Small Magnetic Bipoles Emerging in a Filament Channel

THE ASTROPHYSICAL JOURNAL, 548 :497È507, 2001 February 10( 2001. The American Astronomical Society. All rights reserved. Printed in U.S.A.

SMALL MAGNETIC BIPOLES EMERGING IN A FILAMENT CHANNEL

JONGCHUL CHAE,1,2 SARA F. MARTIN,3 H. S. YUN,2 JUNGHOON KIM,2 SANGWOO LEE,2 PHILIP R. GOODE,1TOM SPIROCK,1 AND HAIMIN WANG1

Received 2000 April 13 ; accepted 2000 September 7

ABSTRACTObservations have shown that quiescent prominences or Ðlaments have a hemispheric magnetic

pattern of chirality. Motivated by the question of whether the Ðlament chirality is of subsurface origin ornot, we have studied small magnetic bipoles that emerged in a quiescent Ðlament channel at latitudeN45¡. During our 5 day observing run, performed in 1999 October, a huge Ðlament erupted andreformed shortly in the same Ðlament channel. Using high-cadence, long-integration line-of-sight magne-tograms taken at Big Bear Solar Observatory, we identiÐed a total of 102 bipoles that showed anaverage total Ñux of 2.8 ] 1019 Mx, an average separation of 7400 km at the time of full development,and an emergence rate of 430 hr~1 per the entire solar surface area. These properties indicate that mostof the bipoles are ephemeral regions. The most important Ðnding in the present study is that the mag-netic axes of the bipoles emerging in the Ðlament channel are systematically oriented ; a negative(trailing) pole is observed to be located preferentially to the south-east of its companion positive (leading)pole. This preferred orientation does not match either the Hale law of active region orientation or atheory that attributes the axial Ðeld of a Ðlament to emerging bipoles. We propose two possible sub-surface Ðeld conÐgurations of bipoles consistent with the observed preferential orientation and discussphysical implications of our results for understanding Ðlament magnetic Ðelds.Subject headings : Sun: Ðlaments È Sun: magnetic Ðelds È Sun: photosphere È Sun: prominences

1. INTRODUCTION

The orientations and patterns of magnetic Ðelds on theSun, whether large scale or small scale, hold implicationsabout the role of helicity in solar magnetic structuresÈwhich are in turn relevant to the nature of solar eruptionsand the global balance of helicity on the Sun. The sample ofsmall bipoles discussed in this paper were observed in anarea in, and to each side of, a Ðlament channel containing aquiescent Ðlament that erupted. Because such bipoles arenow recognized as having potential relevance to the helicityin Ðlaments and their structure and formation, we beginwith a brief review of studies on the helicity of Ðlaments andpossible roles of small bipoles in their environment.

The evidence for helicity in Ðlaments is their magneticpatterns of ““ handedness,ÏÏ or chirality. A Ðlament for whichthe magnetic Ðeld along its axis is rightward to an observerat the positive-polarity side is called dextral ; one with itsaxial Ðeld leftward from the same perspective is called sinis-tral. Filament environments, like channels and coronalarcades, also display magnetic patterns that have a one-to-one correspondence with Ðlament chirality (for review seeZirker et al. 1997 ; Martin 1998a, 1998b). An interestingÐnding about Ðlament chirality is that quiescent Ðlamentsin the northern hemisphere are predominantly dextral andthose in the southern hemisphere are predominantly sinis-tral. This hemispheric pattern of chirality for quiescent Ðla-ments has been supported by two kinds of independentobservations : Ha morphological observations of Ðlamentson the disk (Martin, Marquette, & Bilimoria 1992 ; Martin,Bilimoria, & Tracadas 1994) and Hanle measurements of

1 Big Bear Solar Observatory, New Jersey Institute of Technology,40386 North Shore Lane, Big Bear City, CA 92314.

2 Astronomy Program, School of Earth and Environmental Sciences,Seoul National University, Seoul, Korea.

3 Helio Research, 5212 Maryland Avenue, La Crescenta, CA 91214.

magnetic Ðelds in prominences at the limb (Leroy,Bommier, & Sahal-Brechot 1984 ; Bommier & Leroy 1998).

There are basically two ways of explaining the helicity ofmagnetic Ðelds above the photosphere. One way is for mag-netic Ñuxes to carry helicity with them when they emergethrough the photosphere (van Ballegooijen & Martens1990 ; Rust & Kumar 1994 ; Priest, van Ballegooijen, &MacKay 1996 ; MacKay et al. 1998 ; Seehafer 1998). Theother way is for the shearing photospheric Ñow, such as thesolar di†erential rotation, to act on magnetic Ðelds that areinitially untwisted and thereby produce magnetic helicity(Kuperus 1996 ; Zirker et al. 1997 ; van Ballegooijen, Cart-lege, & Priest 1998 ; van Ballegooijen 1999). A possible wayto see whether the hemispheric dependence of the chiralityof quiescent Ðlaments is of subsurface origin or not is tostudy emerging magnetic Ñux, which is expected to keepsubsurface information for a while after emergence.

Van Ballegooijen & Martens (1990) were the Ðrst topredict a signiÐcant role of small bipoles in the formation ofÐlaments. A more comprehensive variation of this modelwas developed by Priest et al. (1996). The model proposesthat the axial Ðeld may be produced by di†erential rotationacting on a transverse Ðeld below the photosphere and pre-dicts that magnetic axes of bipoles emerging in a Ðlamentchannel should be systematically oriented in the direction ofÐlament axial Ðelds. The prediction could be easily checkedobservationally and has motivated the present study.

We will examine the statistics of small bipoles emergingin a Ðlament channel based on high-cadence, long-integration (deep) line-of-sight magnetograms taken with adigital camera (Wang et al. 1998) at Big Bear Solar Obser-vatory (BBSO). SpeciÐcally we will investigate (1) whetherbipoles are potentially important in Ðlament magneticÐelds, (2) whether they have a preferred orientation, and (3)whether the preferred orientation, if any, matches theoreti-cal predictions.

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498 CHAE ET AL. Vol. 548

2. OBSERVATION AND DATA REDUCTION

BBSO and Helio Research jointly observed a quiescentÐlament channel in the northern hemisphere (N45¡) for 5days in 1999 October. BBSO used the 26 inch reÑector withthe Zeiss Ha Ðlter and the 12 bit OSL digital camera andtook a sequence of high-resolution Ha images at threewavelengths, [0.25, 0, and ]0.25 Helio ResearchA� .repeated faster spectral scans and took high-resolution Haimages at seven wavelengths with a 0.30 step from [1.0A�to 1.0 A� .

The magnetograph observations were performed usingthe BBSO 10 inch refractor with a new digital camera. TheÐeld of view is 340@@] 340@@. A total of 4096 frames havebeen integrated to obtain each magnetogram, which sets thecadence of the observations to be about 2.5 minutes and thenoise level to be 6.3 G at each detector pixel of size 0A.67

Averaging over the neighborhood of 3] 3 pixels,] 0A.67.which is comparable to the seeing-limited resolution, yields4.0 G for a more meaningful estimate of the noise level.

The conversion of observed circular polarization into Ñuxdensity has been done under the weak-Ðeld approximationwith a reference to a simultaneously taken National SolarObservatory at Kitt Peak (NSO/KP) full-disk magneto-gram. All of the NSO/KP magnetograms taken since 1992April 22 are calibrated against the 8688 Fe I 512-channelA�magnetograph observations. The reason for this is that the512-channel magnetograms were well calibrated in units ofgauss. We compared the Ñuxes of many magnetic concen-trations from both the BBSO digital magnetograph andNSO/KP full-disk magnetograph. The linear regressionproduced 40,000 as the calibration coefficient for BBSOmagnetograms, which means that an observed 1% circularpolarization corresponds to the line-of-sight Ñux density of400 G. The probable error in the coefficient, which has beenestimated from the comparison of results using di†erent setsof data, is about 10%.

Each sequence of magnetograms has been aligned withthe cross-correlation method. The accuracy of this methodis mainly limited by the spatial resolution of data, which istypically 2A.

3. BIPOLE IDENTIFICATION AND PARAMETERS

We are interested in small bipoles formed by Ñux emer-gence. These bipoles are physically distinct from bipolesformed by collision of Ñuxes of opposite polarity. Emergingbipoles are identiÐed from the time evolution of the spatialdistribution of magnetic Ñux. Two neighboring Ñux concen-trations of opposite polarity are identiÐed as an emergingbipole if they appear at the same time and display the char-acteristic that the distance between the two Ñux concentra-tions, as well as the Ñux, increases with time (Martin 1988,1989). In principle, the amount of magnetic Ñux of the posi-tive concentration should balance that of the negative one.In reality, however, emerging Ñux is often unbalanced. Thismay be possibly due to the limitation of magnetographsensitivity or to Ñux cancellation (Martin 1989). Anotherpossibility is that the observed Ñux imbalance may reÑectthe di†erence of magnetic Ðeld tilt with respect to the line ofsight between the two poles, which may be important inhigh-latitude observations. Therefore, we do not strictlyimpose the Ñux balance condition and allow a Ñux ratio ofup to a factor of 2.

Figure 1 illustrates the time evolution of an emergingbipole. As seen from the Ðgure, at the beginning of emer-gence, the magnetic Ðelds are weak and the positive andnegative Ñuxes are not well separated from each other (seethe magnetogram at 15 :53 UT). As emergence proceeds,both the positive and negative Ñuxes increase ; the Ñux con-centrations become stronger and are well separated fromeach other (see the magnetogram at 16 :55 UT).

Once a bipole is identiÐed, its three physical parameters,total Ñux, polarity separation, and orientation, are deter-mined at the time of its full development, namely, afteremergence stops. We Ðnd that the two poles are oftenobserved more than 1 hr, moving much less than the dis-tance between the poles without signiÐcant Ñux change.Thus, choosing a speciÐc instant for the full development isnot critical in the measurement of bipole parameters.

To measure the total Ñux of a bipole, we Ðrst deÐne asmall area enclosing the bipole only. Summation of the Ñuxover the pixels in this area that display positive magneticsignals above the noise level yields the positive Ñux of thebipole. The negative Ñux also is measured in a similar way.The total Ñux of the bipole is deÐned as the absolute sum ofthe positive and negative Ñuxes. Flux measured in this wayis the line-of-sight Ñux, that is, an integration of the line-of-sight Ðeld component over the plane of the sky. Since weobserve an area away from disk center, this value is di†erentfrom the vertical Ñux. Therefore, we correct the line-of-sightÑux for geometrical projection e†ects to obtain the verticalcomponent. There are two corrections to be taken intoaccount. First, the area in the plane of the sky is smallerthan the solar surface area, because of the geometrical fore-shortening, by a factor of cos t, where t is the heliocentricangle of each bipole. In addition, the magnitudes of a mea-sured line-of-sight Ðeld component are likely to be smallerthan that of the vertical Ðeld component since photosphericmagnetic Ðelds of bipoles are expected to be predominantlyvertical at the time of full development. If we assume thatbipole magnetic Ðelds are completely vertical at the surface,the correction factor should be an additional factor of 1/cos t. Therefore, the measured Ñux should be multiplied bya factor of 1/cos2 t to correct for both of these o†Èdisk-center e†ects.

What would be the observable limit of total Ñux of verysmall bipoles? Suppose the intrinsic diameter of each pole ismuch smaller than the spatial resolution. Then the observeddiameter of each bipole on the plane of the sky is equal tothe FWHM \ 2@@ of observations. With the choice of 3p \ 12 G for the detection limit of Ñux density, we obtain2 ] n(FWHM/2)2] 3 p \ 4 ] 1017 Mx for the observablelimit of the line-of-sight Ñux. The corresponding limit of thevertical Ñux is obtained by multiplying the correctionfactor, 1/cos2 t\ 2, to the above value and is found to be8 ] 1017 Mx.

We deÐne the magnetic axis vector of a bipole as a vec-tor whose origin is at the centroid of the positive pole andwhose endpoint is at the negative pole centroid. To deter-mine the centroid of each pole, we select only the pixels thathave Ñux density greater than or equal to one-half of thepeak value. The selected pixels usually form a small andregularly shaped island. We take Ñux-weighted averages ofthe pixel coordinates for the centroid. The error of the cen-troid of each pole has been empirically determined fromnine successive magnetograms. The rms value of each coor-dinate ranges It is not surprising that centroid0A.3È0A.5.

15:53 UT 16:18 UT

16:43 UT 16:55 UT

No. 1, 2001 MAGNETIC BIPOLES 499

FIG. 1.ÈSequence of magnetograms showing the time evolution of an emerging bipole. The black and white patches indicate Ñux densities of [50 and 50G, respectively. The Ðeld of view is 34@@] 34@@. The arrow in the lower right panel represents the magnetic axis vector of the bipole as viewed on the plane ofthe sky.

positions have been determined with subpixel precisionsince we take a Ñux-weighted average of the pixel coordi-nates around each peak. Figure 1 shows the magnetic axisvector of a bipole as viewed on the plane of the sky. Theimage coordinates of each centroid are then transformedinto the heliographic latitude and longitude (l, b) to takeinto account the viewerÏs perspective.

The polarity separation, d, of the two poles is deÐned bythe magnitude of the magnetic axis vector and is given by[(*l cos b)2] (*b)2]1@2 where *l and *b are the rela-R

_,

tive coordinates of the negative pole measured with respectto the positive pole. is the solar radius. Note that, forR

_active regions in the northern hemisphere belonging to thepresent solar cycle 23, the positive Ñux is leading, and thenegative one, trailing. The bipole orientation h is deÐned bythe directional angle of the magnetic axis vector, which isgiven by tan~1 (*b/*l cos b). This angle is measuredcounterclockwise with respect to solar west. Solar north isthus 90¡, and solar east, 180¡. We use the same conventionfor specifying the orientation of Ðlament axial Ðelds andbipolar active regions, too. The error in each measuredbipole orientation has been empirically determined fromnine successive magnetograms. The rms value ranges from3¡ to 5¡, with a tendency of being larger for smaller polarityseparation.

Finally, we want to mention that some bipoles displaychange in the orientation during emergence. We examined a

few cases and found that the change in orientation amountsto up to about 30¡, compatible with previous reports (e.g.,Wang, Wang, & Shi 1992). Systematic investigation oforientation change during bipole emergence is worthwhilebut is beyond the scope of the present paper.

4. RESULTS

4.1. Filament ChannelFigure 2 presents the relevant Ðve full-disk Ha images

taken by the BBSO Singer Ha full-disk telescope (Denker etal. 1999) and the NSO/KP full-disk magnetogram taken onOctober 13. The Ðlament channel is centered at latitudeN45¡. It is characterized by the existence of a dark and bigquiescent Ðlament on October 11 and 12 and aligned Ðbrilson October 13 as seen in Ha images. A polarity inversionline (PIL) as seen in the magnetogram separates the north-ern negative-Ñux region and the southern positive-Ñuxregion. The PIL is inclined about 25¡ to the east-west line.This orientation is typical of high-latitude PILs and isknown to be due to the transport of magnetic Ñux bysurface Ñows such as convective di†usion, di†erential rota-tion, and meridional Ñow (e.g., DeVore, Boris, & Sheeley1984 ; Sheeley, Wang, & DeVore 1989 ; Sheeley 1992 ; vanBallegooijen et al. 1998).

High-resolution Ha images taken on October 12 revealthat the Ðlament had very complex structures and large-

15:24 Oct 13

15:15 Oct 12BBSO

15:05 Oct 12

Enlarged View (Oct 12)

Enlarged View (Oct 13) 15:43 Oct 13NSO/KP

15:29 Oct 14 15:50 Oct 15


FIG. 2.ÈThe Ðve BBSO Ha full-disk images and one NSO/KP full-disk magnetogram. Only the northern hemisphere is shown. The Ðeld of view of ourhigh-resolution observations is indicated by a rectangle each day.

scale internal rotation, indicating that it was highly active.The Ðlament erupted the next day. The Solar and Helio-spheric Observatory (SOHO) EUV Imaging Telescope (EIT)195 movie shows that the Ðlament began to rise rapidlyA�around 7 :00 UT and erupted around 9 :30 UT. The associ-ated coronal mass ejection (CME) began to be visiblearound 10 :30 UT in the LASCO/C2 movie. The Ha obser-vations performed on October 14 and 15 reveal a new Ðla-ment forming in the same Ðlament channel.

The chirality of the Ðlament channel can be determinedfrom Ðlament morphology, Ðbril pattern, or coronal arcadeskew (Martin 1998a). First of all, the Ðlament has right-bearing barbs as seen in the October 12 full-disk Ha image.Second, well-aligned Ðbrils are seen along the PIL onOctober 13. Some of the Ðbrils emanate from plagettes in thepositive-Ñux side and are directed rightward for an observerfacing the PIL at the plagettes. Finally, the coronal arcadeis about 50¡ left skewed with respect to the PIL. This skewhas been determined from pairs of Ha footpoints that werefound to be connected by X-ray loops seen in the Yohkohsoft X-ray full-disk images taken a few hours after the Ðla-ment eruption on October 13. All of these characteristics

are consistent with the dextral chirality of the channel,which is compatible with the well-known hemisphericchirality preference (Martin 1998b).

4.2. Emergence Rate and Average FluxFigure 3 shows some of the bipoles, which emerged in this

region. We have identiÐed a total of 102 bipoles from the 5day observations with a total of 25 hr duration. Table 1provides a summary of observed bipoles. To be sure that weobserve the entire bipole at its full development, werestricted the area of study to the inner 280@@] 280@@. Thephysical size of the inner region has been determined withthe geometrical foreshortening e†ect taken into account,which ranged from 0.88% to 1.05% of the solar surface area,changing a little from day to day because of di†erent disklocations. We Ðnd that the emergence rate did not varysigniÐcantly from day to day. The emergence rate averagedover the 5 observing days is equivalent to about 430 emerg-ing bipoles hr~1 over the entire solar surface area. Wechoose this area unit for the emergence rate for comparisonwith other observations. Its usage does not imply that thislocalized rate persists all over the Sun.

E40

E30

E30

E20

E20

E10

E10

N30

N40

N40

N50

N50

N60


FIG. 3.ÈBBSO high-resolution line-of-sight magnetogram of the Ðlament channel taken on 1999 October 12. The superposed ellipses mark some of theidentiÐed bipoles in this channel. The brightest and darkest levels of the gray scale represent the Ñux densities of ^50 G, respectively. Drawn over themagnetogram are the heliographic latitudes and longitudes.

The average total Ñux, which is the sum of the absolutevalues of the Ñuxes of all the bipoles divided by the totalnumber of bipoles, is found to be about 2.8 ] 1019 Mx.Individual bipoles showed a large range of total Ñux, 0.7È33 ] 1019 Mx (see ° 4.3 below). The average total Ñuxvaried little from day to day, with the exception of October12, which was just prior to the Ðlament eruption. The highaverage value on this day is due to the occurrence of threebig bipoles having Ñuxes greater than 10] 1019 Mx. The

biggest region emerged on the polarity inversion line abovewhich the Ðlament was located, so the Ðlament eruptionmay have been related to the emergence of this region. ThatÐlament eruptions are often associated with emergingbipoles was previously reported by Feynman & Martin(1995) and Wang & Sheeley (1999).

The average total Ñux together with the emergence rate of4.1 bipoles hr~1 indicates a new Ñux emergence rate of11 ] 1019 Mx hr~1 in the 280@@] 280@@ observed area.

TABLE 1

SUMMARY OF THE IDENTIFIED BIPOLES IN THE FILAMENT CHANNEL OF AREA 280@@] 280@@ ON DIFFERENT DAYS

Duration Observed AreaaDate (hr) Number (%) Emergence Rateb Average Total Flux Position Channel Characteristic

Oct 11 . . . . . . 6 23 1.05 370 2.6] 1019 Mx E29¡N43¡ A dark and big ÐlamentOct 12 . . . . . . 7 29 0.95 430 4.1] 1019 Mx E19¡N43¡ The Ðlament became activeOct 13 . . . . . . 1 5 0.91 550 1.7] 1019 Mx E03¡N43¡ The Ðlament disappearedOct 14 . . . . . . 6 26 0.88 490 2.1] 1019 Mx W07¡N43¡ A new Ðlament began to formOct 15 . . . . . . 5 19 0.92 410 2.3] 1019 Mx W20¡N43¡ The formation continued

a Solar surface area percentage.b Per hour per the entire solar surface area.

0 5 10 15 20 25 30 Separation (Mm)

0

10

20

30

40

Nu

mb

er

18 19 20 21log Total flux (Mx)

0

10

20

30

40

50


Meanwhile, the total vertical Ñux of the same area keepsnearly the same value of about 8.7 ] 1021 Mx throughoutthe observing days. Therefore, Ñux should cancel and disap-pear at the same rate as Ñux emergence, and the preexistingÑuxes should be replaced by new Ñux every 80 hr. Thistimescale is short enough, so the Ñux replacement by emerg-ing bipoles may be important in prominences.

Table 2 shows that the emergence rate and average totalÑux of bipoles in the present study are comparable to thosefrom previous studies of ephemeral regionsÈwhich havebeen known to be small bipolar active regions with lifetimesof 1 day or shorter (Harvey & Martin 1973). Therefore,most of the bipoles we identiÐed in the present study seemto be ephemeral regions. Since all the results were obtainedfrom observations of small areas at di†erent locations atdi†erent times and are based on small numbers of bipoles,the di†erences among them seem to reÑect local dependenceand associated statistical Ñuctuation as well as di†erences ininstruments and analysis techniques. As far as the emer-gence rate and average total Ñux is concerned, the study ofHarvey (1993) seems to be the closest to our result. Shereported a total Ñux range of 0.2È33 ] 1019 Mx, which isalso very close to ours. Table 2 also compares the Ñuxreplacement timescale with some previous estimates andindicates that our value is not atypical. A recent SOHOMDI study of the magnetic Ñux in the quiet Sun also pro-duced a comparable value of 40È70 hr for the Ñux replace-ment time (Schrijver et al. 1998).

4.3. Polarity Separation and Flux DistributionFigure 4 shows the number distributions of polarity

separation and magnetic Ñux for all the identiÐed bipoles.Each of them reveals a single-peaked smooth distribution.The number decreases at the small-scale end of each dis-tribution, possibly because of the limited chance of identify-ing very small bipoles. Since all the observed bipoles haveÑuxes above the conservatively estimated observable limitof 8 ] 1017 Mx, the possible selection e†ect, if any, may notbe due to the Ñux detection limit. The selection e†ect maybe attributed to the difficulty of identifying small bipolesemerging in a region crowded by network magnetic concen-trations and intranetwork elements that have sizes andÑuxes comparable to those of the bipole. It is very hard toevaluate this kind of selection e†ect. Recently, Hagenaar(2000) applied an automatic bipole detection method toMDI full-disk magnetograms and found a tendency forsmaller bipoles to outnumber bigger ones. Even if we arenot fully sure that all the bipoles they found are emergingbipoles, their result suggests that the number decrease at the

FIG. 4.ÈNumber distributions of (left) polarity separation and (right)total magnetic Ñux for all the identiÐed emerging bipoles.

small-scale end in Figure 4 might be due to observationalselection e†ects.

The average polarity separation and Ñux of the bipolesstudied in the present paper are about 7.4 Mm and2.8] 1019 Mx, respectively. The size is about one-half thesupergranulation diameter 13È18 Mm, the value recentlydetermined by Hagenaar, Schrijver, & Title (1997). Theaverage total Ñux is signiÐcantly bigger than the mediantotal Ñux value, 1.8] 1019 Mx. This means that the Ñuxcontribution by big bipoles is important even though theirnumber may be small. We have found a positive correlationbetween Ñux and polarity separation.

4.4. Orientation DistributionFigure 5 shows that the orientation distribution of

bipoles has a peak component centered around 235¡. Thismeans that bipole magnetic axes are preferentially orientedin the solar south-east direction. We could not Ðnd anycorrelation between Ñux and orientation of bipoles.

Figure 6 is a graphical presentation of the orientationdistribution compared with known orientations of otherfeatures. The Ðgure shows that the preferred orientation ofbipoles is much di†erent from that described by the Halepolarity law of active region orientation (Hale et al. 1919).The trailing polarity of a bipole (i.e., the negative pole inanalogy to active regions) is preferentially located equator-ward of its leading positive pole, whereas, according to JoyÏslaw, the trailing polarity of a bipolar active region should belocated poleward of the leading polarity. The active region

TABLE 2

COMPARISON OF EMERGENCE FREQUENCY AND AVERAGE TOTAL FLUX OF EPHEMERAL REGIONS AMONG DIFFERENT STUDIES

Work Number Emergence Ratea Average Total Flux Renewal Timeb Observing Year/Instrument/Field of View

Wang (1988a) . . . . . . . . . . . 16 220 1.5 ] 1019 Mx 130 1985/BBSO Video Magnetograph (VMG)/Quiet area at disk center

Martin (1989) . . . . . . . . . . . 11 170 2.5] 1019 Mx 90 1988/BBSO VMG/quiet area at S15¡Wang et al. (1992) . . . . . . 31 110 5.0 ] 1019 Mx 200 1989/BBSO VMG/unipolar region at N30¡Harvey (1993) . . . . . . . . . . . 44 690 3.3] 1019 Mx . . . 1975/KP 512-channel Mag/quiet area near equatorSchrijver et al. (1997) . . . . . . 230 1.3 ] 1019 Mx . . . 1996/SOHO MDI/quiet area near disk centerPresent study . . . . . . . . . . . 102 430 2.8 ] 1019 Mx 80 1999/BBSO DMG/Ðlament channel at N45¡

a Per hour per the entire solar surface area.b The Ñux replacement time, which is deÐned as preexisting Ñux divided by new Ñux emergence rate.

Coronal Arcade Orientation

Axial Field Directionof the Dextral Filament

West

North

AR OrientationNorthern Hemisphere

(a)



West

North


(b)



West

North



FIG. 5.ÈNumber distribution of orientation for all the bipoles identi-Ðed in the Ðlament channel. West is 0¡ in our deÐnition of the orientation,north, 90¡, and so on.

orientation in the Ðgure shows a tilt angle of 12¡ for thehighest latitude active regions at N35¡ (Hale et al. 1919).Figure 6 also shows that the preferred orientation of bipolesis perpendicular to the Ðlament axial Ðeld direction within10¡ and has only a small parallel component. Moreover,even the direction of the small parallel component isopposite to the axial Ðeld direction, contrary to the predic-tion of van Ballegooijen & Martens (1990). It appears thatthe preferred orientation is nearly inverse to the orientationof background preexisting Ñuxes as implied by the magne-togram and the coronal arcade orientation.

To see whether or not the orientation of bipoles may beassociated with Ðlament eruption, in Figure 7 we comparedthe bipole orientation distribution before and after theprominence eruption. We found no signiÐcant change in thepreferred orientation. The major di†erence, if any, is thatthe preeruption distribution of bipole orientation displays astronger tendency of the preferred orientation. It is very

FIG. 6.ÈGraphical representation of the bipole orientation distribu-tion. The length of an arrow is proportional to the number of bipoles thatare oriented in that direction. The thick arrow represents the preferredorientation.

difficult to put a physical signiÐcance on this di†erence sincewe cannot exclude the possibility that this might be due tothe poorer seeing condition on the latter days, which mighthave hindered the unambiguous identiÐcation of smallbipoles. However, if the di†erence turned out to be intrinsic,it could imply that the degree of alignment of bipole mag-netic axes may be important in the Ðlament eruption.

4.5. Spatial DistributionFigure 8 shows the spatial distribution of the bipoles and

the correlation between their spatial position and orienta-tion. The position of a bipole is measured from the PIL,being positive if the bipole is in the positive-Ñux side andnegative otherwise. The Ðgure shows that bipoles emergednot only on the PIL but also in the positive- and negative-Ñux regions and that spatial location and orientation arenot correlated. We could not Ðnd any signiÐcant physicaldi†erence among bipoles in di†erent locations. Since

FIG. 7.ÈComparison of bipole orientation (a) before and (b) after the prominence eruption

-200 -100 0 100 200 Position (Mm)

0

5

10

15

20

25

30

Nu

mb

er

-200 -100 0 100 200 Position (Mm)

0

100

200

300

Ori

enta

tion

(deg

rees

)


FIG. 8.ÈL eft : Number distribution of bipole position from the polarityinversion line. Right : Correlation of orientation and displacement. Positive(negative) positions are for bipoles in the positive (negative) Ñux side.

bipoles appear anywhere and are obviously of mixedpolarity, they are likely to be a potential source of theminority-polarity Ñuxes that are found in either side of theÐlament channel. Minority-polarity Ðelds are regarded asimportant to Ðlament barbs (e.g., Martin 1998a).

5. DISCUSSION

5.1. Ephemeral Regions5.1.1. Comparison with Previous Results

The statistical properties of bipoles in the present studysuggest that they are mostly ephemeral regions. Here wecontrast our results on the preferred orientation of thebipoles with previous studies of ephemeral region orienta-tion. The early study by Harvey, Harvey, & Martin (1975)revealed that the ephemeral region orientation is almostrandom. Later Martin & Harvey (1979) found that ephem-eral regions during solar minimum tend to have ““ proper ÏÏand ““ reverse ÏÏ orientations, as seen in active regions.Harvey et al. (1975) also compared each property of ephem-eral regions with that of active regions and found no signiÐ-cant di†erence other than size. The distribution of theorientations of active regions systematically broadens fromlarge to small ones such that the orientations of the smallestactive regions are close to random. In this sense, the nearlyrandom orientation of ephemeral regions Ðts the pattern ofactive regions. Hence, Harvey et al. concluded that ephem-eral regions represented the small-scale end of the distribu-tion of active regions. Further support for ephemeralregions being part of the active region population was pro-vided by Martin (1989) in comparing the emergence ofephemeral regions and active regions of various sizes ; shefound that the emergence process of large ephemeralregions is characterized by the successive appearance ofbipolar magnetic units called ““ elementary bipoles.ÏÏ Shefound no distinction between the elementary bipoles oflarge ephemeral regions and those of new active regions.Harvey (1993) examined a much larger sample of ephemeralregions from NSO/Kitt Peak daily magnetograms over asolar cycle and compared their overall orientations withactive regions. Although ephemeral regions had a large dis-persion in orientation, they still exhibited a weak statistical

tendency for east-west orientations consistent with activeregions. This, along with other properties of ephemeralregions, including the centering of their latitude distributionaround the activity belts of active regions in each hemi-sphere, led her to conclude again that ephemeral regions arethe small-scale end of a wide spectrum of active regions.

Why is our result on bipole orientation di†erent fromthese numerous previous studies? The present study usedbipoles in a small area of a Ðlament channel, whereas pre-vious studies based on Kitt Peak full-disk magnetogramsgathered bipoles from all over the visible hemisphere of theSun. Therefore, the two opposing results seem to suggestthat bipoles may have a preferred orientation in a speciÐcsmall region only and have large dispersion in orientationwhen they are sampled in a big area.

5.1.2. Small Bipoles : Ephemeral Regions or Intranetwork Fields?

There is a possibility that some of the bipoles in thepresent study might not be ephemeral regions but insteadintranetwork bipoles. One way of distinguishing ephemeralregions from intranetwork bipoles is to look at the locationsand velocity patterns with respect to the geometry of thelocal supergranules. Ephemeral regions tend to appear atnetwork boundary (Wang 1988b) and usually display bidi-rectional diverging motions. On the other hand, intranet-work magnetic Ðelds originate in the centers ofsupergranules, become visible as discrete poles of magneticÑux of either polarity, and Ñow toward the boundaries ofthe supergranules (Martin 1988, 1989 ; Zhang et al. 1998).Another way of distinguishing most ephemeral regions fromintranetwork Ðelds is by the amount of Ñux. Previous datasets, however, revealed an overlap in the distributions of theÑux of ephemeral regions, intranetwork Ðelds, and networkmagnetic Ðelds (Martin 1989). Wang et al. (1995) showedthat intranetwork elements have Ñuxes up to a few 1018 Mx,which is for only one pole. Thus, total Ñuxes of large intra-network bipoles may be up to several 1018 Mx, which iswithin the bipole Ñux range we determined. Thus, wecannot fully exclude the possibility that some of the bipoleswe identiÐed are intranetwork bipoles, even if they may notbe the majority. A rigorous distinction between very smallephemeral regions and large intranetwork Ðelds is not atrivial task and would require careful determination ofsupergranulations and Ñow Ðelds, which is beyond thescope of and not the main theme of the present paper.

5.1.3. Possible Subsurface Magnetic ConÐgurations

The orientation of bipoles may contain useful informa-tion on subsurface magnetic conÐgurations. We proposetwo di†erent pictures of subsurface magnetic Ðelds forephemeral regions. We Ðrst focus on our result that thepreferred orientation is nearly opposite to the magnetic axesof coronal loops that connect the positive and negativenetwork magnetic Ðelds on the two sides of the Ðlament.Figure 9 presents a possible subsurface Ðeld conÐgurationconsistent with this property. Subphotospheric horizontalÐelds connect the network magnetic Ðelds on the two sidesof the PIL and are approximately parallel to the planes ofthe coronal loops. The emergence of parts of this sub-photospheric component results in bipoles having theorientation inverse to the orientation of overlying coronalloops. According to this picture, ephemeral regions arelikely to appear mostly in the PIL. However, the picturedoes not exclude the possibility that ephemeral regions

+ ++ -- -+ - +-

ER > 15 Mm

ER < 15 Mm



FIG. 9.ÈSubsurface Ðeld line connection model of ephemeral regions.The subsurface Ðelds connect the two sides of a polarity inversion linebelow surface.

appear in the two sides of the PIL as we observed (see Fig.8), especially if we relax the translational invariance alongthe third direction. The translational invariance is a conse-quence of limitation of this kind of two-dimensional modelbut is not the essential feature of the picture. This modelpredicts that the orientations of ephemeral regions willdepend on the local magnetic conÐguration. If this were thecase, a sample of ephemeral regions taken from a very largearea would display random orientation since such a samplewould not reveal any local dependence. This could explainwhy Kitt Peak full-disk data displayed random orientationin ephemeral regions.

What would be the physical origin of subsurface horizon-tal Ðelds if this picture holds? The horizontal Ðelds mightoriginate from the rising of a toroidal Ñux bundle in a U-shaped loop between two )-shaped loops that emergedearlier, as proposed by Spruit, Title, & van Ballegooijen(1987). The polarity distribution of the Ðlament channelimplies that the channel may have been formed in the inter-face of two active region remnants rather than inside oneactive region, supporting this idea. Alternatively, the hori-zontal magnetic Ðelds might originate from subsurfacereconnection of two Ñux bundles rooted at di†erent regions,possibly at the base of convection zone, for example, by themechanism provided by Schrijver & Title (1999).

Another possible picture of subsurface magnetic Ðelds ofephemeral regions has been conceived by our comparisonof our results with the result of Wang et al. (1992) on theorientation of ephemeral regions in a unipolar area within acorona hole. From high-resolution magnetograms taken atBig Bear Solar Observatory and Huairou Observatoryabout one solar cycle before our observations, they foundthat ephemeral regions with large separations (greater than15 Mm) tended to have a preferred north-south orientationwith the leading polarity poleward and the following equa-torward. Figure 10 presents our illustration of their result.Comparing with Figure 6, we Ðnd that both active regionsand ephemeral regions have reversed their orientations overthe 10 yr period. Interestingly, we Ðnd that the relativeorientation between ephemeral regions and active regionshas about the same value of 70¡ in both our observationsand theirs. This coincidence suggests the possibility that the

FIG. 10.ÈResults of Wang et al. (1992) on bipole orientation in thenorthern hemisphere from his 1989 observations around the solar cycle 22maximum.

relative orientation between ephemeral regions and activeregions might be invariant over the solar cycle. Figure 11provides a picture of subsurface magnetic Ðeld conÐgu-ration that is based on this possibility. Each ephemeralregion is considered to be part of a thread that originatesfrom global toroidal magnetic Ðelds having the same originas active region magnetic Ðelds. If the thread is assumed tobe left helical because of unknown reasons, the modelexplains the observed relative orientation of small bipoleswith respect to active region orientation. Note that, whenthe polarity is reversed with the solar cycle, both the activeregion orientation and ephemeral region orientation arereversed, so the relative orientation does not change and thethread remains left helical. This picture predicts the exis-tence of a systematic alignment of ephemeral region mag-netic axes over the entire solar surface. This prediction ishard to reconcile with previous studies that have reportedthe almost random orientation of ephemeral regions. In thissense, this picture is less convincing than the other picturepresented in Figure 9.

5.2. Filament Chirality5.2.1. Axial Fields

Several unsolved mysteries about the nature of solarprominences could be relevant to any Ðnding of a preferredorientation of ephemeral regions. Current questions arewhether the observed preferred orientation of ephemeralregions could a†ect or be related to the chirality and forma-tion of Ðlament channels. Van Ballegooijen & Martens(1990) proposed a model in which the axial Ðelds of a Ðla-ment originate from the emergence of magnetic Ðelds insmall bipoles that are signiÐcantly aligned along the direc-tion of Ðlament axial Ðelds. What we Ðnd is that bipolemagnetic axis vectors tend to be almost perpendicular tothe observed direction of the Ðbril alignment in a dextralchannel. Moreover, if the minor component parallel to thePIL is taken into account, the observed bipole orientationwould better match a sinistral Ðlament channel rather than

Oblique view

Side view

Top view

surface

ER OrientationHale Law of

AR Orientation


FIG. 11.ÈLeft-helical Ñux thread model of ERs in the northern hemi-sphere. The thread originates from global toroidal magnetic Ðelds thatfollow the Hale polarity law and is left helical because of unknown reasons.

a dextral one. Therefore, our result does not support theprediction made by this model.

5.2.2. Barbs

It has been demonstrated that the barbs of Ðlaments arerelated to small patches of magnetic Ðelds known as““ minority polarities ÏÏ in the context of being opposite to themajority polarity of the network magnetic Ðelds on eachside of a Ðlament. Some of these minority polarities havebeen identiÐed as ephemeral regions (Martin 1998a). Theexistence and distributions of the minority polarities (andby implication, ephemeral regions) in a Ðlament channelhave been shown to be important in recent models of Ðla-ment barbs (Aulanier & DeMoulin 1998 ; Aulanier et al.1998 ; MacKay, Longbottom, & Priest 1999 ). Therefore,our Ðnding of the preferred bipole orientation may impor-tant in the study of barbs. Whether and how the preferredorientation found in the present study could be connectedto Ðlament barbs should be further investigated.

5.2.3. Magnetic Helicity

Finally, we address the most important question :whether Ðlament chirality is of subsurface origin or not.Obviously, our observation of bipoles does not support themodel (van Ballegooijen & Martens 1990 ; Priest et al. 1996)that explains the Ðlament chirality as a result of emergenceof horizontal magnetic Ðelds sheared by subsurface di†eren-tial rotation. Nevertheless, it appears premature to con-

clude that Ðlament chirality is not of subsurface origin. Thebasic reason for this is that we do not know the magnetichelicity budget of bipoles. If the observed preferred orienta-tion implied that magnetic helicity carried by Ñux emer-gence is not random, then the contribution of emergingbipoles to the magnetic helicity of a Ðlament would not benegligible because of the high rate of Ñux emergence. Thehelical thread model for the orientation of bipoles depictedin Figure 11 suggests this kind of possibility.

6. SUMMARY

We have studied small bipoles that emerged in a quiesc-ent Ðlament channel at N45¡ from 5 day observations witha total of 25 hr duration. The Ðlament channel is dextral,and 25¡ inclined to the east-west line. Our results are sum-marized as follows.

1. The bipoles have an average total Ñux of 2.8] 1019Mx and an average separation of 7400 km. The emergencerate of bipoles is 430 hr~1 if it is extrapolated to the entiresolar surface, and the new magnetic Ñuxes of bipoles canreplace the preexisting Ñuxes in the Ðlament channel in 80hr.

2. The statistical properties of bipoles we studied in thepresent paper are consistent with those of ephemeralregions that were previously studied. However, we cannotfully rule out the possibility that a minor fraction of thebipoles could come from another population of small mag-netic features known as intranetwork magnetic Ðelds.

3. The bipoles have a preferred orientation ; the negative(trailing) pole of a bipole is preferentially located southeastof the positive (leading) pole.

4. The preferred orientation is opposite to the orienta-tion of the local overlying coronal Ðelds, and much di†erentfrom either the Ðlament axial Ðeld direction or the activeregion orientation. We have proposed two di†erent kinds ofsubsurface magnetic Ðeld conÐgurations consistent with theobserved preferred orientation : one depending on localmagnetic conÐguration and the other depending on theglobal toroidal magnetic Ðelds, which may have the sameorigin as active region Ðelds.

5. Our results do not support the model of van Balle-gooijen & Martens (1990). It is, however, premature toexclude the possibility that Ðlament chirality is of sub-surface origin.

We conclude that further studies need to be done for avariety of quiet-Sun and active-Sun conditions before adeÐnitive interpretation can be ascribed to the preferredorientation found in our data set.

We thank the referee for valuable comments and sugges-tions, which helped in improving the paper. We also thankA. van Ballegooijen, K. Harvey, and C. Schrijver for theirkind help and encouragements. This work is supported inpart by the US-Korea Cooperative Science Program(KOSEF 995-0200-002-2, NSF INT-98-16267), by NASAunder grant NAG5-9682, NAG5-7837, by NSF under grantATM 97-14796, and by the BK21 project of the Koreangovernment. The contribution of S. F. M. is supported byNSF grant ATM 98-08063. We acknowledge the use ofNSO/Kitt Peak full-disk magnetograms. J. Chae thanksGod, who gave him the joy of investigating the topic treatedin this paper.


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Small Magnetic Bipoles Emerging in a Filament Channel

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