral field lines, assuming the field lines to be radiaL Measurements of the Doppler field were also assembled in a reregistered film. These give us our best view so far of the Evershed flow. In the Doppler frame (Figure 4) we see a strong peak of the receding Evershed flow (dark) at the limbward edge of the spot. Similarly, the near boundary is white, reflecting the blue-shifted flow on the near side. Note, however, that the inner penumbra does not show the opposite pattern; in fact, it shows no wave-length shift at all. This may be because the light level is low, or because the apparent inflow in the inner penumbra is some kind ofmoving excitation, rather than a true motion. The Evershed flow, as can be seen on the illustration, is not a uniform flow, but a series of moving elements which match almost perfectly the regions of strongest reversed sign magnetic field in the outer penumbra on the far side. This means that they are closer to the horizontal than the 22°central distance. Penumbral structures that are not reversed in apparent sign do not show the strong Evershed flow, and this makes it less obvious on the near side. Even there the best fit is with reversed features, leading one to suspect that they are downflowing. The proper motion velocity of the Evershed elements is 0.5 1.0. kim’sec, roughly the same as the Doppler velocity. The Evershed elements may also be related to the larger MMF’s. There are a few anomalous motions in the penumbra, as in the upper left of Figure 4; these all correspond to field inclusions. The occurrence of “flags” discovered by Bumba (1960) /6/ can be explained by the combination of the stationary background with the moving Evershed elements. -
High-Resolution Sunspot Observations
(5)227
Some may wonder if the close comiection between the Evershed flows and the magnetic structure is due to our use of a Zeeman-sensitive line. It is generally held in the solar physics that one must use a line of g=0 for Doppler measurements. While this may be desirable for absolute confidence, there is no published evidence of any anomalies associated with the use of a Zeeman-sensitive line. Certainly our results in thousands of Dopplergrams have revealed no effect. Qose examination of the frames shows the match exists only on one side of the penumbra, and not, for example, in the orphan penumbra or the various other features. Further, we have many observations of the Evershed effect; its amplitude intensifies strongly toward the limb, while the magnetic signal decreases. There is a prominent neutral line at the right of the image; this is marked in the Doppler flow by a fairly sharp velocity gradient, with the negative polarity receding at 0.2 Km/sec and the positive approaching. Most neutral lines are found to have a convergence, and this is well-displayed in the movie, which shows continuous convergence in proper motion, substantial amounts offlux flowing into the neutral line. In this case almost all the flow is on the penumbral side, with strong flow from the naked penumbra into the neutral line, and apparent cancellation with the negative flux. Points of strong down flow appear here from time to time, lasting typically for about an hour. Whether ornot they are connected with the disappearance of flux here we cannot as yet say. CONCLUSIONS The dynamic events that occur around a stable sunspot are indeed surprising, and must play an important role in the physics of the spot. Clearly the strong motion is confined to regions where the field has some horizontal component. This component is not large; from the reversal of apparent longitudinal sign we find the tilt of field lines to the horizontal to be 22°or more out to the edge of the penumbra. So the motion may be upward along a Wilson effect dish, because it is unlikely that it crosses field lines. However the physical significance is beyond the scope of the paper. We found a strong radial flow in the penumbra of this round spot, with the inner penumbra moving inward and the outer penumbra, outward. While the outward flow was confirmed by both proper motion and Doppler shifts, the mwarrl flow did not show a Doppler shift and might be only an excitation effect. The second, or orphan penumbra, shows motions similar to the normal penumbra. This work was supported by the NSF through ATM 8816007 and NASA NAGW-1972.
REFERENCES 1. Muller, R. 1973, Solar Phys. 29 55. 2. Zirin H. and Wang, H. 1989. Solar Phys. 245, 119. 3. Sheeley, N. R. 1969. Solar Phys. 9, 347. 4. Title, A., Tarbell, T., Simon, G. and the SOUP Team, 1986. In Adv. Space Res. 6,253. 5. Harvey, J. W. and Harvey, K.L 1973. SolarPhys. 2861. 6. Bumba, V. 1960. Izv. Kr. Astr. Obs. 23,253.
(5)228
H Zirin and H. Wang
Fig. 1. A picture of BB16800 in the green, showing our normal image of umbra, penumbra and granulation.
High-Resolution Sunspot Observations
Fig. 2. The same in a bA window at the K line, showing outfiowing bright points.
(5)229
(5)23(J
1 I. Zirin and H. \Vang
a
Fig. 3. The same in longitudinal magnetic field. This frame and Fig. 4 are contracted horizontally by 4:3 by our imageprocessor. We have kept the ratio here to maintain the field. We have marked the matching features here and in Fig. 4.
High-Resolution Sunspot Observations
(~~)231
a
Fig. 4. Doppler image.