Formation and Dynamics in an Observed Preeruptive Filament

The formation of filaments/prominences is still a debated topic. Many different processes have been proposed: levitation, injection of cool plasma, merging filaments, and cooling plasma in hot loops. We take the opportunity to make a multiwavelength analysis of the formation of an active-region filament, combining several UV and EUV observations including the new Ne vii 465 Å filtergrams provided by the Solar Upper Transition Region Imager on board the Space Advanced Technology satellite. The filament is mainly observed at the limb for 3 hr. It is progressively formed through a series of stages, including emergence and cooling of hot loops, reconnection between small filaments, material transfer in a large filament channel, and reconnection between filaments and emerged hot loops. From the observations at 465 Å, we find that the new-formed filaments show bright structures as in 304 Å, while the long-lived stable filaments display dark morphology as in 211 Å. This suggests that the plasma around 0.5 MK would be an essential component of new-formed filaments and the material temperature in filaments would be variable during their evolution. The filament formed by the recombination of two filaments and an emerged hot loop finally erupts. After reconnection, the final filament shows a highly twisted structure of both bright and dark strands, which is surrounded by several weak and dispersive looplike structures. This eruptive filament has a complex multichannel topology and covers a wide range of temperatures.


Introduction
Filaments (or prominences) are very common phenomena in the solar atmosphere, which are composed of dense and cool material in hot corona (Labrosse et al. 2010;Parenti 2014;Chen et al. 2020).Until now, several models have been proposed for the formation of filaments/prominences, which are mainly focusing on two scenes: the surface and subsurface mechanisms in single dipole or multiple dipoles (Mackay et al. 2010;Parenti 2014;Vial & Engvold 2015).The subsurface models mainly rely on the emergence of twisted flux ropes or U-type loops, which may rise to coronal heights and drag cool photospheric or chromospheric plasma with it (Lites 2005;Mackay et al. 2010).In the surface mechanisms, many different magnetic activities, such as differential rotation (Dai et al. 2021), flux cancellation (Yang et al. 2016;Yardley et al. 2016), shearing motion (Chae et al. 2001;Yan et al. 2015), magnetic reconnection (Welsch et al. 2005;Zhang et al. 2022a;Li et al. 2022b), converging flows (Yang et al. 2016), flux emergence (Yeates et al. 2008;Yang & Chen 2019), and so on, are proposed to be the primary cause for the formation of filaments.
The topological structure of filaments always changes during their evolution.The filament connections could be changed by the magnetic reconnection in the upper atmosphere during their formation.For example, the interaction between two filaments could reconfigure their structures and produce new ones with different distribution (Guo et al. 2021;Zhang et al. 2021;Zheng et al. 2022).The magnetic reconnection between filament and its nearby magnetic structures could also change the filament connections (Li et al. 2016;Dai et al. 2022).Dai et al. (2022) found that the slow magnetic reconnection between filament and adjoining loops made the filament split into three groups of threads vertically.Yang et al. (2016) reported the rapid formation of a filament caused by magnetic reconnection between two sets of dark threadlike structures.In addition, Zhang et al. (2022b) presented that the filament split into two S-shaped branches and produced a double-decker filament structure as a result of the internal magnetic reconnection.Besides the rearrangement of filament topology, both external and internal magnetic reconnection processes can also lead to the eruption of filaments (Shen et al. 2012;Zhu et al. 2015;Zheng et al. 2019).
The origin and distribution of denser and cooler plasma in the filament or filament channel is another key question during the evolution of filaments.In the injection model, the intermittent reconnection, presented as jets or surges, at the low altitude of solar atmosphere made the cool plasma move upward to the filament channel (Chae et al. 2000;Chae 2003;Tian et al. 2018;Wang et al. 2019;Panesar et al. 2020).In the evaporation-condensation models, the heating in the vicinity footpoints of the filaments would evaporate the chromosphere plasma into the corona and then condensate as cool filament material (Deng et al. 2002;Winebarger et al. 2002;Wei et al. 2020;Yang et al. 2021).Huang et al. (2021) tried to unify the above two mechanisms into one single frame, which depends on the location of the heating in the lower or the upper Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence.Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
chromosphere.Based on high-resolution observations, material has also been found to transfer between the filament and the neighboring filament or coronal structure by magnetic reconnection (Xue et al. 2014;Yan et al. 2020;Li et al. 2022a).
With Hα, UV, and EUV observations, plenty of new features of filaments in the chromosphere and corona have been presented perfectly.The solar transition region (TR) is the transition layer from the collisional and partially ionized chromosphere to the collisionless and fully ionized corona, which is a crucial region for the mass and energy transport in the evolution of filaments and also other solar eruptions.The temperature of TR ranges from 0.02 to 0.8 MK and the emission lines are in the spectral range of 400-1600 Å.Now we have the full-disk image at 304 Å, which is proposed to be emitted from the plasma at about 0.05 MK from the lower TR (0.02-0.1 MK).But in solar activities and flares, it is not a pure TR line.For the upper TR (0.1-0.8 MK), the direct imaging observation has never been routinely obtained (Tian 2017).The Solar Upper Transition Region Imager (SUTRI) on board the Space Advanced Technology demonstration satellite (SATech-01; Bai et al. 2023) uses a unique filter centered at the Ne VII 46.5 nm (465 Å) line to obtain a new observing wavelength of the Sun, which could fill in the current observation gap at the range of 0.1-0.8MK.It was launched to a Sun-synchronous orbit at a height of about 500 km in 2022 July.Ne VII 46.5 nm is proposed to be emitted by the plasma at a temperature regime of 0.5 MK (Tian 2017), which could cover the space from the lower TR to the corona.Thus, the observations at 465 Å of SUTRI provide a great opportunity to learn the consecutive material transfer of filaments from the chromosphere and the TR to the corona.And it could present some new observation features to understand the temperature distribution and evolution of filaments.
On 2022 September 23, SUTRI observed an eruptive filament in NOAA active region (AR) 13110 at the east limb of the Sun.We made a multiwavelength analysis on the formation of the AR filaments, combining with other UV and EUV observations.The large-scale eruptive filament has been recorded to evolve from several small filaments, which experienced a series of complicated processes of structure reorganization and material transfer.The new observations at 465 Å present many new features during the evolution of AR filaments.The paper is organized as follows: Section 2 shows the observation features.The analysis and results are included in Section 3. Finally, the summary and discussion are shown in Section 4.

Observations
The eruptive AR filament appeared in AR 13110 at the east limb of the Sun on 2022 September 23, which is accompanied by an M1.7-class flare at 17:58 UT and a fast coronal mass ejection (CME) consequently.We investigate the formation and dynamics of the eruptive filament from 14:00 to 17:45 UT.During this period, GOES soft X-ray flux increases to the C-class level and records a C5.6-flare from 14:26 to 14:59 UT.We use UV and EUV observations from the Atmospheric Imaging Assembly (AIA; Lemen et al. 2012) on board the Solar Dynamics Observatory (SDO; Pesnell et al. 2012).The full-disk observations of AIA provide multiple and simultaneous EUV images of a 12 s temporal cadence (24 s for UV images) and an image scale of 0 6 pixel −1 .The observations at 193 and 304 Å from the Extreme Ultraviolet Imager (EUVI) of the Solar Terrestrial Relations Observatory (STEREO-A; Howard et al. 2008) also record the whole evolution of this AR filament.The separation angle to Earth of STEREO-A on 2022 September 23 is about 17°.953, which observed this AR in a different viewing angle.The EUVI observations have a 150 s cadence and an image scale of 1 6 pixel −1 .SUTRI takes the full-disk solar images at Ne VII 465 Å with a 30 s temporal cadence and the field of view of SUTRI image is 41 6.The image scale is about 1 22 pixel −1 and the spatial resolution is about 8″.Due to the Earth eclipse time of the satellite accounts for about 1/3 of SATech-01ʼs orbit period, there are several data gaps for the observations at 465 Å.Even so, SUTRI covers almost all of the key phases in the formation of the large eruptive filament.In our multiwavelength analysis of this event, we convert the coordinate from the detector coordinate system of EUVI and SUTRI images to the coordinate system of AIA/SDO.The images from different satellites could present the same spatial location of the AR.The observations of EUVI are in a different viewing angle from AIA and SUTRI, which enables us to study the topology structure and material distribution from both the top and side view of the filaments.

The New-formed Filament F1 after the Cooling of Hot
Loop L1 Due to its separation angle to the Earth (about 17°.953 on 2022 September 23), STEREO-A could record this limb AR earlier than AIA/SDO (193, 304, and 1600 Å) and SUTRI (465 Å).From EUVI observations, it is found that there is a small stable filament in AR 13110 in the early phase.It appeared at about 08:00 UT and evolved gradually until about 14:00 UT.During this period, it showed no obvious change on both length and topology.Figures 1(a)-(h) presents this stable filament (labeled as F0 with yellow arrows) at 195,193,304, and 465 Å.At 1600 Å, F0 could not be distinguished (Figures 1(i)-(j)).From 14:09:28 UT, a tiny bright emission appeared at the north area of AR 13110 in 1600 Å images.Then a bright looplike structure (labeled as L1 with the green arrow in the left panel of Figure 1) grew and rose upward gradually.At 465 Å, the whole bright loop could be also distinguished.This new loop might be related to flux emergence, which cannot be a quantitative description with the magnetogram because this AR is too close to the limb.Several minutes later, L1 began to expand and diffuse.It turned into a dark structure at 195 and 193 Å as seen in Figures 1(b) and (d), but presented as a relatively weak bright structure in 304, 465, and 1600 Å images (Figures 1(f), (h), and (j)).At this time, a new filament (labeled as F1 by yellow arrows) was formed.This AR had two small-scale filaments: the sigmoid-like filament F0 and the arch-shaped filament F1.
We make a time-distance plot along the slice AB in 1600 Å images (dotted line in Figure 1(i)), which follows the moving direction of L1. Figure 2(a) shows that the compact bright loop rose up very fast at first.Then it slowed down gradually and the emission there was also weakened.We calculate the differential emission measure (DEM) of the selected region in L1 at 14:22 UT (the black box B1 in the left panel of Figure 1).The result shows that the temperature of B1 is about 6.5 MK (Figure 2(b)).Thus, the new emerged loop L1 is a typical hot loop.We select box B2 (white box in the right panel of Figure 1) in L1 and F1 to study the emission there.B2 is shifted southward from B1 to exclude the mixed area of F1 and F0. Figure 1(d) shows the locations of B1 and B2.The It can be seen that the emission at all wavelengths was enhanced impulsively from 14:19 to 14:24 UT when L1 rose up.This indicates that the emerged loop L1 contains both hot and cool plasma.Due to expansion and diffusion, the emission at all wavelengths decreased.The emission at 131, 304, and 465 Å decayed later than that of 193 and 211 Å.After 14:24 UT, the emission of B2 at 131, 304, and 465 Å was smoothly enhanced.The emission at 211 and 193 Å was weaker than the background, and the emission at 304 Å presents a slower decay than that of 465 Å.This kind of slower decay of cool plasma emission could be the result of plasma cooling in L1.

The Reconnection of Two Small-scale Filaments
After about 30 minutes, F1 had a larger-scale and more compact structure.The underlying filament (F0) remained an S-shaped structure from the EUVI images at 195 Å (Figure 3(a)).At about 15:10:00 UT, F0 expanded upward and interacted with F1.Then bright emission appeared along F0, denoted by white solid arrow in Figures 3(b) and (c).The bright emission first appeared at the top of F0 (Figure 3(b)) and then covered the whole structure (Figure 3(c)).About 3 minutes later, two new filaments were formed, which are denoted by F2 and F3 with yellow arrows in Figure 3(d).F3 presented as a larger sigmoid structure.The small and archlike dark structures are denoted as F2.At this moment, this AR had two filaments: the overlying larger-scale filament (F3) with a sigmoid configuration and the underlying small filament (F2) with the archlike shape.

The Material Transfer in a Large Filament Channel
At about 15:40:16 UT, a bright jet appeared around the north area of the AR as seen in 1600 Å images (denoted by the green arrow in Figure 4(n)).At 304,193,and 465 Å (Figures 4(a), (d), (g), and (j)), this jet could be found under F3.In 1600 Å images, we found the ejecta moved upward intermittently and there were many bright structures above the jet (Figure 4(n)).In 193, 304, and 465 Å image, some dark structures could be also distinguished above the bright jet.With the continuous material injection near the jet, the bright/dark structures began to grow and extend southward.In the middle panel of Figure 4, it is found that the main part of a large filament was filled and built, which was denoted by F4 with yellow arrows.In Figure 4(p), we found that the leg of filament F4 had a brighter and thinner structure than the overlying main body.At 16:21:00 UT, the whole structure of F4 was outlined at all wavelengths, which was more than 100″ (the right panel of Figure 4).Simultaneously, F2 grew and expanded southward and the structure of F3 was partly destroyed.
We use the AIA/SDO data at 1600 and 211 Å to make timedistance plots for the north leg (slice CD) and the overlying main body (slice DE) of F4 from 15:19 to 16:49 UT.The locations of slice CD and DE are shown in Figure 4(q) with red dotted lines.Slice CD originates from the footpoint and ends at the top of the leg.Slice DE starts from the top of the leg and stops at the south end of the main body.We set the width of slice CD to be 5 pixels and that of the slice DE to be 10 pixels to cover the whole filament structure.From the time-distance plot of slice CD at 1600 Å in Figure 5(a), we find that the bright jet (denoted by white arrow) first appeared at about 15:40 UT.After the jet, several bright beams of material injection shifted upward along the north leg (denoted by yellow arrows).We select one beam of bright shifting structure (green dotted line) and find the linear fitting speed is about 85 km s −1 .
In the time-distance plots of slice DE at 1600 Å, two branches of bright structure are found to extend from D to E ("I" and "II" in Figure 5(b)).In 304 Å, bright shifting structures also moved from D to E in F4, but there is no clear distinction of two branches.At 193 and 211 Å, the time-distance plots of slice DE present more detail (Figures 5(d)-(e)).In the first branch, several pieces of unidirectional dark channels are found to extend southward from D to E along the main body of F4.These dark channels intermittently appear and shift southward at the similar speed.In the initial position of slice DE, a darker compact structure does not change size, which corresponds to the stable structure of F2.In the second branch, the darker structure of F2 is enlarged due to its continuous expansion, which can be also shown in the images at 304 Å in Figure 4(c).In the south part of slice DE, we also could find the dark structure shifting southward as before.
We choose one typical sample of these shifting structures (green dotted lines in Figures 5(c)-(e)) to estimate its shifting speed.This shifting channel originates at 15:58:50 UT and shifts from the point of 48″ to the point of 116″ along slice DE.It is found that the dark structure at 193 and 211 Å and the bright structure at 304 Å present the same shifting speed of about 82 km s −1 .But at 1600 Å, we cannot distinguish the same structure as that at 193 and 304 Å.We choose part of the sample, which originates from the same time and location as that of 304, 193, and 211 Å and obtain the shifting speed of about 80 km s −1 .It can be seen that the speed of the material injection in the north leg (slice CD) and structure extension in the main body (slice DE) shows the similar moving speed, which suggests that the material in F4 is injected intermittently from the root by the interaction between the leg of F4 and jet and then transferred into the whole body.

The Recombination of Active-region Filaments
The material transfer in F4 lasted for more than 1 hr and during this period, F2 and F4 expanded to a larger size.F2 was still under F4, which can be seen in 304 and 195 Å images of EUVI (Figures 6(d) and (g)).At this moment, the structure of filament F3 cannot be distinguished completely.From 17:11:52 UT, another new loop emerged under F2 and F4, which is denoted as L2 with the green arrows in the middle panel of Figure 6.When this loop encountered with the overlying filaments F2 and F4, complex reconnecting process took place among them.Simultaneously, the whole filament system grew upward and expanded rapidly.Finally, these structures formed a larger twisted filament after reconstruction, which is denoted as F5 in the right panel of Figure 6.In 304 Å images of EUVI, we find F5 had a very compact core, which was twisted with bright and dark channels.Surrounding this core, there were several weak and dispersive dark structures.During the expansion and reconstruction process, bright emission is found to travel form the interaction site to the south root in a twisted way along the core.After continuous expansion of F5, the surrounding weak structures were stretched and destroyed at about 17:50:00 UT.Then the twisted filament erupted upward much faster and the north leg deviated from the root.An M1.7-class flare took place near the north root area and a fast CME was recorded by LASCO later.

Summary and Discussion
With the observation data of SUTRI, we obtain the image of the AR filaments at a new wavelength of 465 Å. Combining with UV and EUV observations form AIA and EUVI, we investigate the formation and dynamics of AR filaments in AR 13110 on 2022 September 23.In the initial phase, this AR only has a stable small-scale filament (F0).Then this AR experiences a series of complex processes, including emergence of small hot loop L1, cooling and formation of filament F1, reconnection between filaments F0 and F1, material transfer inside the large filament channel F4, and the recombination of filaments F2 and F4 and another new loop L2.Finally, the twisted filament F5 is reconfigured and erupts outward.smooth peak, the emission in F1 at 465, 304, and 131 is enhanced obviously.But there is no increase for the emission at 211 and 193 Å of hot plasma.This may suggest that the hot loop begins to turn into a new filament (F1), which is shown as a bright structure at 1600, 304, and 465 Å and a dark feature at 193 and 211 Å.Thus, the temperature of the cool plasma inside the new-formed filament ranges from 0.01 MK (1600 Å) to 0.5 MK (465 Å).The emission at 304 Å has a similar evolution to that of 465 Å, but it has a much slower decay phase.The decreased emission at 465 Å indicates the continuous cooling of plasma around 0.5 MK, which increases the amount of the cooler plasma and produces a slower decay at 304 Å.But for the stable filament F0 and the leg of filament F3, they show as dark structures in 465 Å images.The lower-temperature plasma inside would produce strong absorption lines in 465 Å observations.Thus, the plasma in the long-lived filament would contain cooler plasma than the new-formed filaments (i.e., F1 and F4).Hence, the temperature of material in filaments would be different during their evolution.
During the formation of this eruptive filament, material transfer is another crucial feature.Before the appearance of the tiny jet near the north leg of F4, we could not find any observation feature of F4.When the jet comes up, both the bright structure at 1600, 304, and 465 Å and the dark structure at 211 and 195 Å of F4 present a continuous and synchronous extension from the north to the south root.The material injection in the north leg presents a speed of about 85 km s −1 , and the material in the main body shifts at about 82 km s −1 .These features indicate that the plasma flows produced by the reconnection between the jet and the leg of F4 transfer along the empty channel of F4.The plasma temperature of the injected material also ranges from 0.05 to 0.5 MK, or even to a higher level.Thus, the new observation of 465 Å provides us a new characteristic of filaments, i.e., the plasma at the temperature of about 0.5 MK could be transferred from the lower atmosphere to the corona along the filament channel.The plasma around 0.5 MK may be the essential component of filaments, in particular in the new-formed filaments.Therefore, we may propose that the material in filaments would cover a wider temperature range, which could extend to the range of megakelvins.
In this AR, the final eruptive filament (F5) is also a newformed one, which is recombined by three structures of different scales.When the small new emerged hot loop L2 rises up, it encounters and interacts with filaments F2 and F4.All of them have a similar orientation.Then they reconnect and form a new filament F5.Before recombination, all of them do not show an obvious twist structure.After reconnection, F5 presents a highly twisted structure with both dark and bright channels at all wavelengths, surrounded by several weak and dispersive looplike structures shown at 304 Å.The weak surrounding looplike structures do not show any twist feature and they are stretched and broken by the eruptive twisted core.The reconstruction of F5 could be an extremely complicated process and the reconnection process may promote the twist of new filament core.Thus, the classical single flux rope or U-type structure was unsuitable for F5, which needs a more complex topology to be composed of more than one channels.
In addition, F5 also presents a complex pattern of temperature distribution.All the images in the right panel of Figure 6 present both bright and dark structures along the twisted core except for that at 1600 Å.The bright emission travels in a spiral way from the interaction site to the south leg.The bright structures at 211 and 193 Å or even at 94 Å indicate the presence of a component of hot plasma above 1 MK in the twisted core of F5, which could be heated near the reconnection region and then moves along the twisted structures.Simultaneously, F5 also contains plasma at lower temperatures, which is represented by absorption dark structures.The temperature of the material in F5 may cover a wider range from 0.01 MK (1600 Å), 0.05 MK (304 Å), 0.5 MK (465 Å) to above 1 MK (195 and 211 Å).Therefore, we conclude that the temperature distribution in eruptive filaments is more complicated than the temperature distribution in stable filaments.The inside or outside reconnections during their eruption process would produce hot plasma (Huang et al. 2019), which would enrich the temperature components of plasma in filaments.
In conclusion, the new observations at 465 Å show us a new temperature component of the AR filaments, which helps us to understand the multithermal distribution of the new-formed filaments.The emergence of hot loops seen in the chromosphere and corona is slightly imprecise.The material injection process, the complicated reconnection process of filaments (F2 and F4), the new hot loop L2, and the formation of the twist core of F5 is conjectured by the emergence of new magnetic flux.However, due to the location of this event at the eastern limb, we cannot obtain reliable photospheric magnetograms to study the evolution of the magnetic field in AR 13110.Our main result is based on the formation of filament F1 from the hot loop (L1) after cooling and the formation of F5 driven by the rise of the hot loop L2.This limb event enables us to study the formation of filament both by the cooling process and by material transfer from the low atmosphere to the whole body of the filament in the corona.

Figure 1 .
Figure 1.The multiwavelength observations of the emerged hot loop (L1, denoted by green arrows in the left panel), the new-formed filament (F1, denoted by yellow arrows in the right panel) and the preexisting stable filament (F0).After coordinate transforming, the images at 195 Å of EUVI and 465 Å of SUTRI show the same spatial location as the AIA images.The boxes B1 (black) and B2 (white) are the selected regions to calculate the temperature of L1 at 14:22 UT and the emission light curve of L1 and F1 from 14:18 to 14:34 UT.The white dotted line in (i) is the slice AB, which follows the rising direction of L1.

Figure 2 .
Figure 2. (a) The time-distance plot of slice AB at 1600 Å.The bright emission along slice AB emerges and moves upward.(b) The DEM result of B1 in L1 at 14:22 UT.The typical temperature of L1 is about 6.5 MK.(c) The normalized flux of B2 of L1 and F1 at 131 Å (blue), 171 Å (green), 193 Å (red), 211 Å (orange), 304 Å (black), and 465 Å (blue dotted line).The first impulsive peak from 14:19 to 14:24 UT corresponds to the emergence of L1.During the second smooth peak, F1 is formed after the cooling and expansion of L1.The emission at 304 Å presents a slower decay than that of 465 Å.

Figure 4 .
Figure 4. Material transfer from the north root to the whole body of the new large-scale filament (F4).F2 and F3 are denoted by yellow arrows in EUVI images at 304 Å.The EUVI images at 304 Å, SUTRI images at 465 Å are transformed into the AIA coordinate system and they cover the same region.Left panel: in the 1600 Å image, a bright tiny jet (denoted by the green arrow) appears near the north root of F4.Middle panel: part of F4 is built of a bright structure at 304, 465, and 1600 Å and a dark one at 193 Å.Right panel: the whole structure of F4 is formed.The slice CDE (red dotted line) is along the north leg and the main body of F4.

Figure 5 .
Figure 5. (a) Time-distance plot of slice CD at 1600 Å.The jet and material injections are denoted by white and yellow arrows, respectively.(b) Time-distance plot of slice DE at 1600 Å.Two branches of bright shifting structure are denoted by I and II.(c) Time-distance plot of slice DE at 304 Å. Bright shifting structures are shown in F4 and the shifting speed is about 82 km s −1 .(d)-(e) Time-distance plot of slice DE at 193 and 211 Å.Many tiny dark channels appear intermittently in F4 and shift at the speed of about 82 km s −1 .

Figure 6 .
Figure 6.Observations of the recombination of filaments F2 and F4, the new emerging loop L2, and the eruptive twisted filament F5 in multiwavelengths.The EUVI/ STEREO images at 304 and 195 Å are transformed into the the Earth-based coordinate, which show the same spatial location as AIA at 211 and 1600 Å. Left panel: F4 and F2 expand to a larger size.Middle panel: a new loop (L2, denoted by green arrows) emerges under the filaments F2 and F4, and three structures interact with each other.Right panel: the eruptive filament (F5) has a twisted structure mainly visible in emission.In 304 Å we note some dispersive looplike dark structures.