JINGXIU WANG,1,2,3 WEI LI,1,2 CARSTEN DENKER,2 CHIKYIN LEE,2 HAIMIN WANG,2 PHILIP R. .... Arrow ââ 3 ÅÅ marks a site of a few macrospicules.
THE ASTROPHYSICAL JOURNAL, 530 : 1071È1084, 2000 February 20 ( 2000. The American Astronomical Society. All rights reserved. Printed in U.S.A.
MINIFILAMENT ERUPTION ON THE QUIET SUN. I. OBSERVATIONS AT Ha CENTRAL LINE JINGXIU WANG,1,2,3 WEI LI,1,2 CARSTEN DENKER,2 CHIKYIN LEE,2 HAIMIN WANG,2 PHILIP R. GOODE,2 ALAN MCALLISTER,3 AND SARA F. MARTIN3 Received 1998 February 12 ; accepted 1999 October 1
ABSTRACT The eruption of miniature Ðlaments on the quiet Sun has been analyzed from time sequences of digital Ha Ðltergrams obtained at Big Bear Solar Observatory during 1997 September 18È24. The 2 days with the best image quality were selected for this initial study. During 13 hr of time-lapse observations on these 2 days, in an e†ective 640@@ ] 480@@ area of quiet Sun close to the disk center, 88 erupting miniature Ðlaments were identiÐed. On average, these small-scale erupting Ðlaments have a projected length of 19,000 km, an observed ejection speed of 13 km s~1, and a mean lifetime of 50 minutes from Ðrst appearance through eruption. The total mass and kinetic energy involved in a miniature Ðlament eruption is estimated to be 1013 g and 1025 ergs, respectively. They are distinguished from macrospicules by the same criteria that large-scale Ðlaments, before and during eruption, are distinguished from surges. Prior to eruption, one end, both ends, or the midsection of a miniature Ðlament is superposed over a polarity reversal boundary on line-of-sight magnetograms. We conclude that miniature Ðlaments are the small-scale analog to large-scale Ðlaments. Subject heading : Sun : Ðlaments 1.
INTRODUCTION
survey of quiet-Sun Ha Ðltergrams taken at Big Bear Solar Observatory (BBSO). In their rediscovery of miniÐlaments, they did not make the association with previous observations under the name ““ macrospicule ÏÏ and described them as ““ small-scale eruptive Ðlaments on the quiet Sun.ÏÏ They concluded that the eruptive miniÐlaments are the small-scale analog to large-scale eruptive Ðlaments. We present evidence here that eruptive miniÐlaments and macrospicules are two di†erent types of activity on the quiet Sun. Hermans & Martin further emphasized the correlation between eruptive miniÐlaments and canceling magnetic features (Livi et al. 1985 ; Martin et al. 1985). They called attention to the fact that miniÐlaments are associated with one or two small canceling magnetic features. The current paper is the Ðrst of a set of studies based on the latest Solar and Heliospheric Observatory (SOHO) campaign on miniÐlament eruption on the quiet Sun. It summarizes the observations at the Ha central line and serves as an introductory work to the whole series. The motivation of this work is to clearly deÐne this phenomenon and clarify its dynamic nature. In ° 2 we brieÑy describe the observations and data reduction. In ° 3 we summarize the basic parameters of eruptive miniÐlaments identiÐed in this study. Section 4 describes and illustrates morphological variations among eruptive miniÐlaments. Section 5 makes the distinction between eruptive miniÐlaments and macrospicules. The last section is the summary and advisory for further studies.
It has become a common consensus that the quiet Sun is never quiet. Numerous small-scale events, such as spicules and microÑares, are chromospheric indicators of an exceedingly dynamic layer of mixed-polarity magnetic Ðelds everywhere on the Sun. This sea of solar small-scale magnetic activity is sometimes not seen amid large areas of unipolar network Ðelds which dominate most current-day fulldisk magnetograms. Magnetograms of higher sensitivity (threshold of 10È20 G), moderate spatial resolution (1AÈ3A), and moderate temporal resolution (1È10 minutes) are the modest requirements to detect the continuously changing, mixed-polarity background. Above this sea of magnetic activity, Ha Ðltergrams of comparable spatial and temporal resolution have revealed many small-scale dynamic features which appear to be the counterparts of large-scale dynamic features such as Ñares and surges. Among the small-scale features, found in time series of Ha Ðltergrams, are miniature erupting Ðlaments which appear to be analogous to large-scale erupting Ðlaments. The properties of these miniature erupting Ðlaments have only been sparsely described in the literature and are the subject of this paper. A comprehensive description is provided here from Ha Ðltergrams of higher spatial resolution than used in previous papers. The term miniature Ðlament will be shortened to ““ miniÐlament.ÏÏ The earliest description of miniÐlament eruptions at the limb was given by Moore et al. (1977), while the earliest description of disk events was made by Labonte (1979). However, these authors classiÐed the miniÐlament eruption as one form of macrospicule. Macrospicules were Ðrst discovered from space observations made in the He II j304 line (Bohlin et al. 1975). Hermans & Martin (1986) made a more detailed study of the eruption of miniÐlaments, based on a
2.
OBSERVATIONS
Time lapse sequences of digital Ha Ðltergrams with high spatial and moderate temporal resolution were taken 1997 September 18È24 at BBSO. The observations of the quiet Sun were made on a rectangular area consisting of four overlapped Ðelds of view centered at 52¡.0 north and on the solar rotation axis. Each Ðeld of view covered an area of approximately 400@@ ] 270@@ and partly overlapped with the three adjacent Ðelds of view. The data were recorded by a Kodak 1536 ] 1024 CCD camera with a pixel size of 0A. 263. Figure 1 shows an example of the magnetogram and Ha Ðltergram, taken from one Ðeld of view, centered at 126¡.0
1 Beijing Astronomical Observatory, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, China. 2 Big Bear Solar Observatory, New Jersey Institute of Technology, Big Bear City, CA 92314. 3 Helio Research, 5212 Maryland Avenue, La Crescenta, CA 91214.
1071
1072
WANG ET AL.
Vol. 530
16:14
17:19
FIG. 1.ÈMagnetogram (upper panel) and Ha Ðltergram from the Ðeld of view, centered at W126¡.0 and S62¡.3. North is on the top and east is on the right (the same for the following Ðgures). Arrow ““ 2 ÏÏ indicates an activated miniÐlament whose appearance is superposed on the magnetogram in white. Arrow ““ 1 ÏÏ points to the site of an eruptive miniÐlament which appeared later at 17 : 56 UT and is also illustrated in Fig. 8. Arrow ““ 3 ÏÏ marks the site for a later macrospicule which is illustrated in Fig. 10.
west and 62¡.3 south on September 23. The magnetogram (upper part in the Ðgure) was acquired from integrating 4096 video frames. The sensitivity of the magnetograms is 3 G (Varsik 1995). To maintain a good temporal resolution of Ha Ðltergrams, only two magnetograms were taken at
approximately 16 : 00 and 23 : 00 UT, respectively, right before and after the Ha observations at BBSO. The lower part of the Ðgure is the Ha Ðltergram. At the lower right corner, there is a white bar indicating the scale of 20A. The same scale bar is presented in the other Ðgures
No. 2, 2000
MINIFILAMENT ERUPTION ON QUIET SUN
except Figure 9. Most of the Ha Ðltergrams reach 1A spatial resolution or even better. Arrow ““ 2 ÏÏ in the Ðltergram points to an activated miniÐlament which was the darkest feature in this frame. We superposed this Ðlament on the magnetogram (white color) to demonstrate its association with the magnetic environment. Arrow ““ 1 ÏÏ in the Ðgure points to a site of a miniÐlament that appeared after 17 : 56 UT, an example which is described in ° 4.3 and also shown in Figure 8. Arrow ““ 3 ÏÏ marks a site of a few macrospicules that appeared after 17 : 46 UT. We illustrate the macrospicules at this site in Figure 10 and describe them in ° 5. The four Ðelds of view were observed every 2 minutes at Ðxed heliocentric coordinates throughout the observing day without change to compensate for features entering and exiting the Ðeld of view as a result of solar rotation. Because a purpose in this study is to unambiguously deÐne this class of eruptive events, we only use the Ðltergrams of September 22È23 when the seeing was not worse than 1A during the time of the observations. Because of solar rotation, the area at the east and west edges of the four contiguous Ðelds of view was not observed continuously. The Ðnally registered frames were reduced to only the area that was observed continuously throughout the observing day. The e†ective window of the quiet Sun used for this study covered an area of approximately 640@@ ] 480@@. The frames for each Ðeld of view were carefully coaligned by maximizing the crosscorrelation between successive images. 3.
BASIC PARAMETERS OF ERUPTIVE MINIFILAMENTS
In 13 hr of time lapse observations covering 2 days and in an e†ective area of 640@@ ] 480@@, 88 erupting miniÐlaments were identiÐed. Our sample consists of all events, which showed clear darkening, expansion, and acceleration in lateral displacement when viewing in movie mode. The projected length, width, area, and lateral displacement were measured by procedures available in Interactive Data Language (IDL). The duration of activation and eruption phase and lifetime were visually estimated. By ““ activation ÏÏ we mean the darkening, broadening, and ascending of the miniÐlament before eruption. The length of the miniÐlament was measured along the Ðlament axis, and the area was measured by tracing the irregular periphery of a Ðlament. The mean width was determined, then, from dividing the area by the length. It is difficult to make an unambiguous estimation of ejection speed because the appearance of miniÐlaments in the core of the Ha line su†ers from projection e†ects. Additionally, the apparent speed can be inaccurate if part of the Ðlament disappears as a result of Doppler motion exceeding the passband of the Ðlter (half-width 1 Ó passband). Seen against the solar disk, 4 is the lateral displacement of dark the ejection speed material divided by the interval of time during which the
1073
rapid apparent motion takes place prior to eruption. When there is an ejection in two directions, the fastest ejection is chosen for the estimation. In Table 1 we listed the average parameters of the eruptive miniÐlaments in this study. We may estimate the total number of miniÐlament eruptions on the quiet Sun by assuming a uniform distribution of these events on the solar surface. Then, on the whole Sun there would be approximately 6200 miniÐlament eruptions each day. This is 4 times that estimated for macrospicules by Labonte (1979) and more than 10 times the conservative estimate by Hermans & Martin (1986) for eruptive miniÐlaments. Although it should be kept in mind that our estimate was made during the early rise of the current solar cycle, we attribute our large number primarily to images of higher spatial resolution and to digital data which is more readily amenable to elaborate data analyses. However, the temporal resolution of our data, ^2 minutes, is not higher than in previous studies. We successively observed four Ðelds of view in order to improve our sampling each day at the expense of higher temporal resolution. It is of interest to have a rough idea of the total mass and kinetic energy of miniÐlament eruptions. We assume that the miniÐlaments are looplike in geometry with the diameter equal to the projected width and that their density is not too far from that in dark mottles. Adopting the latest estimation of density for dark mottles (Tsiropoula & Schmieder 1997), 10~13 g cm~3, we found that the total mass in an eruptive miniÐlament is on the order of 1013 g. Then, roughly, the total energy required to power each miniÐlament eruption is on the order of 1025 ergs. 4.
PATTERNS OF MINIFILAMENT ERUPTION
MiniÐlaments on the quiet Sun show a rich and varied morphology when erupting. As the eruption geometry may provide clues about the magnetic nature of miniÐlaments, a rather detailed description of morphology variations is presented. The sample of eruptive miniÐlaments in this study revealed the following patterns. 4.1. Complete Eruption after Breaking Open Earlier studies (Moore et al. 1977 ; Hermans & Marin 1986) have shown that the most common morphology of the eruptive miniÐlament is an erupting arch which breaks open at its top followed by the disappearance of the remnant (see an example in Fig. 1 of Hermans & Martin 1986). Our observations conÐrm their discovery. We further found that circle-like miniÐlaments are also common on the quiet Sun. These archlike or circle-like Ðlaments often erupt completely after they appear to break open somewhere close to their tops. An example of this type is illustrated by the time sequence of Ha Ðltergrams in Figure 2. Two magnetograms in the
TABLE 1 PARAMETERS OF ERUPTIVE MINIFILAMENTS Term
At Formation
Before Eruption
Projected length (103 km) . . . . . . . . . . . . . . . . . Projected width (103 km) . . . . . . . . . . . . . . . . . . Projected area (107 km2) . . . . . . . . . . . . . . . . . . Projected ejection velocity (km s~1) . . . . . . Activation duration (minutes) . . . . . . . . . . . . . Eruption duration (minutes) . . . . . . . . . . . . . .
19.0 ^ 6.0 1.6 ^ 0.5 3.1 ^ 1.4
23.0 ^ 8.0 2.2 ^ 0.7 5.1 ^ 2.1
During Eruption
13 ^ 11 21 ^ 23 28 ^ 25
1074
WANG ET AL.
Vol. 530
16:10
18:45
20:01
20:53
16:55
18:57
20:07
21:23
17:21
19:07
20:15
22:00
17:37
19:25
20:25
22:24
18:03
19:49
20:27
22:36
18:09
19:53
20:37
22:40
18:25
19:55
20:47
22:46
18:39
19:59
20:51
23:28
FIG. 2.ÈTime sequence of Ha Ðltergrams showing a semicircular miniÐlament. The Ðrst and last panel are magnetograms of the line-of-sight component to demonstrate the gross magnetic conÐguration at the photosphere below and adjacent to this eruptive miniÐlament.
Ðrst and last frames, respectively, show the gross correlation between the magnetic conÐguration and miniÐlament eruption. In this example, a semicircular thin Ðlament, indicated by an arrow at 16 : 55 UT, outlines the boundary of several network elements of positive polarity (brighter patches in the center of magnetogram at 16 : 10 UT). At the east and northwest of the periphery of the positive Ñux, there is negative Ñux (darker patches) of lower Ñux density. Although 4096 video frame integrations are used, the magnetogram is still not sensitive enough to clarify if the positive Ñux is wholly embedded by negative Ðelds with still lower Ñux density. To compare the magnetic Ðeld conÐguration and Ðlament structure, an enlarged view of the Ðrst two panels of Figure 2 is shown in Figure 3. In addition, we plotted the
potential transverse Ðeld, extrapolated from the observed line-of-sight magnetogram, on the top of the latter with short arrows with length proportional to the Ðeld strength. We also superposed the gross Ðlament structure on the vector magnetogram by white contours. The Ðlament clearly outlines the boundary of positive Ñux patches and is located along the boundary between the opposite polarities at the east. Moreover, the miniÐlament ran mostly perpendicular to the extrapolated transverse Ðeld, showing clearly a nonpotential conÐguration. The activation phase started immediately after the appearance of this miniÐlament. It was characterized by expanding and darkening of the northern part (top) of the Ðlament. Bright features, resembling a two-ribbon or multiribbon Ñare as typically associated with the eruption of a
No. 2, 2000
MINIFILAMENT ERUPTION ON QUIET SUN
1075
FIG. 3.ÈAn enlarged view of the Ðrst two panels of Fig. 2. In the observed magnetogram, potential transverse Ðeld was superposed by arrows (white arrows : above the negative line-of-sight Ñux ; black arrows : above positive Ñux).
large-scale Ðlament, appeared during the activation phase (indicated by an arrow at 19 : 53 UT). The apparent breakage of the Ðlament occurred at 19 : 55 UT, accompanied by pronounced brightening. The western part of the Ðlament ejected toward a strong network element in the west before 20 : 01 UT ; the eastern part ejected toward another strong network element in the east from 20 : 47 to 20 : 53 UT. Bright Ñare points appeared during the miniÐlament eruption. In this case, another miniÐlament of the same morphology formed by 21 : 23 UT and ejected approximately at 22 : 00 UT. A similar Ðlament appeared later and erupted at 22 : 46 UT. By comparing the magnetograms at 16 : 10 and 23 : 28 UT, we found indication of Ñux cancellation along the polarity boundary occupied by the miniÐlaments. The miniÐlament was complicated in shape. It resembled magnetic loops which are highly nonpotential. Inside the circle-like Ðlament there was a dark core which was repeatedly
activatedÈdarkening, brightening, and disappearing. We will go back to discuss this in ° 4.4. Often the breakage of a miniÐlament was preceded by a kink at the top of the Ðlament, sometimes forming a cusplike structure. A case was marginally seen at 19 : 53 UT in the example shown in Figure 2. In Figure 4 we illustrate an example of the kink development before the breakage and eruption of a miniÐlament. At 16 : 44 UT a miniÐlament was seen, shown as a semicircle and indicated by an arrow. It increased greatly in both area and absorption immediately after its Ðrst appearance. The loop seemed to proÐle a negative Ñux element in the lower middle of the Ðeld of view. A cusplike structure developed since 17 : 32 UT. An arrow at 17 : 48 UT points at this. The Ðlament broke open after 17 : 42 UT and ejected toward a strong network element in the northwest. Limited by the current temporal and spatial resolution of the data, we cannot be sure if the top of the Ðlament was twisted before eruption. However, it is quite
1076
WANG ET AL.
Vol. 530
16:26
17:08
17:22
17:34
17:44
16:44
17:14
17:24
17:36
17:48
16:54
17:16
17:26
17:38
18:00
17:00
17:18
17:28
17:40
18:02
17:04
17:20
17:32
17:42
23:39
FIG. 4.ÈExample of miniÐlament with kink or cusplike structure which developed before its eruption
evident that the kink development is part of the opening process of the miniÐlament. 4.2. Ejection from One End One type of miniÐlament eruption has been illustrated by previous investigations : a Ðlament rises at one end and ejects approximately in the plane of the Ðlament axis. Two examples are shown in Figure 3 in Labonte (1979) and Figure 2 in Hermans & Martin (1986). Hermans & Martin (1986) noted that the Ðlament had one end terminating above a site of Ñux cancellation in the corresponding magnetograms. In Figure 5 we show an example of this type of eruption. An arrow at 16 : 41 UT indicates a miniÐlament. From overlaying an Ha image with the magnetogram at 16 : 17 UT, we identiÐed that one end of the Ðlament appeared to terminate at the contact line between two magnetic Ñux patches of
opposite polarity, rather than at either the positive or negative polarity. This is common for large-scale Ðlaments in both quiet and active regions. The same could be generally true for Ðlament barbs or legs, although current evidence favors rooting in one polarity. However, the error in overlaying the Ha images with the magnetograms might make this identiÐcation uncertain. The opposite polarities were identiÐed as a canceling magnetic feature, located at a conjunction of several network cells. Note the mutual Ñux disappearance seen from magnetograms between 16 : 17 and 23 : 31 UT. At 16 : 41 UT the Ðlament was already in the preeruptive ascending phase. Prominent brightening was seen at the end where the ejection started ; it appears to be a scaleddown version of a two-ribbon Ñare typically related to large-scale Ðlament eruption. Arrows at 17 : 19 UT point to small chromospheric Ñare patches (Ñare loop footpoints) on
No. 2, 2000
MINIFILAMENT ERUPTION ON QUIET SUN
16:17
18:05
18:55
19:23
16:41
18:21
18:57
19:25
16:47
18:37
19:01
19:27
16:55
18:39
19:05
19:29
17:05
18:41
19:07
19:31
17:19
18:43
19:09
19:33
17:37
18:49
19:11
19:35
17:43
18:53
19:13
23:31
1077
FIG. 5.ÈExample of miniÐlament which showed continued activation. The ejection began with the rising of one end of the Ðlament. The rising end was located above the boundary between opposite polarities of a canceling magnetic feature.
both sides of magnetic polarity reversal boundary. This Ðlament was continuously changing. There was persistent outward mass motion or ejection from the east end, where opposite polarity magnetic Ñux was compressing and canceling in the photosphere beneath the Ðlament. One period of outward ejection was from 18 : 05 to 18 : 55 UT. The Ðlament recovered, or a new Ðlament segment formed at the east end and reactivated after 18 : 57 UT. It erupted again at 19 : 23 UT with greatly enhanced brightening at the ejecting end. Examples of this type of miniÐlament eruption usually are relatively large events with pronounced brightening at the chromosphere near the ejecting end. Two of all three illustrations of miniÐlament eruption on the quiet Sun in
the solar literature were of this variation. Although no concurrent magnetograms were available to show the detailed evolution of the magnetic conÐguration in this example, the obvious Ñux loss is seen by comparing the magnetograms at 16 : 17 and 23 : 31 UT. It appears that the miniÐlament disappeared when the canceling magnetic feature identiÐed at 16 : 17 UT no longer existed. We further remark that the ejection seemed to be along the boundary between the magnetic Ðelds of opposite polarity. A few events exhibit evidence of whiplike motion during the eruptive phase. In these events, the Ðlament usually appeared to detach from the chromosphere at one end while the other end, the rooting end, appeared to remain Ðxed ; then, the whole Ðlament started to swivel around the
1078
WANG ET AL.
Vol. 530
16:10
22:35
22:45
22:55
22:29
22:39
22:47
22:57
22:31
22:41
22:49
22:59
22:33
22:43
22:53
23:11
FIG. 6.ÈEruptive miniÐlament which showed obvious whiplike motion during eruption
rooting end, to have an opposite curvature of that before disappearance. This motion is the same as the whiplike motion that is sometimes observed during the eruption of larger Ðlaments. We interpret the above pattern of mass ejection as indicating that magnetic reconnection has occurred in the low corona or in the chromosphere near the rising end of the miniÐlament. The evidence of reconnection is the ejection of the miniÐlament mass along a new apparent magnetic path with reverse curvature from that of the original miniÐlament. Figure 6 shows one of these examples. At 22 : 29 UT an arrow points to a very thin Ðlament. Because magnetograms were only recorded at BBSO at the beginning and end of each observing day, we could not know precisely the magnetic Ñux distribution in the Ðlament environment at the time of the event. However, the magnetogram at 23 : 11 UT shows that this miniÐlament outlined the magnetic polarity reversal boundary between weak negative Ñux close to the center of the Ðeld of view and its surrounding weaker positive Ñux. The Ðlament appeared to break open at 22 : 39 UT at the site, indicated by arrows. This site was characterized by signiÐcant brightening of the chromo-
sphere on both sides of the Ðlament ; this brightening resembles a two-ribbon Ñare related to a large-scale Ðlament eruption. After the start of the tiny Ñare, the miniÐlament started to rotate counterclockwise while expanding and ejecting. We have superposed the original appearance of this tiny Ðlament on frames of 22 : 43, 22 : 47, and 22 : 59 UT, respectively, to show the Ðlament rotation. Its orientation at 22 : 59 UT was co-aligned with the polarity reversal boundary between weak Ñux of opposite polarities. Because Ha observations were stopped at 23 : 00 UT, we could not know the Ðnal observable stage of the ejecting Ðlament. Figure 7 shows a miniÐlament which appeared to rotate about one end by more than 100¡ in the course of eruption. An arrow indicates the miniÐlament at 21 : 01 UT. In successive frames, the miniÐlament is seen expanding and darkening. The upper part of the Ðlament started to rise and begin the whiplike motion after 21 : 07 UT, while the lower, rooting end was Ðxed. To learn the angular extent of the whiplike motion, we superposed the appearance of the Ðlament prior to the motion with several later frames. In Figure 7 the miniÐlament positions are shown as white features. During the period of 30 minutes from 21 : 01 to 21 : 31 UT, the rooting end remained close to the same bright
No. 2, 2000
MINIFILAMENT ERUPTION ON QUIET SUN
21:01
21:11
21:19
21:27
21:03
21:13
21:21
21:29
21:05
21:15
21:23
21:31
21:07
21:17
21:25
21:33
1079
FIG. 7.ÈExample of miniÐlament in which one end rotated around its other, ““ rooting ÏÏ end more than 100¡ during eruption
plage, while the upper ““ tail ÏÏ of the Ðlament rotated almost 180¡ with respect to the rooting end. No simultaneous magnetograms are available to Ðnd associated changes in the magnetic Ðeld. The di†erent appearance of the miniÐlament eruption in Figures 5, 6, and 7 could be attributed to whether or not the ejection planes are parallel with the line of sight. The whiplike motion might only be seen when the eruption plane is largely transverse to the line-of-sight direction. 4.3. Blown o† and Bodily Ejected Outward Several miniÐlaments in our sample showed the following morphological evolution : immediately after formation, the Ðlament started to grow in both area and absorption ; it seemed to be blown o† and wholly eject upward in the direction perpendicular to the axis of the Ðlament. An example of this variation is illustrated in Figure 8 (see ° 2 and arrow ““ 2 ÏÏ in Fig. 1). A tiny Ðlament appeared at 17 : 56 UT in a brighter plage area (see the arrow in the Ðgure), dividing the magnetic Ðelds of weak positive and negative Ñux. It started a quick growth after 18 : 12 UT. The whole Ðlament was gradually blown o† and bodily ejected
outward in the direction perpendicular to the Ðlament axis. It is very hard to separate the activation phase and ejection phase for this form of miniÐlament eruption. The miniÐlament darkened, expanded, and erupted bodily simultaneously. As the result, the concave inward curvature became concave outward curvature before the disappearance of the miniÐlament. Bright points appeared in the course of Ðlament eruption on both sides of the erupting Ðlament. In the panel of 18 : 49 UT, we superposed the tiny Ðlament at 17 : 56 UT in light gray to show how much the Ðlament had grown during the activation and eruption and to show the site and direction of the Ðlament eruption. From the magnetogram at 23 : 16 UT, we found big changes in the local magnetic conÐguration. Initially, there were closely contacted weak magnetic Ðelds of opposite polarity which might be interpreted as a site of magnetic Ñux cancellation. By 23 : 16 UT, the cancellation site no longer existed. If canceling magnetic Ðelds are necessary conditions for the very existence of the miniÐlaments, then the disappearance of the magnetic Ðelds is a possible reason for the eruption of the Ðlament. At the least, a rearrangement of the local magnetic Ðeld is implied as a cause for the
1080
WANG ET AL.
Vol. 530
16:14
18:16
18:30
18:46
17:56
18:18
18:32
18:49
18:00
18:22
18:36
18:53
18:04
18:24
18:38
18:55
18:08
18:26
18:40
18:57
18:12
18:28
18:42
23:16
FIG. 8.ÈExample of miniÐlament which was blown o† bodily during eruption
eruptive miniÐlament. This seems to be uncommon in largescale Ðlaments because the cancellation always takes place between many pairs of small-Ñux fragments of opposite polarity ; hence, the removal of one canceling feature is not sufficient to completely alter Ðlament environment (see the example of Wang, Shi, & Martin 1996). 4.4. A Discussion on Dark Core Filament In ° 4.1 we mentioned a tiny structure, a dark core, separate but within the semicircular Ðlament shown in Figures 2 and 3. In Figure 9, we selected frames to particularly show this dark core. It presents all the properties of a Ðlament except for the tiny scale and some peculiar appearance. Sometimes, the semicircular Ðlament and the dark core appeared as a nearly closed circle together with a dark nut within but separate from the outline of the miniÐlament, like that at 17 : 21 UT ; sometimes is was a complete circle, like that at 18 : 25 and 18 : 27 UT ; if the circle became Ðlled in, the Ðne structure of the dark core could no longer be seen, such as at 18 : 09, 19 : 27, and 20 : 01 UT. At other times, the dark core brightened, such as at 17 : 39, 18 : 39, 18 : 57,
and 21 : 23 UT. All the morphological changes are signatures of its activity. Current observations are still not sufficient to reveal whether or not the dark core structure is a separate tiny Ðlament or an intrinsic part of the main miniÐlament. In any sense, the dark core is a Ðlament in nature. Moreover, it is found that the dark core, at least, repeated the history of darkening, expanding, brightening, and disappearing for two or three times in the duration of the main miniÐlament. It is of interest to understand the magnetic nature of the dark core. Fortunately, aimed by the extrapolated transverse magnetic Ðeld shown in Figure 3, we identify that the dark core appeared above an area embedded in positive Ñux, but at almost exactly the saddle point, or singular point, of the transverse Ðeld. More events need to be studied to know if this is a common magnetic Ðeld pattern associated with the dark core Ðlament. The current set of observations has shown that the dark core Ðlament is quite common on the quiet Sun. In Figure 6 there is another example of dark core Ðlament which was within the miniÐlament indicated at 22 : 29 UT. The activity
No. 2, 2000
MINIFILAMENT ERUPTION ON QUIET SUN
16:10
18:27
19:27
20:53
17:21
18:39
19:53
21:23
17:39
18:45
19:55
22:00
17:45
18:57
19:59
22:36
18:09
19:01
20:01
22:46
18:25
19:25
20:27
23:28
1081
FIG. 9.ÈEnlarged view of a dark core in the Ðlament shown in Fig. 2
of the dark core Ðlament can be clearly seen from 22 : 29 to 22 : 39 UT and also from 22 : 47 to 22 : 49 UT. This is the Ðrst time this type of Ðlament activity on the quiet Sun has been revealed. No similar structure has been reported for largescale Ðlaments. Further observations with higher spatial and temporal resolutions are required to clarify the nature of the dark core structure. Because we do not know if the dark core is part of the miniÐlament or totally separate from the miniÐlament, we have not counted the dark core among the sample of miniÐlaments. 5.
DISTINCTION BETWEEN ERUPTIVE MINIFILAMENT AND MACROSPICULE
In an earlier description of Ha macrospicules, Labonte (1979) classiÐed the miniÐlament eruption as one form of an Ha macrospicule and di†erentiated the forms of ““ Ðlamentlike eruption ÏÏ and ““ surgelike macrospicule.ÏÏ According to him, the former ““ darkens, erupts in loops, and disappears ÏÏ and the latter is distinguished from a normal spicule by its ““ larger length, width, and lifetimes, composite structures, and . . . signiÐcant brightening at the base.ÏÏ From the survey of Hermans & Martin (1986) and our current observations,
we conÐrm that miniÐlament eruptions and macrospicules are signiÐcantly di†erent not only in their morphological appearance, but also in their associations with magnetic Ðelds and ejection patterns. Here we further distinguish these two types of eruptive phenomena. In Figure 10, we present typical examples of macrospicules to show how they di†er from miniÐlaments. At 17 : 46 UT, a macrospicule appeared as two parallel spikes. They darkened concurrently with signiÐcant brightening at their base and disappeared before 18 : 08 UT. The second macrospicule appeared before 18 : 08 UT, was about the same length as the Ðrst one, and disappeared after 18 : 22 UT. Projected against the disk, the second macrospicule was in between the two spikes of the Ðrst one. Inspecting the magnetic conÐgurations at 16 : 14 and 23 : 06 UT, we found that macrospicules were not lying above the magnetic neutral lines in the photospheric magnetograms but emanated from a strong network element of negative polarity, although at 23 : 16 UT a tiny positive Ñux patch was seen at the previous macrospicule base. In fact, it could not be excluded that macrospicules (or spicules) represented the interaction between strong network and very weak intranetwork Ðelds (Wang et al. 1995, 1998 ; Zhang et al. 1998). Although at
16:14
18:04
18:18
17:37
18:08
18:22
17:46
18:10
18:26
17:56
18:12
18:30
18:00
18:16
23:16
FIG. 10.ÈTypical examples of macrospicules ejected from a strong network element
TABLE 2 DISTINCTION BETWEEN ERUPTIVE MINIFILAMENTS AND MACROSPICULES Parameter
Eruptive MiniÐlament
Macrospicule
1 .......... 2 .......... 3 ..........
Has a preeruption phase Curved or closed in shape Entire length lies along a polarity reversal boundary between any small-scale Ðelds of opposite polarity Implied horizontal magnetic Ðeld Related magnetic Ðeld could be very weak Exhibits lateral displacement in addition to Ñow along its length Longer lifetime : D50 minutes
No preeruption phase Spike or surgelike in shape with slight curvature One end associated with a polarity reversal boundary bordering a patch of network magnetic Ðeld Implied nearly vertical magnetic Ðeld Always associated with strong Ñux concentration Has mass motion only along its length
4 .......... 5 .......... 6 .......... 7 ..........
Shorter lifetime : \20 minutes
MINIFILAMENT ERUPTION ON QUIET SUN their base there may appear opposite polarities, as a whole the macrospicules at any time are not seen superposed along the polarity reversal boundary as for the eruptive miniÐlaments. This means that the macrospicules have a large vertical component when Ðrst seen, whereas erupting miniÐlaments are mostly horizontal relative to the chromosphere. Moreover, macrospicules do not exist prior to their ejection. Their short lifetime is 20 minutes or less. From what was meant by its name, the macrospicule is a jetlike, nearly linear feature, and the mass ejection is purely along its length. No lateral displacements are found in macrospicules. Hence, macrospicules are thought to be mass ejection along existing magnetic Ðeld lines with little or no sideways displacement. In contrast, the lateral displacement of erupting miniÐlament indicates that their magnetic structure is changing conÐguration throughout their eruption. The identiÐcation of macrospicules from disk observations tends to be problematic. The main reason for this comes from the difficulty in distinguishing a spicule from a macrospicule. No quantitative criterion is available to differentiate a macrospicule from numerous spicules when viewing against the disk. Adopting a criterion that they are giant spicules, 15,000 km in length or longer, from current time sequences of Ha Ðltergrams, we identiÐed about 20 deÐnitive macrospicules. In Table 2 we summarize the main distinctions between miniÐlament eruptions and macrospicules. It is necessary to state that the distinction between eruptive miniÐlament and macrospicule lies in all the aspects listed in the table, but not necessarily in any single example. 6.
SUMMARY AND FURTHER WORK
As the Ðrst in a planned series of papers, we have analyzed the time sequences of digital Ha Ðltergrams obtained at BBSO during a SOHO campaign on the eruption of miniÐlaments. Some basic parameters and morphological variations are deduced and described from a sample of 88 events. The following properties are quite common in miniÐlament eruptions and need to be emphasized. 1. Similar to large-scale Ðlaments in both quiet and active regions, the eruptive miniÐlaments reside above the magnetic polarity reversal boundaries between adjacent opposite polarity Ðelds. They are spatially correlated to canceling magnetic features at the same boundaries. However, during some erupting miniÐlaments, the magnetic Ñux cancellation results in the complete disappearance of one or both polarities. Accordingly, the condition for the very existence of a miniÐlament is removed because there is no longer a polarity inversion at the position of the Ðlament in the preeruptive state, a requisite for Ðlament formation and maintenance. In this circumstance, another miniÐlament does not form at the site of the erupted miniÐlament. The complete removal of a polarity reversal through magnetic Ñux cancellation is not common for large-scale Ðlaments because there is usually a succession of canceling features related to the sustenance of large Ðlaments. 2. Like large-scale Ðlaments, many but not all miniÐlaments increase in area (see Table 1 and an example in Fig. 8) and in absorption during the early eruptive phase. With almost all of the miniÐlament eruptions, there appear tiny Ñares in the activation phase or the eruptive phase or both. But the most pronounced brightening takes place during the activation phase. The spatial pattern of the
1083
brightening is very similar to two-ribbon or multiribbon Ñaring associated with large-scale erupting Ðlaments. However, unlike their large-scale analog, eruptive miniÐlaments rarely appear stable ; the activation phase begins immediately after the Ðlament appearance. They are continuously changing in form and brightness from their initial appearance. 3. With respect to the line-of-sight Ðeld distribution, miniÐlaments are di†erent from the phenomenon of the Ðeld transition arches which directly join opposite polarity Ðelds within newly developing active regions (Zirin 1988). As described in ° 4, the morphological evolution implies that miniÐlaments are most likely magnetic loops. One might conclude that those loops joining the opposite polarity Ðelds at the two sides of the miniÐlament must have been highly sheared or twisted to bring the appearance of running along the magnetic neutral lines in the chromosphere (Rust & Kumar 1994 ; Low 1996). We favor the alternative that they have a magnetic Ðeld that is or becomes separate from the Ðeld of the polarities on two sides of the Ðlament. Magnetic reconnection is implied by the breakage, rotation, and concurrent Ñaring associated with the eruptive miniÐlaments. 4. Most miniÐlaments are ejected toward nearby strong network elements, which means that not all the mass in the miniÐlament is ejected into corona and perhaps most of the mass is transported to other magnetic structures. This has been clearly demonstrated by examples shown in Figures 2 and 4. Among solar plasma features, usually the mass can move only along or together with the magnetic lines of force. Accordingly, the mass transport can be possible only parallel to existing magnetic Ðelds or along the reconnected magnetic Ðelds where magnetic reconnection altered the original magnetic connectivity and conÐguration. The mass transport and the rapid change in conÐguration indicate that magnetic reconnection is probably taking place when they erupt. 5. The observed properties of miniÐlament eruptions show many di†erences from macrospicules, discovered in 1970s. Therefore, we suggest that miniÐlament eruptions and macrospicules are two di†erent types of physical phenomena on the quiet Sun. The biggest remaining question about the eruptive miniÐlaments is their physical nature. MiniÐlaments are aligned with polarity reversal boundary, and as they erupt, they ascend and appear as loops with a vertical component until their mass is completely expelled to a new location or drains down the legs of the erupting miniÐlament. The detailed study of their magnetic Ðeld distribution and evolution could provide the key to a thorough understanding. The magnetic nature of eruptive miniÐlaments should be analyzed in future studies. This paper can serve as an introduction to a more comprehensive study of this explosive phenomenon. Work can be done in the following directions : (1) association of miniÐlament eruption with the local magnetic Ðeld conÐguration and evolution at both photospheric and chromospheric levels : this can be done by analyzing magnetograms obtained at Kitt Peak and SOHO ; (2) diagnosis of dynamic and thermodynamic processes in miniÐlament eruption from SOHO spectrogram observations for some of the events ; (3) UV, EUV, and X-ray appearance of eruptive miniÐlaments ; (4) MHD processes which result in the curious appearance and eruption of miniÐlaments on the quiet Sun ; and (5) the possible role of
1084
WANG ET AL.
miniÐlament eruption in the mass and energy supply to the corona. Data are available for additional multiwavelength analyses following this introductory work. Observation and primary data reduction for this work was made during the visit of J. Wang at Helio Research and BBSO. He thanks Bill Marquette, Randy Fear, and Je† Nenow for very good observations ; he also thanks Jong-
chul Chae for valuable suggestions to improve the paper. His work at National Astronomical Observatories is supported by NSFC under grant 19791090. J. Wang and W. LiÏs visit to BBSO is supported by NSF under grant INT-9603534. BBSO observations were supported by NASA awards NAG5-3536 and NAG5-4919 and by NSF award ATM-9796196. The project is supported by a SOHO guest investigation program by A. H. McAllister and S. F. Martin under NASA/GSFC grant NAG5-4572.
REFERENCES Bohlin, J. D., Vogel, S. N., Purcell, J. D., Sheeley, N. R., Jr., Tousey, R., & Tsiropoula, G., & Schmieder, B. 1997, A&A, 324, 1183 Van Heosier, M. E. 1975, ApJ, 197, L133 Varsik, J. R. 1995, Sol. Phys., 161, 207 Hermans, L. M., & Martin, S. F. 1986, in Coronal and Prominence Wang, H., Johannesson, A., Stage, M., Lee, C., & Zirin, H. 1998, Sol. Phys., Plasmas, ed. A. I. Poland (NASA CP-2442), 369 178, 493 Labonte, B. J. 1979, Sol. Phys., 61, 283 Wang, J., Shi, Z., & Martin, S. F. 1996, A&A, 316, 201 Livi, S. H. B., Wang, J., & Martin, S. F. 1985, Australian J. Phys., 38, 855 Wang, J., Wang, H., Tang, F., Lee, J., & Zirin, H. 1995, Sol. Phys., 160, 277 Low, B. C. 1996, Sol. Phys., 167, 217 Zhang, J., Lin, G., Wang, J., Wang, H., & Zirin, H. 1998, A&A, 338, 322 Martin, S. F., Livi, S. H. B., & Wang, J. 1985, Australian J. Phys., 38, 929 Zirin, H. 1988, Astrophysics of the Sun (Cambridge : Cambridge Univ. Moore, R. L., Tang, F., Bohlin, J. D., & Golub, L. 1977, ApJ, 218, 286 Press) Rust, D., & Kumar, A. 1994, Sol. Phys., 155, 69
