Close Correlation among Hα Surges, Magnetic Flux

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Surge activities were observed at the edge of an emerging flux region. ... that the surge activities in Hα and the brightenings in TRACE 1600 ˚A images correlate ...

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Close Correlation among H Surges, Magnetic Flux Cancellations, and UV Brightenings Found at the Edge of an Emerging Flux Region Article  in  Publications- Astronomical Society of Japan · January 2003 DOI: 10.1093/pasj/55.1.313

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PASJ: Publ. Astron. Soc. Japan 55, 313–320, 2003 February 25 c 2003. Astronomical Society of Japan. 

Close Correlation among Hα Surges, Magnetic Flux Cancellations, and UV Brightenings Found at the Edge of an Emerging Flux Region Keiji YOSHIMURA The Institute of Space and Astronautical Science, Yoshinodai, Sagamihara, Kanagawa 229-8510 [email protected]

Hiroki K UROKAWA Kwasan and Hida Observatory, Kyoto University, Sakyo-ku, Kyoto 606-8502

Masumi S HIMOJO Nobeyama Radio Observatory, National Astronomical Observatory, Nobeyama, Minamimaki, Minamisaku, Nagano 384-1305

and Richard S HINE Lockheed-Martin Solar and Astrophysics Laboratory, Palo Alto, CA, USA (Received 2002 September 13; accepted 2002 December 10)

Abstract Surge activities were observed at the edge of an emerging flux region. We studied the relations between the features around the surges in various data sets: magnetogram, Hα, G-band, UV, EUV, and soft X-rays. We showed ˚ images correlate well in both time and space that the surge activities in Hα and the brightenings in TRACE 1600 A with the cancellation of magnetic fluxes around an emerging flux region. In particular, at the onset of surge activity, a close correlation among them was clearly found. These facts are consistent with the magnetic reconnection model. The released energy through magnetic reconnection, which is estimated to be 1028 erg, is sufficiently large to produce surge activities. No prominent brightenings were observed in soft X-ray and EUV images during the surge activities. This may suggest that the energy releases occurred at a layer of high densities. Key words: Sun: chromosphere — Sun: magnetic fields — Sun: UV radiation 1. Introduction Surges are collimated ejections of chromospheric matter into coronal heights. They can be seen as dark features in Hα images. They move upward at 20–200kms−1 , reach heights of up to 200000 km and last for 10–20 min. The ejected matter often return back along almost the same path as in the rising phase (Bruzek, Durrant 1977). The relationship between the surge activities and the photospheric magnetic features were presented by Rust (1968). He showed that surges occurred at neutral points above the satellite polarity. Roy (1973) pointed out that surges occurred in regions of evolving magnetic features. Kurokawa (1988) as well as Kurokawa and Kawai (1993) showed that Hα surges are the first active manifestation of magnetic flux emergence in some emerging flux regions (EFRs), and suggested that the various morphological features of those surges could be explained by a magnetic reconnection model. By an MHD simulation, Yokoyama and Shibata (1996) actually produced X-ray jets and Hα surges with reconnection between the EFR and the preexisting coronal magnetic field. In this model, reconnections between emerging magnetic fluxes and preexisting fields drive the ejections of chromospheric matter to corona by magnetic tension. In order to verify such models, the spatial and temporal relations between the magnetic fields and the surges must be examined in detail by observations. S. Sano (2001, private communication) found that the photospheric magnetic flux around many surges increased after the surge activities. Chae et al. (1999) and Zhang et al. (2000)

studied the surges with EFRs, and found magnetic cancellations between the preexisting field and the newly emerging one at the footpoints of the surges. However, the detailed relationship between the emerging magnetic fluxes and the surge activities is still unclear. Our motivation for this study is to clarify it with a high spatial and temporal data set. Since, actually, the high-resolution magnetogram data from SOHO / MDI were available through the surge active phase in this study, we could see the detailed variations of the magnetic field. 2.

Data

The emerging flux region (EFR) which we studied appeared on 1999 June 10 near the center of the solar disk (N19 W04 in heliocentric coordinates). It was located about 25000 km distance from the nearest sunspot. The surge activities, which lasted for 2 hours, were observed around the EFR (figure 1). As the summary of data set that we used in this study is given in table 1. The observation covered the whole duration of the surge activities including their onset. Hα and G-band images were taken with the Swedish vacuum solar telescope at La Palma by a Lockheed Solar Astrophysics Laboratory team under fairy good seeing conditions. SOHO / MDI magnetogram data with high resolution mode were available for this region. TRACE also observed ˚ and UV (1600 A). ˚ The 195 A ˚ the EFR in EUV (195, 171 A) passband has mainly been used for EUV observations until ˚ passband after 10:00 UT. A TRACE 10:00 UT, and the 171 A

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˚ in order from left to right. An EFR was Fig. 1. Hα surges around 10:20 UT at five different wavelengths: −0.75, −0.35, ±0.0, + 0.35, and + 0.75 A located at the lower right corner in each image.

Table 1. Summary of data set.

Cadence Hα (line center) Hα (blue / red wing) G-band Magnetogram ˚ UV (1600 A) ˚ UV (1550, 1700 A) ˚ EUV (171, 195 A) SXR

30 s 2 min 25 s 1 min 30 s 30 min 30 s 1–2 hr

˚ passband usually shows the strucobservation with a 1600 A tures of a 4.0–10 × 103 K plasma (Handy et al. 1999). However ˚ which are emitthe intensities of the C IV lines (1548, 1550 A), ˚ ted from a plasma of about 105 K, and included in the 1600 A passband, vary by fairly large factors in energetic phenomena like flares (Brekke et al. 1996). The fraction of the C IV inten˚ passband images are able to be estimated sities to the 1600 A ˚ passband images (Handy from the set of 1550, 1600, 1700 A et al. 1998). Since the EFR was located outside of the area of the Yohkoh / SXT partial frame observation, only full-frame soft X-ray (SXR) data of lower cadence and lower spatial resolution are available. 3.

Results

3.1. Characteristic Features Found at the Onset of Surge Activities Thanks to the high temporal and spatial resolution of the data, we can study the detailed evolution of various features related to the magnetic flux emergence from below the photosphere at several wavelengths. It is found that an area of negative magnetic field appeared around 9:10 UT, which is enclosed by the black circle in

Spatial sampling 

0. 1/pix 0. 1/pix 0. 1/pix 0. 6/pix 0. 5/pix 0. 5/pix 0. 5/pix 10 /pix

Observation site La Palma La Palma La Palma SOHO / MDI TRACE TRACE TRACE Yohkoh / SXT

figure 2. The noise level of MDI magnetogram data is about 14 G for each pixel (Hagenaar 2001). Though the signals of the magnetic field strength in each pixel of the area around 9:10 UT were no higher than the noise level, the spatial integration over the area resulted in a nontrivial signal. We can identify the increase in the negative fluxes around 9:15 UT (figure 3). The time slice of sequential magnetograms along the line indicated in figure 2 was made, and shown in figure 4. Notice that the emerging negative fluxes collided with the preexisting positive fluxes after around 9:20 UT. After that time, the neutral line where collisions occurred moved toward the northeast with a velocity of 0.7 km s−1 . The emergence of a positive flux in the area encircled in figure 2 was unclear. Assuming a symmetric loop emerging from below the photosphere, it is expected that one of its foot points, which is located nearer to the solar disk center, is less visible in the longitudinal magnetic field map than the other, because the angle between the foot and the line of sight is closer to 90◦ . Since the foot points which show positive polarity were located nearer to the center of the solar disk in this case, the difference in visibility is reasonable. In Hα blue-wing images we can see the first dark feature of surge activities at 9:20 UT around the place where the two polarity collided. Although it was not so prominent at that time, it evolved to be larger and darker. In Hα red-wing images, the

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Fig. 2. Magnetogram at 9:25 UT. An EFR appeared in the area enclosed by the black circle.

Fig. 4. Time slice of magnetogram at the position indicated by the white line in figure 2.

Fig. 3. Temporal variation of the total negative magnetic flux integrated over the area encircled in figure 2. Notice a rapid increase in the negative magnetic flux from 09:15 UT, which is evidence of the flux emergence from below the photosphere. The error bars are estimated from the noise level given in Hagenaar (2001).

dark features were found around 9:35 UT at almost the same place as where those in the blue-wing images had appeared. Around 9:42 UT, the dark features in the red wing extended to above the EFR. These two dark features confirm the conclusion of Kurokawa and Sano (2000), who found two kinds of falling dark features in the Hα red wing; one is falling along the same magnetic field lines as the rising surge, and the other is along different field lines, which were newly formed as a result of magnetic reconnection. ˚ started around We could find that the brightening in 1600 A

˚ brightening appeared at the neutral Fig. 5. Time profile of the 1600 A line where the features of opposite magnetic polarity collided.

9:20 UT (figure 5) at the place where opposite polarities collided. It became brightest around 9:36 UT. Several exploding granules were first found around 9:15 UT in G-band images at the site where the EFR appeared. Some small dark threads, which are thought to be evidence of emerging fluxes, could be identified there at that time. Then, photospheric features (in magnetogram and G-band images) were the first signature of the magnetic emergence in this case.

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˚ images are overlaid with black contours on the Fig. 6. Time evolution of the EFR at the early phase in various data. The brightest parts of 1600 A magnetograms and Hα images to indicate the spatial correlations.

3.2. Spatial Relationships Figure 6 shows the evolution of the region for each wavelength. The spatial relationships among the Hα dark features, magnetic field and the UV brightenings are examined in this figure. The accuracy of the coalignment is about 1 . We could find that both of the UV brightenings and the foot points of

surges were located around the magnetic neutral line where we can see the collisions between the positive fluxes, which had already existed at the beginning of the observation, and the emerging negative ones. This set of three features kept the same spatial relationships with each other through all of the surge activities, while the neutral line moved northeastward. For this kind of spatial correlation between the magnetic

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Fig. 8. Time profile of the total negative magnetic fluxes around feature “B”.

Fig. 7. Time profiles of (a) the surges lengths, (b) the total flux of ˚ emission around the brightenings, (c) the total of positive mag1600 A netic fluxes in the area, which is shown as the rectangle in the uppermost panel in figure 6, and (d) the total of positive magnetic fluxes inside the feature “A” shown in figure 6.

neutral lines, where opposite-polarity elements collided, and foot points of surges were already found in previous studies (Kurokawa, Kawai 1993; Chae et al. 1999; Zhang et al. 2000). ˚ brightenings were also located at Note the fact that the 1600 A almost the same parts.

in the positive fluxes must be a good index for the magnetic cancellation. All surge activities and most of the brightening at ˚ were observed during the declining phase of the posi1600 A tive magnetic fluxes. We can see that the cancellation started at around 9:20 UT when the collisions of the opposite magnetic features were observed, as described above. Figure 7d shows the decrease of the magnetic fluxes in feature “A”, which is enclosed by the black circle in the uppermost ˚ appeared panel of figure 6. All of the brightenings in 1600 A around feature A. The decrease, i.e., cancellation, rate changed slightly at around 10:05 UT from −1.6 × 1015 Mx s−1 (9:40– 10:00 UT) to −2.8 × 1015 Mx s−1 (10:00–10:30 UT). During ˚ and the the latter phase the largest brightening in 1600 A longest surges were observed. The cancellation stopped at around 11:10 UT, when the positive magnetic feature “A” had disappeared. After that time we could not see any surge activities. Thus, the correlation among these three features seems to be very good in this small area. Here, we add one more observational feature related to the activities during 10:00–10:40 UT. Figure 8 shows the time profile of the total negative magnetic fluxes in feature “B”, which is one of preexisting negative magnetic features, and is enclosed by the white circle in figure 6. A decrease in the negative fluxes was clearly observed during 10:00–10:40 UT. This means that magnetic cancellations between the preexisting negative and positive polarities also occurred near the surge region.

3.3. Temporal Relationships

3.4. Contribution of C IV Emission to the Intensities of the Brightening Points in 1600 A˚

Figure 7 shows temporal variations of the lengths of surges ˚ emission (b), and the total pos(a), the total counts of 1600 A itive magnetic flux in the area enclosed by the rectangle in the uppermost panel of figure 6 (c). Since few positive magnetic fluxes seemed to have newly emerged in this area, the decrease

Using the set of three different UV passband images, we can estimate the emissions from the C IV and UV continuum in the ˚ (Handy et al. 1998). Since we don’t have range 1540–1559 A ˚ during the largest brightenings any other UV data than 1600 A (10:14–10:35 UT), we adopted Handy’s method to the data

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˚ images. The lower panels shows negative images. The arrows point toward the falling matter with slight Fig. 9. Surges in the EUV (171 and 195 A) brightening.

taken before and after this time range. In the UV images taken around 10:39 UT we can see a small brightening at the place where the largest brightenings were observed. Its emission in C IV and the continuum at that time was estimated to be 1.9 × 1016 and 4.5 × 1014 photons s−1 cm−2 str−1 , respectively. On the other hand, at 10:05 UT when we couldn’t see any special brightenings, the emissions from the same part were ˚ estimated to be 2.6 × 1015 (C IV) and 1.4 × 1015 (1540–1559 A continuum) photons s−1 cm−2 str−1 . These results indicate that a large fraction of the emissions from the prominent ˚ images may be produced by C IV. brightenings in the 1600 A ˚ brightenings must This means that the plasma in the 1600 A be heated up to 105 K. Note that the rapid variation of the intensities in the UV images possibly causes large errors in the estimated value, because single sets of UV images were not taken simultaneously. 3.5. Motions of the Brightening Points at 1600 A˚ ˚ which were 500–1500 km All of the brightenings at 1600 A, in diameter, were located on neutral lines at the edge of the EFR. Almost all of them showed motions along the neutral lines at 3–15 km s−1 through about 103 km distance. We could not find any preference in the directions of the motions. These motions must correspond to the changes in the energy release site along the neutral lines. 3.6. EUV and SXR Images In EUV images we could see dark features, which are almost identical with the Hα surges, while no bright feature was found

until about 10:15 UT. At that time, some parts of the surges looked slightly bright in EUV images. After the surge activities decreased, we found a bright region covering the EFR (figure 9). In soft X-ray images taken by Yohkoh / SXT we could not find any brightenings around the EFR during this observation (figure 10). Also, the subtracted images show no change in brightness around the EFR. However, since the SXR data did not cover the main phase of the surge activities, we can not mention whether the surge activities produced any transient SXR brightenings or not. 4.

Discussions

4.1. Onset of Surges We could find various features at the first emergence of the magnetic fluxes at several wavelengths. Before the surge activities, the emergence of magnetic features was observed in this EFR. Zhang et al. (2000), on the contrary, described that the surge had been observed for almost one hour before the appearance of magnetic fluxes. Such different results may depend on the detectability of the magnetic features at a very early phase of the EFR. Two observational facts which were found in this study should be pointed out. One is the weakness of the magnetic fields at the first phase of the EFR. Since their signals were almost under the noise level, we could not find any nontrivial signals without spatial integration. Another one is the effect of magnetic cancellation. The emerging magnetic flux which collided and was cancelled with the preexisting fields

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Fig. 10. Upper panels: SXR images taken by Yohkoh / SXT at 9:04:20 UT (left), 10:53:40 UT (middle), and 14:04:36 UT (right). Lower panels: Differential images between the above three. The position where the EFR was located is shown as a white circle in each image.

in our case. Thus, the increases in the magnetic fluxes are expected to be lower than the total emergence flux from below the photosphere. It is therefore possible that Zhang et al. (2000) could not catch any signals of the magnetic flux emergence, even if they had already appeared before the surge. 4.2. Surges Driven by Reconnection In this study we found a clear relation among the surge activity, the magnetic flux cancellation and the brightening in ˚ images. Those are: (1) the foot points of surges and the 1600 A ˚ appeared at the neutral line where magbrightenings in 1600 A ˚ netic cancellation occurred; (2) the surge activity and 1600 A brightenings were observed simultaneously with a decrease in the preexisting positive magnetic flux; (3) after the magnetic cancellation stopped, no more surges were observed. These observational facts suggest that the released energy from magnetic cancellation drove the surge activities. We provide a quantitative examination of the energetics of surges in the following. 4.3. Energy Estimation Can the released energy accelerate enough chromospheric matter? Here, we estimate the total energy released through magnetic dissipation and the kinetic energy of the surge activities. The amount of released energy, E, through the magnetic reconnection is the total Poynting flux flowing into the reconnection region. It is estimated by the following equations: B2 vH L∆t, (1) 4π ∆Φ 1 , (2) v= ∆t BL where B, v, ∆t, and ∆φ are the magnetic field strength, the velocity of inflow toward the neutral line, the duration of cancellation, and the total flux cancellation, respectively. L and H are the length along the neutral line and the height of the dissipation region, respectively. First we adopt the parameters of the cancellation presented in figure 7c: B ∼ 100 (G), ∆Φ ∼ 2 × 1019 (Mx), ∆t ∼ 7 × 103 (s), L ∼ 4 × 108 (cm), H ∼ 3 × 108 (cm). Then, we get E ∼ 5 × 1028 (erg) and v ∼ 7 × 104 (cm s−1 ). The velocity, E=

v, is almost the same as that of the neutral line movement (0.7 km s−1 ). Garc´ıa de la Rosa et al. (1989) reported a velocity of 0.76 km s−1 for the magnetic cancellations in a quiet region. Next, we adopt the parameters of the cancellation around feature “A” (figure 7d): B ∼ 300(G), ∆Φ ∼ 7×1018 (Mx), ∆t ∼ 7 × 103 (s), L ∼ 1 × 108 (cm), H ∼ 3 × 108 (cm). Then we get E ∼ 5 × 1028 (erg) and v ∼ 3 × 104 (cm s−1 ). It may be noticed that in both case the same values as the released energies are obtained. However, since this is just an order estimation, it does not necessarily mean that almost all of the energy release occurred around feature “A”. We estimated the order of the total kinetic energy contained in the surges as follows. In order to estimate it, the heights of the surges should be measured. We can guess the direction of the surge ejections by assuming that it should be along the ambient magnetic field lines, which are able to be extrapolated from the photospheric magnetic fields. We found that the surges cross the line of sight at about 50◦ and the local vertical line at about 30◦ . Thus, the highest surge reached up to 4.5 × 104 km, when its curvature could be ignored. The total mass of the surges was estimated from the chromospheric density (∼ 1011 cm−3 ) and the volume of the surge at the highest state. In order to bring the total mass up to a height of 4.5 × 104 km an energy of 1027 erg is needed. The surge activities in this study consisted of many single events. Even if we take it into account, the total kinetic energy of these surges is less than the order of 1028 erg. Hence, the magnetic field reconnection can provide sufficient energy to produce these surge activities. 4.4. Intermittency of Activities In figure 7c we can see a nearly constant decrease in the mag˚ images and surge netic flux, though the brightenings in 1600 A activities were localized in time and space, especially during 10:00–10:40 UT. We showed in the previous section that there is a correlation between the changes in the cancellation rates and the surge activities in a limited small area, i.e. in “A” and “B”. However, the change in the cancellation rates was rather small (figure 7d). Since the released energy can be proportional to the cancellation rate, it is questionable that the change in the cancellation rate is directly related to the drastic change

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˚ brightenings. in surges and 1600 A Regarding this question, we mention two possible factors: the “magnetic structure around the dissipation region” and the “non-uniformity in a small-scale structure”. Actually, the cancellation rate around feature “B” is not very large. However, the local magnetic structure around features “A” and “B” must change drastically when these two approach each other. Even if the energy release rate does not vary, the difference in the structure around the energy release site may change the appearance of the events at various wavelength images. Small-scale magnetic structures less than the resolution size of MDI (∼ 0. 6) may have caused the intermittent energy re˚ bright points along the neutral lease. The motions of 1600 A lines may be evidence of such small-scale structures. Such structures must be studied in detail with the Solar-B mission. 4.5. No Brightening in EUV and SXR Images The relations between the surges and higher temperature phenomena, for example “X-ray jet”, have been studied by many authors. Some of them have reported on coincidental events (Shibata et al. 1992; Canfield et al. 1996). In particular, Asai et al. (2001) found a good correlation between Hα surge ˚ bright ejection in a light bridge of a sunspot. and a 171 A However, Rust et al. (1977) showed that most of the Hα surges do not associate with any X-ray emissions. In our case, no bright structure in SXR and no large brightening in EUV was observed during the active phase of the surges around the EFR. Yohkoh / SXT had found many small events. For example, Yoshimura and Kurokawa (1999) reported on SXR brightenings which contained an energy of 1027 erg, which occurred above the EFRs. Also, the thermal energies of X-ray jets were 1027 –1029 erg (Shimojo, Shibata 2000). Assuming an ˚ energy of 5 × 1028 erg was released during the brightest 1600 A emission, the energy release rate of the magnetic cancellation in this study would not be smaller than that of other small X-ray events.

The energy release which drove the surges in this study is considered to have occurred in a high-density layer, maybe the chromosphere. Thus, the magnetic dissipation could not supply enough energy to heat up the ambient plasma with a large mass to the coronal temperature, though there must have been a plasma of 105 K which emitted the C IV line emissions. There is, of course, a possibility that the EUV dark features hid some large brightenings in EUV, and that we failed to find the bright features in SXR due to the low cadence. 5.

Conclusions

We found close correlations both in time and space among the Hα surges, magnetic-flux cancellations, and UV brightenings on a magnetic neutral line at the edge of an emerging flux region. These results support the surge model driven by magnetic reconnection. The energy released through magnetic reconnection could be large enough to produce the observed surge activities. The origin of the intermittent activities of the surge is still unclear. They may provide us with some information about the detailed structure of the energy-release site. Though the plasma could be heated up to 105 K during the surge activities, which is indicated by the UV brightenings, plasma of higher (> 106 K) temperature has not been clearly observed. This may be evidence that the energy release occurred at a high-density layer. The extrapolation of the magnetic field was calculated using a software package developed by T. Sakurai. The authors would like to thank the TRACE, SOHO / MDI, and Yohkoh teams for providing data. SOHO is a project of international cooperation between ESA and NASA. Yohkoh is a mission of the ISAS, and involves many domestic institutions, with participation from the US and UK. We would also thank to A. Title (LMSAL, USA) who supported our visiting to the LMSAL for this work.

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