SOHO/SUMER Observations of Transition Region Explosive Events in Prominence

ZHANG Min， WANG Dong， DENG Yan

1. Department of Mathematics and Physics, Anhui Jianzhu University, Heifei 230601， China

2. School of Earth and Space Science, University of Science and Technology of China, Heifei 230026, China

SOHO/SUMER Observations of Transition Region Explosive Events in Prominence

ZHANG Min1,2， WANG Dong1,2， DENG Yan1

1. Department of Mathematics and Physics, Anhui Jianzhu University, Heifei 230601， China

2. School of Earth and Space Science, University of Science and Technology of China, Heifei 230026, China

Explosive events (EEs) are small-scale dynamic phenomena often observed in the solar transition region (TR). EEs are characterized by non-Gaussian and broad profiles with enhancements in the blue/red wings with an average line-of-sight Doppler velocities of ～100 km·s-1. They have a small spatial scale of about 1 800 km and a short lifetime of about 60 s on average. EEs are often found to be associated with magnetic cancellation and reveal bi-directional flows with high velocities comparable to the local Alfvén velocity; they are generally regarded as the consequence of small-scale fast magnetic reconnections. Since the launch of SOHO spacecraft, the SUMER (solar ultraviolet measurements of emitted radiation) spectrograph has been widely used to study EEs. With high spatial and spectral resolution, and wide spectral coverage, SUMER was a powerful tool of ultraviolet spectroscopy and it has greatly increased our knowledge of EEs. Relationship between EEs and other small-scale events observed in the transition region, such as blinkers and EUV spicules have also been investigated during the SOHO era. However, the association between EEs and large-scale events such as prominence remains unclear. A sit-and-stare mode observation made by SUMER is selected for the study. We investigate the properties of EEs in a prominence. EEs are identified with analysis of the width of Si Ⅲ line (111.3 nm). The Si Ⅲ lines with a width greater than three standard deviations (3σ) were singled out for further visual inspection to finally determine the occurrence of EEs. It is found that the vast majority of explosive events concentrate in the bright knots of the prominence. EEs in the core of the prominence occur repetitively with a period of about 20 mins. It is proposed that the explosive events caused by small-scale fast magnetic reconnections are triggered by magnetic flux loops in the core of prominence. The blue shift of the explosive events is significant and possibly related to the initiation of a CME.

Sun: Prominence; Sun: Explosive events; Sun: UV line

Introduction

Explosive events (EEs) are small-scale dynamic phenomena often observed in the solar transition region (TR). They can be observed in far and extreme ultraviolet (FUV/EUV) spectral lines with a formation temperature ranging from 1×104～5×105K and best seen in typical transition-region lines (e.g., Si iv; C iv; O vi). As turbulent events and jets, they are characterized by non-Gaussian and broad profiles with enhancements in the blue/red wings, especially with the line-of-sight Doppler velocities of the blue wing reaching 100 km·s-1[1-2]. EEs are often found to be associated with magnetic cancellation and reveal bi-directional flows with high velocities comparable to the local Alfvén velocity, they have been suggested to be a consequence of small-scale fast magnetic reconnections[3-5].Analysis of the energetics of EEs indicates that the energy flux released by these events might be insignificant for heating the solar atmosphere globally[6]. However, the mass flux carried by such events could be a significant source of the solar wind.

Prominences are very splendid phenomena in the solar atmosphere. They can be classified into quiet prominences (QPs), active prominences (APs) and eruptive prominences (EPs). Eruptive prominences are usually the most intense ones with the ascending speed of several hundred kilometers per second[7]. The close relationship between EPs and coronal mass ejections (CMEs) has been discussed and reviewed by many authors[8]. Prominences have fine structures such as thin threads with about 210 km in width and 3 500～28 000 km in length. They are also highly dynamic phenomena. The velocities of plasma in prominences are about 2～35 km·s-1in Hα line and slightly higher in EUV lines. Since prominences are cool and dense plasma in the corona, they are suggested to be the result of the injection of chromospheric plasma through siphon-effect, or the condensation of coronal plasma by thermal instability[9-10].

In the past, EEs were mainly studied in the quiet-Sun (QS) region on the solar disk, while their properties above limb have been poorly investigated. There have been a lot of investigations of the association between EEs and other TR small-scale events such as blinkers and EUV spicules. However, the association between EEs and large-scale events such as prominence remains unclear. In this paper, we present results of EEs in the prominenceobserved by SOHO/SUMER for the first time.

1 Observations and data analysis

In this paper, we analyze a data set taken by SOHO/SUMER[11-12]from 20:50 UT on 25 Sep 2000 to 8:17 UT on the next day. A slit with length of 300″ and width of 4″ was pointing at a prominence above east limb of the Sun (x=-980″,y=-250″). In Fig.1, we overplot the sumer slit on

Fig.1 Location of the SUMER slit (the narrow black vertical bar) on images of He Ⅱ (30.4 nm) (upper panel) and Fe Ⅻ (19.5nm) (lower panel)observed by SOHO/EIT. In the upper panel, the prominence periphery is showed in white-box, the bright spot is in green-box, the strong jet is showed in blue-triangle in the first map while the bright surge is showed in blue-box in the last map

the He Ⅱ(30.4 nm) and Fe Ⅻ(19.5 nm) images from SOHO/EIT. Highcadence SUMER observations were carried out with an exposure time of 162 s in the wavelength range between 109.8 and 113.8 nm. We select the strong TR Si Ⅲ line (111.3 nm, ～5×104K) for this study. As a common practice, we applied the standard procedure for correcting and calibrating the SUMER raw data which includes decompression, field-field, dead-time, local-gain and geometrical corrections. The method used to deduce the line parameters (line radiance, central position of the spectral line and width) is described in detail by Dammasch[13]. In order to deduce the Doppler velocity more reliably, an additional line-position correction was performed to remove spurious spectral line shifts caused by thermal deformations of the instrument and the residual errors systematically varying along the slit. As previous studies have shown that chromospheric lines have very small systematic line shifts on average in SUMER observations, we use cold chromospheric C Ⅰ (110.9 nm) line as a reference to derive the rest wavelength of Si Ⅲ line in the prominence[1,14]. In this paper, EEs were identified by Si Ⅲ profiles. We disregarded the noisy profiles with a peak intensity below the half-peak intensity of the average profile. Then the profiles with a width greater than three standard deviations (3σ) were singled out for further visual inspection to finally identify EEs. Our method is similar to those used by Teriaca[1].

2 Results

Fig.1 shows EIT images with a cadence of 6 hours in the He Ⅱ (30.4 nm) and Fe Ⅻ (19.5 nm) lines. In the upper panel we can see an arc-like prominence with the length of 300″ locating above the east limb and extending to the south. The central position of its original footprint on the disk is about 0″ in Y (north-south)- direction. The SUMER slit above the solar limb was crossing the prominence exactly. To the right of the slit there is a singular bright spot (green-box in the top left of Fig.1) on the disk and the bright spots move westwards with solar rotation. Strong jets (blue-triangle in the top left of Fig.1) inject into the corona straightly from the southeast corner of the solar disk at 13:18 UT Sep. 25 and existed till 7:17 Sep. 26. The prominence periphery (white-box in the top left of Fig.1) brightens strongly and the footprint of the prominence on the disk is not distinct at 19:17 UT Sep. 25 which is close to the beginning of SUMER observation. Six hours later, the arc-like prominence erupts partially with some structures still visible. Loops appear across the SUMER slit, which can also be seen in Fe Ⅻ 195 image. The prominence fades gradually and two bright surges (blue-box in the top right of Fig.1) burst through the slit at 7:17 UT Sep. 26. The EIT Fe Ⅻ 195 images with a cadence of 6 hours are also showed in the lower panel of Fig.1. The singular bright spot and the strong jets can also be seen in Fe Ⅻ (19.5 nm)line and exist throughout the entire observation duration. At 1:11 UT Sep. 26, large amount of plasma ejected into higher layers and the coronal loops appear simultaneously. A blob-like CME observed by LASCO C2 at 2:50 UT Sep 26 has been reported to be associated with this prominence[15].

The intensity evolution of the prominence obtained by the Si Ⅲ line is shown in the upper panel of Fig.2. The intensities are shown in logarithmic scale to increase the contrast and enhance the bright area with high intensity. Because our observation was taken in sit-and-stare mode, the figures show the evolution in time of the prominence which falls into the SUMER field-of-view. The bright knots with enhanced emission around -150″ are identified as the core of the prominence. The bright knots have a few ten arcsecs along the slit. They mainly survive before 1:25 UT Sep 26, although they can be seen again occasionally after 04:20 UT Sep 26 with much smaller size. The fine thread-like structures of the prominence are varying with time along the slit, which can also be observed by Si Ⅲ line. The Doppler shift of the prominence is shown in the middle panel of Fig.2. The blue shift (negative values) represents flows moving to the observer, while the red shift (positive values) indicates flows moving away from the observer. By calibrating with the cold chromospheric C Ⅰ (110.9 nm) line and eliminating the effect of solar rotation, the line-of-sight velocity of Si Ⅲ in the prominence is blue shifted and its average blue shift is 2.76 km·s-1. In the Dopplergram, the dynamics of prominence can be seen and some significant blue patches can be found. The blue patches with relative higher velocity concentrate from -200″ to -130″ along the slit. The velocity of the thread of the prominence alternates between blue and red. In order to facilitate the study, the Dopplergram map is overlaid by the black contours of the intensity to outline the bright knots of the prominence. We found that most of the bright knots appear in regions where large blue shifts are found. The width of the Si Ⅲ line is also showed in the bottom panels of Fig.2. The line width of the bright knots at around -150″ along the slit, which is identified as the core of the prominence, is usually large, while the bright knots in other regions has smaller line width.

The identified events are marked as the black “+” in intensity maps of Si Ⅲ line (see Fig.3). 337 pixels with EE-like profiles were detected in the prominence. It is obvious that the black “+” is not randomly distributed. Some of them can form a small group and neighboring EE pixels in each spectral line can be regarded as a single event. The occurrence rate of EEs in the prominence is about 4×10-16m-2·s-1, which is comparable to the occurrence rate obtained by Teriaca[1]in a quiet sun (QS) region and Zhang[15]in both QS and polar coronal hole (PCH) region. We find the vast majority

Fig.2 Intensity(logarithmic scale), Doppler shift and line width maps of the Si Ⅲ line.

Fig.3 EEs are marked with black “+” in the intensity map of the prominence seen in the Si Ⅲ line.

of the EEs concentrate in the region from -200″ to -130″ along the slit and most of the EEs lie in or on the edge of the bright knots. Some time, EEs occur repeatedly in the same region. Four regions marked with A,B,C and D in which EEs recur are showed in Fig.3. The center of A-region is -150″ and the repetitive occurrence here lasts for 110 mins. We find seven EEs during this period and the quasi-periodicity is about 16 mins. The center of B-region is slightly lower than that of A-region and the repetitive occurrence also lasts for 120 mins. In this region, we find six EEs and the quasi-periodicity is about 22 mins. The central positions of C and D regions are approximately the same as that of A-region and there are six and five EEs respectively. The quasi-periodicity of EEs is about 21 mins in both C and D regions.

Fig.4 shows three typical EEs with bi-directional flows. Each spectrum of EEs has several arcsecs along the slit and all spectrums have a strong intensity in the wings. The positions of these EEs are around -170″ along the slit. The profile of each EE with bi-wings which have well-defined bursting velocities is greater than 150 km·s-1. The average line-of-sight Doppler velocities of the wings are up to ～150 km·s-1. The shift of the EE1 obtained by single-Gaussian fitting is -19.04 km·s-1and the blue shift is obvious. In contrast, EE2 and EE3 have very small shifts, with the velocities of 3.03 and 5.23 km·s-1respectively. Although with different shifts, the profiles of all events have non-Gaussian shapes and obviously enhanced wings in both the red and blue sides.

Fig.4 Three slit spectrum are showed in the Si Ⅲ line, two solid black lines marked the lacation of the EEs

3 Conclusions and Discussions

We study the TR EEs in a prominence above the east limb. The prominence core is located from -200″ to -130″ along the slit and erupts at about 1:17 UT Sep. 26. A small flare and CME are observed subsequently. The blue-shift of the prominence core observed by the Si Ⅲ line is significant. But when we show the time evolution of the prominence, the red-shift of the prominence is also observed in the core. As the prominence is arclike and the main part of it is along the slit, the red-shift may be characterized as the visual-direction component of the downflow. This is consistent with the scenario that some prominence materials fall back to the disk after ascending to a certain height. Through diagnosing the line width of the profile, we select 337 EE pixels. The occurrence rate of EEs in the prominence is 4×10-16m-2·s-1which is similar to that in QS. Most TR EEs with high emission often occur in or on the edge of the core of the prominence. We studied three typical bi-directions EEs in more detail. They all have broad width and the shift of each two wings can reach 150 km·s-1.

The repetitive occurrence of EEs was first observed in regions undergoing magnetic cancellation by Dere[16], which had been confirmed by other authors. Innes[17]found that EEs could occur repeatedly several times and lasts about 30 minutes. Ning[3]found that the repetitive occurrence of EEs was not random, but had a quasi-periodicity of three or five minutes. In the coronal hole boundary, Doyle[5]found that the period of the EEs can be increased from three minutes at beginning to five minutes in the end. In our data, the period of the repetition is around 20 min, which is larger than five minutes but is in the range of the quasi-periodicity of the high-speed fine-scale jets (see, e.g., Tian et al., 2011)[18]. These high-speed fine-scale jets might be the coronal counterpart of EEs. EEs repetitively occur in the same region and last for several hours in the prominence. We select four in the time series observation. Although the positions of the four repeated regions with higher emission are slightly difference, they are all in the position from -200″ to -130″ along the slit. As mentioned above, the core of the prominence is in the region. And the same time, we observed the loops appear in the same region from EIT images after the prominence eruption. As we know, EEs are associated with the cancellation of the magnetic flux and the emerging flux can provide the preconditions of magnetic reconnection. Chen et al.[14]described the physical mechanism detailedly and inferred that the reconnection between the emerging flux and the pre-existing coronal magnetic field causes the formation of Hα surges and the oscillation of the prominence which finally triggering the eruption of the prominence. The reconnection appears intermittently which is implied by the repetitive Hα surges. Here, we speculate that the repetitive reconnection can be triggered by some kind of wave mode (i.e. the kink mode) of the flux tubes. Then plasma is heated and accelerated, and we not only diagnosis EEs but also find two bright surges occur from EIT images.

EEs are mainly associated with blue shift in the prominence. Although the outflow speed derived from a single Gaussian fit is roughly in the range of 10～40 km·s-1, the effluent plasma of their wings are splendidly. From the observed phenomenon, we assume that when EEs occur in the prominence, the small-scale magnetic reconnection events generate outflow and the high speed flow can be easily observed. In our study, the high speed flow generated from the EEs in the erupted prominence might play a role in the initiation of the CME or contribute to the solar wind streams following CMEs.

Anyway, our work is just the beginning. New imaging and spectroscopic data, especially high resolution and multi-band spectrum data are needed to understand the ralationship between prominence and EEs in the future.

Acknowledgements： The SUMER project is financially supported by DLR,CNES, NASA, and the ESA PRODEX Programme (Swiss contribution).SUMER is an instrument onboard SOHO, a mission operated by ESA and NASA. We thank Dr. H. Tian for the helpful comments.

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O657.3

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太阳爆发日珥内双向喷流事件的紫外光谱研究

章 敏1,2， 王 东1,2， 邓 燕1

1. 安徽建筑大学数理学院， 安徽 合肥 230601

2. 中国科学技术大学地球与空间科学学院， 安徽 合肥 230026

太阳双向喷流事件是过渡区重要的小尺度现象之一。 双向喷流事件的光谱特征是强的展宽和非高斯形状。 当双向喷流事件发生时， 光谱像的红、 蓝两翼分别或者同时明显增强， 其相应的多普勒速度可达100 km·s-1以上。 双向喷流事件的平均尺度约1 800 km, 寿命约60 s。 双向喷流事件出现在磁对消区附近， 且其速度与当地的阿尔芬速度相当， 普遍认为其产生机制为小尺度快速磁重联。 对其系统、 全面地研究始于SOHO时代。 SOHO/SUMER具有高时空和谱分辨率、 宽的谱线覆盖， 其观测的光谱数据为探究双向喷流事件提供了有力的光谱学诊断工具。 双向喷流事件及其他过渡区小尺度现象的相互联系已被广泛研究， 但双向喷流事件与日珥及其精细结构的关系研究还很少。 文章通过SOHO/SUMER的Si Ⅲ谱线的定点观测， 再现了爆发日珥演化的强度﹑多普勒速度和宽度演化图。 通过Si Ⅲ谱线分析， 找出宽度大于三个标准偏差的Si Ⅲ谱线， 然后进行视像筛选出双向喷流事件， 最终在爆发日珥中诊断出多个双向喷流事件， 且大多数的双向喷流事件以准周期20 min重复出现在爆发日珥的中心区域。 通过讨论， 认为日珥中心磁流管之间的磁重联导致了双向喷流事件的重复出现， 双向喷流事件产生的高速等离子体流可能是日面物质抛射的一部分， 或是跟随日面物质抛射的太阳风的一部分。

太阳日珥； 双向喷流事件； 紫外光谱

2015-10-27，

2016-02-25)

2015-10-27; accepted： 2016-02-25

the National Natural Science Foundation of China(NSFC) under contract 41304134, the foundation for Young Talents in College of Anhui Province (2013SQRL044ZD), colleges and Universities Natural Science Foundation of Anhui Province (KJ2016JD18), the doctoral foundation of Anhui University of Architecture (K02654) and CAS Key Research Program KZZD-EW-01

10.3964/j.issn.1000-0593(2016)08-2679-07

Biography： ZHANG Min, (1980—), female, associate professor in Anhui Jianzhu University e-mail: chengzm@ustc.edu.cn