11institutetext: Yunnan Observatories, Chinese Academy of Sciences, Kunming
Yunnan 650216, PR China
11email<EMAIL_ADDRESS>22institutetext: CAS Key Laboratory of Solar
Activity, National Astronomical Observatories, Beijing 100012, PR China
33institutetext: Center for Astronomical Mega-Science, Chinese Academy of
Sciences, 20A Datun Road, Chaoyang District, Beijing, 100012, PR China
44institutetext: Department of Physics, Yunnan University, Kunming 650091,
People’s Republic of China 55institutetext: School of Earth and Space
Sciences, Peking University, Beijing 100871, People’s Republic of China
66institutetext: School of Physics and Astronomy, Yunnan University, Kunming
650050, People’s Republic of China

# A formation mechanism for the large plumes in the prominence

Jincheng Wang 112233 Xiaoli Yan 1133 Zhike Xue 1133 Liheng Yang 1133 Qiaoling
Li 44 Hechao Chen 55 Chun Xia 66 Zhong Liu 1133

(Received **** **, ****; accepted **** **, ****)

###### Abstract

Aims. To understand the formation mechanism of large plumes in solar
prominences, we investigate the formation process of two such phenomena.

Methods.  We studied the dynamic and thermal properties of two large plumes
using observations from New Vacuum Solar Telescope, the Solar Dynamic
Observatory, and the Solar Terrestrial Relations Observatory-Ahead. We
employed the differential emission measures method to diagnose the
thermodynamical nature of the bubble and plumes. We calculated the Doppler
signals based on observations of H$\alpha$ blue and red wings.

Results. We find that two large plumes observed with high-resolution data are
quite different from previously studied small-scale plumes. They are born at
the top of a prominence bubble with a large projected area of 10-20 Mm2.
Before the occurrence of each large plume, the bubble expands and takes on a
quasi-semicircular appearance. Meanwhile, the emission intensity of extreme-
ultra-violet (EUV) bands increases in the bubble. A small-scale filament is
found to erupt in the bubble during the second large plume. At the point at
which the height of the bubble is comparable with half the width of the
bubble, the bubble becomes unstable and generates the plumes. During the
formation of plumes, two side edges of the top of the bubble, which are
dominated by opposite Doppler signals, approach each other. The large plume
then emerges and keeps rising up with a constant speed of about 13-15 km/s.
These two large plumes have temperatures of $\sim$ 1.3 $\times$ $10^{6}$
Kelvin and densities of $\sim$ 2.0 $\times$ 109 cm-3, two orders hotter and
one order less dense than the typical prominence. We also find that the bubble
is a hot, low-density volume instead of a void region beneath the cold and
dense prominence.

Conclusions. These two large plumes are the result of the breakup of the
prominence bubble triggered by an enhancement of thermal pressure; they
separate from the bubble, most likely by magnetic reconnection.

###### Key Words.:

Sun: prominences – Sun: evolution – Sun: corona

##  1 Introduction

Solar prominences are some of the most spectacular structures in the Sun,
located in the solar limb. They consist of dense, cool plasma suspended in the
hot and tenuous corona, the main potential source of solar storms. When they
are observed in the solar disk, they appear as dark ribbon-like features
composed of bundles of threads, also referred to as solar filaments (Martin,
1998; Mackay et al., 2010). They often lie above magnetic polarity inversion
lines (PILs) in the photosphere and are supported by sheared or twisted
magnetic structures (e.g., sheared-arcade or flux rope) against solar gravity
(e.g., Yang et al., 2014; Vial & Engvold, 2015; Yan et al., 2015; Wang et al.,
2017; Chen et al., 2020). High-resolution observations show that the on-disk
filaments are mainly dominated by horizontal thin threads while the limb
prominences are mainly dominated by vertical thin threads (Chae et al., 2008;
Yan et al., 2015; Li et al., 2018). Further, some horizontal threads or
horizontal motions can be observed in some limb prominences (Ahn et al., 2010;
Shen et al., 2015; Yang et al., 2018). Some authors propose that the vertical
thread structures in the prominences are a pile-up of dips in more or less
horizontal magnetic field lines rather than real vertical magnetic structures
(Schmieder et al., 2010; Dudík et al., 2012). However, the relationship
between these horizontal and vertical threads is still controversial because
of a lack of high-resolution stereoscopic observations.

Observations of the chromospheric spectral line (typically in H$\alpha$ and Ca
II H lines) show that prominence bubbles are quasi-void regions beneath pre-
existing prominences (de Toma et al., 2008; Berger et al., 2008, 2011). In
extreme-ultraviolet (EUV) observations, bubbles are brighter than the main
prominence and exhibit brightness comparable to that of the surrounding corona
(Shen et al., 2015; Berger et al., 2011; Labrosse et al., 2011). Using the
two-filter ratio technique, or differential emission measures (DEM) method,
the temperature of bubble has been estimated to be on the order of $10^{6}$
Kelvin, which is much hotter than its overlying prominence ( $\sim$ $10^{4}$
Kelvin) and similar to the background corona (Shen et al., 2015; Berger et
al., 2011; Awasthi & Liu, 2019). The arcade fields of parasitic bipoles are
thought to be the magnetic structures in the prominence bubbles (Dudík et al.,
2012; Gunár et al., 2014). More recently, the boundaries of two prominence
bubbles were found to correspond to the interface between the prominence barb
and the underlying magnetic loops rooted nearby, and that the bubble can be
formed around a filament barb seen in the solar disk (Guo et al., 2021). Some
authors have suggested that the prominence bubbles are filled with hot and
low-density plasma (Shen et al., 2015; Berger et al., 2011), while others
argue that the prominence bubbles are open windows that allow the passage of
background coronal radiation (Dudík et al., 2012; Gunár et al., 2014). The
nature of prominence bubbles remains controversial and merits further
investigation.

High-resolution observations of prominences are revealing increasing detail in
the prominences, including dark upflow plumes. The upflow plumes appear dark
in the chromospheric lines first reported by Stellmacher & Wiehr (1973) and
propagate initially from large bubbles and disperse into the prominences
(Berger et al., 2008, 2010, 2011). de Toma et al. (2008) reported a rising
bubble with a bright core traversing vertically through a prominence and
suggested that the rising bubble is a void local system of stronger magnetic
field with weak plasma pressure. Using observations of the Solar Optical
Telescope (SOT) on board the Hinode satellite, Berger et al. (2010)
investigated the kinematic characteristics of bubbles in three prominences,
finding a mean ascent speed for plumes of up to 13 - 17 km/s and typical
lifetimes of plumes ranging from 300s to 1000s. Using high-resolution
observations of H$\alpha$ observed with the New Vacuum Solar Telescope, Xue et
al. (2021a) found high levels of turbulence with large Doppler and nonthermal
velocities widely occurring at the plume fronts. These dark upflow plumes are
also an important source of quiescent prominence mass (Berger et al., 2010).
Based on numerous observations and numerical simulations, authors proposed
that dark upflow plumes are generated by the magnetic Rayleigh–Taylor
instability (RTI) taking place at the boundary between the prominence and dark
bubbles that rise into it (Berger et al., 2011, 2010; Hillier et al., 2012).
Kelvin–Helmholtz instability (KHI) at the boundary between the bubble and the
prominence could also drive the occurrence of plumes (Berger et al., 2017;
Awasthi & Liu, 2019; Xue et al., 2021a). Based on these theories and some
observational quantities, the plasma $\beta$ (ratio of gas pressure to
magnetic pressure) was estimated in the prominence (Hillier et al., 2012b).
Therefore, the diverse properties of plumes and bubbles offer a unique toolkit
with which to improve our understanding of solar prominences.

In this paper, we present high-resolution observations of two large plumes
created by the breakup of a prominence bubble beneath a pre-existing
prominence. These provide a rare opportunity to further our understanding of
plume formation. By combining these observations with data from ground-based
and space-based telescopes, we carry out a detailed study of the dynamic and
thermal characteristics of plumes and the related bubble. The sections of this
paper are organized as follows. The observations and methods are described in
Section 2, the results are given in Section 3, and discussions and a summary
are presented in Section 4.

##  2 Observations and Methods

###  2.1 Observations

Our data set is primarily from the New Vacuum Solar Telescope
(NVST)111http://fso.ynao.ac.cn (NVST:Liu et al., 2014; Yan et al., 2020) and
Atmospheric Imaging Assembly (AIA; Lemen et al., 2012) on board the Solar
Dynamic Observatory222https://sdo.gsfc.nasa.gov (SDO: Pesnell et al., 2012).
The NVST is a vacuum solar telescope with a 985 mm clear aperture located at
Fuxian Lake, in Yunnan Province, China. It provides high-resolution
observations of prominence at the H$\alpha$ line (6562.8 $\rm\AA$). The
H$\alpha$ images are recorded by a tunable Lyot filter with a bandwidth of
0.25 $\rm\AA$. The H$\alpha$ blue wing (-0.4 $\rm\AA$), H$\alpha$ center, and
H$\alpha$ red wing (+0.4 $\rm\AA$) images are used in this study. The field of
view of H$\alpha$ images is 150$\arcsec$ × 150$\arcsec$, with a 45s cadence
and a spatial resolution of 0.165$\arcsec$ per pixel. These data are
calibrated from Level 0 to Level 1, and are dark-current subtracted and flat-
field corrected, before a speckle masking method is used to reconstruct the
calibrated images from Level 1 to Level 1+ (Xiang et al., 2016). All NVST
H$\alpha$ observations are normalized by the quiet Sun in line with each other
based on a cross-correlation algorithm (Yang et al., 2015). The AIA instrument
on SDO can provide seven simultaneous full-disk EUV images with a pixel scale
of 0.6$\arcsec$ and a cadence of 12 seconds. The SDO/AIA and NVST observations
are carefully co-aligned by matching specific features observed simultaneously
in both the AIA 171 $\rm\AA$ and NVST H$\alpha$ center channels. We mainly use
the 211, 193, and 171 $\rm\AA$ channels to study the evolution and dynamic
characteristics of bubbles and plumes and 94, 131, 171, 193, 211, 335 $\rm\AA$
images for the DEM calculation. In addition, EUV images from the Extreme
Ultraviolet Imager (EUVI: Wuelser et al., 2004) on board the Solar Terrestrial
Relations Observatory-Ahead333https://stereo.gsfc.nasa.gov (STEREO-A: Kaiser
et al., 2008) are used in this study. We mainly employ 304 $\rm\AA$ and 195
$\rm\AA$ images with spatial resolutions of 1.59$\arcsec$ per pixel and
cadences of 10 and 2.5 minutes, respectively.

###  2.2 Methods

####  2.2.1 Differential emission measures method

To diagnose the thermodynamical natures of bubble and plumes, we employ the
DEM method to derive the emission measure (EM) distribution depending on
temperature (T) (Su et al., 2018). The DEM method is a modified version of the
sparse inversion technique for thermal diagnostics developed by Cheung et al.
(2015). The DEM solutions, which are derived from several popular DEM codes
(such as the regularized inversion code (Hannah & Kontar, 2012), the
XRT$\\_$dem$\\_$iterative2 (Weber et al., 2004), and the sparse inversion code
(Cheung et al., 2015)), show significant emissions at high temperatures, which
is not consistent with observations. The modified method provides some useful
settings of basis functions and tolerances with which to better constrain the
plasma DEMs at high temperatures using AIA data only. The maps of six EUV
wavelengths including 94, 131, 171, 193, 211, and 335 $\rm\AA$ obtained from
SDO/AIA are used in this method. In order to improve the signal-to-noise ratio
(S/N), we average intensities of images within 1 minute for the DEM method.
The uncertainties of EM solutions are estimated by 200 Monte Carlo simulations
with the uncertainties on AIA intensity obtained using the
“aia$\\_$bp$\\_$estimate$\\_$error.pro” routine in SolarSoftWare (SSW:Freeland
& Handy, 1998). The total EM is defined as

${\rm EM}=\sum{\rm EM}_{T}=\int n_{e}^{2}dl,$ (1)

where $n_{e}$ and $l$ are the electron number density and the optical depth
along the line of sight (LOS), respectively. The EM-weighted average
temperatures ($T_{em}$) are also estimated by following the formula (Su et
al., 2018):

$T_{em}=\frac{\sum{({\rm EM}_{T}\cdot T})}{\sum{\rm EM}_{T}}.$ (2)

####  2.2.2 Doppler signals

Based on observations of H$\alpha$ blue (-0.4 $\rm\AA$) and red (+0.4
$\rm\AA$) wings, the Doppler signals (Ds) in the prominence can be calculated
using the following formula:

${\rm D}_{s}=\frac{I_{red}-I_{blue}}{I_{red}+I_{blue}},$ (3)

where $I_{blue}$ and $I_{red}$ are the emission intensities of the blue wing
(H$\alpha$ -0.4 $\AA$) and the red wing (H$\alpha$ +0.4 $\AA$) of the
H$\alpha$ line, respectively. The Doppler signal is a proxy of the LOS
velocity field in chromospheric temperature (Tsiropoula, 2000). In the
formulation, positive values of Ds denote the plasma away from the observer
(redshift) while negative values of Ds denote the plasma close to the observer
(blueshift).

##  3 Results

###  3.1 Overview of the bubble of interest

In 2021 April 14, a prominence appeared at the northeast limb of the solar
disk as viewed from Earth, corresponding to the bottom of a coronal cavity or
the lower side of large-scale coronal loops (see Fig.1 (a)). Figure 1 (a)
shows an overview of the prominence in SDO/AIA 211 $\AA$ at 06:08 UT. Viewed
from STEREO-A, this prominence was observed from the southwest and became an
on-disk quiescent filament (see Fig.1 (b) and (c)). The angle between SDO and
STEREO-A was 53.6$\degr$ at this moment (see the blue box in the Fig.1 (c)).
Figure 1 (d) displays the fine structures of the prominence in the H$\alpha$
line center observed by NVST. In order to better reveal the structure of the
prominence, we rotate the map and reverse the intensity of H$\alpha$
observations (see Fig.1 (d)). The prominence consists of three parts,
including two helmet-like structures and a sparse arch-like structure. The
bubble of interest resides at the bottom of the middle helmet-like structure
that mainly consists of vertical thin threads, and is marked by the white
arrow. The bubble has a sharp arch-like boundary below the prominence with the
projected area of about 35 ${\rm Mm}^{2}$ at 06:08 UT. Panels (b)-(c) show
observations of a prominence from a different viewing angle, at 304 $\AA$ and
195 $\AA$ from STEREO-A/EUVI, respectively. Based on the world coordinate
system (WCS: Greisen & Calabretta, 2002; Thompson & Wei, 2010) and comparing
distinct characteristics between SDO and STEREO-A, we identify the bubble
marked by the white circle in the STEREO-A/EUVI observation. We find that the
bubble exhibits a brightness of cusp shape in 195 $\AA$ and is faded in
comparison at 304 $\AA$ because of strong absorption by the prominence in the
He II line. Combined with the observations from two different viewing angles,
we can identify that the bubble is beneath the main body of the prominence.
More importantly, based on the brightness in the bubble observed by STEREO-A,
we can certify that the bubble is filled with hot plasma (exciting strong
emissions in the EUV line) rather than being a void structure. Panels (e) and
(f) exhibit the prominence in 171 $\AA$ and 193 $\AA$ observations from
SDO/AIA. The prominence appears as a dark object because of the absorption of
the prominence plasma at 171 $\AA$ and 193 $\AA$. However, the bubble appears
as a brightening structure in these two channel observations.

Figure 1: Overview of the prominence. (a) SDO/AIA 211 $\AA$ observation. Solar
limb is plotted using a white line and the black box outlines the field of
view of panels (d)-(f). (b)-(c) Observations of STEREO-A/EUVI in 304 $\AA$ and
195 $\AA$. The white dotted line indicates the solar limb observed from SDO
and the white circle marks the position of the bubble. The blue box in the
bottom-right corner shows the relative locations of STEREO-A, STEREO-B, and
SDO at 06:05 UT. The coordinates of the map in panels (b)-(c) are the
heliographic coordinates from STERO-A. The white plus symbols in panels (a),
(b), and (c) mark the same point on the surface from different viewpoints,
which is derived by the solarsoft WCS Routines in SSW. (d) H$\alpha$
observation from NVST. The black box outlines the field of view of Figs. 2 and
3. The white arrow points to the bubble of interest. (e) SDO/AIA 171 $\AA$
observation. (f) SDO/AIA 193 $\AA$ observation. The solar limb is outline by
white lines in panels (d)-(f).

###  3.2 Formation processes of two large plumes

Figure 2 shows the formation processes of the first large plume originating
from the bubble during the period from 06:07 UT to 06:39 UT. Panels (a1) -
(a5), (b1) - (b5), and (c1) - (c5) display the formation process of the first
large plume in SDO/AIA 171 $\AA$, the NVST H$\alpha$ center, and the blue wing
(H$\alpha$ -0.4 $\AA$), respectively. The corresponding Doppler signals are
exhibited in panels (d1) - (d5). An sharp arch-like boundary between the
bubble and overlying prominence can be identified at around 06:07 UT. The
bubble corresponds to a void region in the H$\alpha$ line (see panels (b1) and
(c1)) and a brightening structure at 171 $\AA$ (see panel (a1)). At the early
phase, the bubble expands gradually and rises away from the surface (see the
first and second columns of Fig.2). During this period, there are some
brightenings occurring in the bubble in 171 $\rm\AA$ observations (see panel
(a2)). Around 06:26 UT, the bubble grows into a quasi-semicircle with a width
of about 13 Mm. Simultaneously, some enhanced emission of H$\alpha$ center
appear at the high part of the bubble–prominence interface, which is marked by
blue arrows in panel (b2). This striking feature was also found in previous
studies (Berger et al., 2010; Awasthi & Liu, 2019; Xue et al., 2021a), which
could be interpreted as accumulated chromospheric materials in that region. We
find that these accumulated chromospheric plasma are dominated by redshifted
signals (see panel (d2)). After that, the bubble becomes unstable and changes
shape. At around 06:32 UT, a large flame-like plume runs out from the top of
the bubble. Meanwhile, two side edges connecting the bubble and plume approach
each other while the left and right side edges are dominated by blue- and
redshifts, respectively (see the breakup region in panel (d4)). At 06:35 UT,
the large plume separates from the bubble and continues to ascend while some
prominence material falls down into the bubble (see panel (b4)). The plume is
brighter than the background prominence at 171 $\AA$, which is comparable with
the boundary between bubble and prominence in brightness (see panels (a3) and
(a4)). As the plume ascends, some material accumulates in the front of the
plume, and exhibits a turbulent shape (see panels (b3) - (b5)). Interestingly,
a slender void thread connecting the bubble and ascending plume is seen in the
H$\alpha$ observation (marked by black arrows in panels (b4) and (b5)), which
corresponds to the bright structure at 171 $\AA$ (see panels (a4) and (a5)).
The left and right sides of this void thread are dominated by strong blue- and
redshifted Doppler signals, respectively (see the zoomed-in region in panel
(d4)). At around 06:39 UT, the plume continues to rise and evolves into
several fragments. Simultaneously, the slender void thread elongates up to 8
Mm in length in H$\alpha$. Its width is about 0.60$\pm$0.12 Mm (five pixels)
in H$\alpha$ (see the blue line in panel (b5)) and 0.87$\pm$0.44 Mm (two
pixels) in the 171 $\rm\AA$ observations (see the blue line in panel (a5)).
The uncertainties come from the spatial resolutions of the observing
instruments. This slender void thread exists for about 7 minutes. After that,
the plume and slender thread disperse into the prominence.

Figure 2: Evolution of the first large plume. (a1) - (a5) SDO/AIA 171 $\AA$
observations. The blue arrow in panel (a2) points to the brightening in the
bubble. White dashed line in panel (a3) marks the position of time–distance
diagrams in Fig.4 (a) - (d). The white curve in panel (a5) depicts the
variation of normalized intensity of SDO/AIA 171 $\rm\AA$ along the white line
in panel (a5). (b1) - (b5) NVST H$\alpha$ center observations. The black line
in panel (b3) marks the slit position of time–distance diagram in Fig.4 (e).
The blue curve in panel (b5) depicts the variation of normalized intensity of
the inverse H$\alpha$ center along the blue line in panel (b5). (c1) - (c5)
NVST H$\alpha$ blue wing (-0.4 $\AA$) observations. (d1) - (d5) Corresponding
Doppler signals. The contours in panels (c1) - (c5) and (d1) - (c5) outline
the corresponding inverse H$\alpha$ center with a level of 0.25 of the normal
intensity. The black horizontal arrows mark the slender void thread in panels
(a4)-(a5) and (b4)-(b5).  Figure 3: Evolution of the second large plume. (a1)
- (a6) SDO/AIA 171 $\AA$ observations. The white dashed line in panel (a4)
marks the position of the time–distance diagrams in Fig.4 (a) - (d). The white
curve in panel (a6) depicts the variation in normalized intensity of SDO/AIA
171 $\rm\AA$ along the white line in panel (a6). (b1) - (b6) NVST H$\alpha$
center observations. The black line in panel (b4) marks the slit position of
the time–distance diagram in Fig.4 (f). The blue curve in panel (b6) depicts
the variation of normalized intensity of inverse H$\alpha$ center along the
blue line in panel (b6). (c1) - (c6) NVST H$\alpha$ blue wing (-0.4 $\AA$)
observations. (d1) - (d6) Corresponding Doppler signals. The contours in
panels (c1) - (c6) and (d1) - (c6) outline the corresponding inverse H$\alpha$
center with a level of 0.25 of the normal intensity. The black horizontal
arrows mark the slender void thread in panels (a5)-(a6) and (b5)-(b6).

Figure 3 displays the evolution of the second large plume originating from the
same bubble as the first plume. In the initial stage, the bubble is small and
an active small-scale filament can be identified at the bottom of the bubble
at around 07:53 UT (see Fig.3 (a1) and (b1)). As this active small-scale
filament erupts, the bubble expands and gets higher (see the second column of
Fig.3). The active small-scale filament exhibits strong blueshifted signal
during its eruption (see Fig.3 (d1) and (d2)). As in the first large plume,
some brightenings are also found in the bubble in the 171 $\rm\AA$
observations during the eruption of the small-scale filament (see panel (a3)).
At around 08:02 UT, the erupting small-scale filament merges into the boundary
between bubble and prominence while the bubble also grows into a quasi-
semicircle with a width of about 12 Mm. In the course of bubble expansion,
some enhanced emission of H$\alpha$ center can also be found at the high part
of bubble–prominence interface. After that, the bubble experiences a similar
episode to the previous one. The bubble becomes unstable and begins to
collapse. At 08:07 UT, a large plume comes out from the top of the bubble
while the two side edges of the plume approach each other. At around 08:12 UT,
the plume separates from the bubble and keeps rising. Simultaneously, we also
identify a slender void thread marked by black arrows in panels (b5) and (b6),
connecting the bubble and ascending plume in the H$\alpha$ observation despite
the poor seeing (see panel (b5)). The slender void thread is also
corresponding to the bright structure in the 171 $\AA$ observation (marked by
the corresponding black arrows in panel (a5) and (a6)). Left and right sides
of this void thread are also dominated by strong blueshifted and redshifted
Doppler signals, respectively (see the zoomed-in region in panel (d5)). At
around 08:14 UT, the plume continues ascending and evolves into several
fragments as in the first plume. The slender thread is also elongated and
distorted. Additionally, its width was measured to be about 0.60$\pm$0.12 Mm
(five pixels) in H$\alpha$ (see the blue line in panel (b6)) and 1.31$\pm$0.44
Mm (three pixels) in 171 $\rm\AA$ observations (see the blue line in panel
(a6)). After that, the plume lifts and disperses into the prominence while the
slender void thread breaks up into the prominence. This slender thread also
lives for about 7 minutes. At around 08:27 UT, the bubble breaks up
completely. It is worth emphasizing that there were many small irregular
plumes in addition to these two large plumes, initially occurring at the
bubble–prominence interface during the formation of these two large plumes.
The appearance of these small plumes is similar to that of some previously
reported plumes (Berger et al., 2008, 2010, 2011; Xue et al., 2021a). These
small plumes were much smaller in size than the two large plumes, and occurred
in random places at the bubble–prominence interface instead of the bubble top.

Figure 4: Time–distance diagrams derived by a sequence of images. (a) NVST
time–distance diagram along white dashed lines in Fig.2 (a3) and Fig.3 (a4).
(b) Corresponding SDO/AIA 171 $\AA$ time–distance diagram. The inset
time–distance diagram in the top right is a zoomed-in image of the region
marked by the blue dashed box. (c) - (d) Time–distance diagram derived by NVST
H$\alpha$ and SDO/AIA 171 $\AA$ images along the black line in Fig.2 (b3). (e)
- (f) Time–distance diagram derived by NVST H$\alpha$ and SDO/AIA 171 $\AA$
images along the black line in Fig.3 (b4). The uncertainties on the velocities
are estimated as the error in the length measurement which is considered to be
about 0.6 arcsec (about one pixel length of SDO/AIA). Figure 5: Thermal
characteristics in two large plumes. (a1) - (a5) 193 $\AA$ image, total
emission measures, EM average temperature, emission measures at log T = 6.0 -
6.3, and emission measures at log T = 6.0 - 6.3 at 06:34:22 UT. (b1) - (b5)
Same as panels (a1) - (a5) at 08:11:22 UT. (c1) - (c2) Average EM curves in
the red and blue boxes of panel (a1) for the first large plume. (d1) - (d2)
Average EM curves in the red and blue boxes of panel (b1) for the second large
plume. The corresponding background EM curves are derived from the
corresponding quiet Sun away from the prominence (red and blue boxes in Fig.1
(a)). The uncertainties were estimated using the Monte-Carlo experiment.

###  3.3 Dynamic properties

In order to investigate the kinetic characteristics of these two large plumes,
we made two time–distance maps derived using time-series images of NVST
H$\alpha$ center and SDO/AIA 171 $\AA$, which are along the same path outlined
by the white dashed lines in Figs. 2 (a3) and 3 (a4). Figure 4 (a) - (b)
exhibit the time-distance maps in H$\alpha$ center and 171 $\AA$,
respectively. Similar behaviors of bubble and plume are seen in these two
time–distance maps. Before the occurrence of the first large plume, the
bubble–prominence interface ascends gradually. This indicates that the bubble
is ascending before the first large plume. Before the second large plume, the
bubble–prominence interface also lifts up sharply, accompanied by some dark
material (marked by the black arrows in panels (a) and (b)) related to the
small-scale filament eruption. Before the bubble–prominence interface rises up
to about 12 Mm, the bubble–prominence interface and the front of the rising
small-scale filament lifts up at a constant speed of about 8.1$\pm$1.2 km/s
(see the inset time–distance diagram panel (b)). The two large plumes travel
almost at a constant speed. The average ascending speeds of the two large
plumes are about 15.3$\pm$0.5 km/s and 13.4$\pm$0.4 km/s, respectively. In
addition, before their appearance, the bubble–prominence interface experiences
several disturbances marked by two black boxes in panel (a). We also used the
time–distance maps to derive the approaching speeds of the two-side edges
connecting the bubble and plume during the formation of the plumes. Figure 4
(c) - (d) shows the H$\alpha$ and AIA 171 $\AA$ time–distance maps for the
first large plume along the black solid line in panel (b3) of Fig.2. The
approaching speeds of the left and right sides are about 8.3$\pm$1.5 km/s and
6.0$\pm$1.7 km/s, respectively. Thus, the relative approaching speed is about
14.3$\pm$3.2 km/s. Figure 4 (e) - (f) shows the H$\alpha$ and AIA 171 $\AA$
time–distance maps for the second large plume along the black solid line in
panel (b4) of Fig.3. The approaching speeds of the left and right sides are
about 6.7$\pm$1.2 km/s and 9.2$\pm$1.2 km/s, respectively. Thus, their
relative approaching speed was about 15.9$\pm$2.4 km/s. Therefore, both large
plumes have almost the same approaching speeds as the two side edges during
their formation. Another feature can be identified in these four time–distance
diagrams, namely the cross-section of the slender void thread. For both large
plumes, the signals of slender void threads, marked by blue arrows in panels
(c)-(f), can be distinguished in both H$\alpha$ and AIA 171 $\AA$
time–distance diagrams.

###  3.4 Thermal properties

Figure 5 shows the thermal characteristics derived using the DEM method in
these two large plumes. Panels (a1) - (a5) display the 193 $\AA$ image,
distribution of total emission measures, EM-weighted average temperature, and
emission measures at log T = 5.7 - 6.0 and log T = 6.0 - 6.3 for the first
large plume at 06:34:22 UT, respectively. Panels (b1) - (b5) are the same
images for the second large plume at 08:11:22 UT. The bubble, plume, and
slender void thread can be seen at that time. The slender void threads are
marked by the black horizontal arrows in panels (a1) and (b1). The bubble
exhibited strong emission measures (see panels (a2) and (b2)). The
distribution of the EM-weighted average temperature, which is on the order of
106 K, is complex. The bubble, plume, and slender void thread have strong
emission measures at log T=6.0 - 6.3 (see panels (a5) and (b5)), while the
bubble also has strong emission measures at log T=5.7 - 6.0 (see panels (a4)
and (b4)). This implies that these structures in H$\alpha$ have strong
emission measures at log T = 5.7 - 6.3. Figure 5 (c1) and (c2) and (d1) and
(d2) show the mean emission measure profiles calculated using the DEM solution
within different regions representing the bubble and plumes for two large
plumes, respectively. Additionally, we estimated the corresponding emission
measure profile in the neighboring quiet corona away from the prominence
marked by the red and blue boxes in Fig.1 (a). The emission measures in these
structures are also found to be mainly dominated by the temperature at log T =
5.7-6.0. They are the subject of large uncertainties at log T =6.4-7.0, and
therefore we only consider the EMs with uncertainties smaller than half of
their value. For the first large plume at around 06:34 UT, the total EM and
$T_{em}$ were estimated to be 8.67$\pm$0.81 $\times$ 1026 cm-5 and
1.23$\pm$0.12 $\times$ 106 K in the bubble, respectively (see panel (c1)).
Meanwhile, the EM and $T_{em}$ in the background were 7.67 $\pm$0.73 $\times$
1026 cm-5 and 1.16$\pm$0.11 $\times$ 106 K, respectively. On the other hand,
the EM and $T_{em}$ in the plume were estimated to be, respectively,
4.63$\pm$1.00 $\times$ 1026 cm-5 and 1.33$\pm$0.28 $\times$ 106 K, while the
corresponding EM and $T_{em}$ in the background were 5.53$\pm$1.05 $\times$
1026 cm-5 and 1.36$\pm$0.25 $\times$ 106 K (see panel (c2)). For the second
large plume at around 08:11 UT, the EM and $T_{em}$ in the bubble were
estimated to be, respectively, 9.77$\pm$0.86 $\times$ 1026 cm-5 and
1.20$\pm$0.10 $\times$ 106 K, while the corresponding EM and $T_{em}$ in the
background were 7.30$\pm$0.70 $\times$ 1026 cm-5 and 1.17$\pm$0.11 $\times$
106 K (see panel (d1)). Panel (d2) shows the profile of EM in the second large
plume. In the second large plume, the EM and $T_{em}$ were estimated to be,
respectively, 4.72$\pm$0.91. $\times$ 1026 cm-5 and 1.31$\pm$0.24 $\times$ 106
K, while the corresponding EM and $T_{em}$ in the background were
5.85$\pm$1.45 $\times$ 1026 cm-5 and 1.28$\pm$0.31 $\times$ 106 K. In general,
we find that the EM in the bubble is in excess of the background, and the EM-
weighted average temperature in the bubble of $\sim$ 1.2 $\times$ 106 K is
comparable to that of the background. However, the EM in the plumes is lower
than that of the background while the $T_{em}$ in the plumes of $\sim$ 1.3
$\times$ 106 K is also comparable to that of the background. This suggests
that these large plumes have a temperature comparable to that of the bubble
but show much lower EMs than the lower surface of the bubble. This shows that
the bubble and plume consist of hot plasma at about 1 - 2 $\times$ 106 K,
which is two orders of magnitude hotter than the overlying prominence (Parenti
et al., 2005). Assuming that the depth ($l$) of the emitting plasma is equal
to the widths of the boxes, the electron number density ($n_{e}$) can be
calculated using the formula: $n_{e}=\sqrt{{\rm EM}/l}$. Based on the above
assumption and derived parameters, the average densities of bubble and plume
can be estimated to be about 1.7 $\times$ 109 cm-3 and 2.0 $\times$ 109 cm-3,
respectively.

Figure 6: Evolution of different parameters in the bubble and plumes. (a1-b1,
c1, d1) Variations of different SDO/AIA normalized intensities by exposure
duration, EM in the red box of Fig.5 (a1), projected areas, and the height for
the first large plume. (a2-b2, c2, d2) Variations of different SDO/AIA
normalized intensities by exposure duration, EM in the red box of Fig.5 (b1),
projected areas, and the height for the second large plume. The blue vertical
dashed line denotes the onset of instability in the bubble top for each large
plume. The horizontal dotted lines in panels (a1)-(b1) and (a2)-(b2) denote
the corresponding average normalized intensity of background at different
wavelengths. The uncertainties in panels (c1)-(c2) are derived directly from
the errors of Fig.5 (c1)-(d1) estimated by the Monte-Carlo experiment.

###  3.5 Variations of different physical parameters

In order to investigate the triggering mechanisms of these two large plumes,
we calculated different physical parameters in the bubble during the formation
of these two large plumes. In addition, we used H$\alpha$ center images to
derive the aspects of the bubble and plume, where we use the contours of
normal intensity at the level of 0.25 plotted in Figs. 2 and 3 as the
boundaries of bubble and plumes. The dashed vertical line denotes the onset of
instability at the top of the bubble for each large plume (around 06:27 UT for
the first plume and around 08:03 UT for the second plume, see the animations 1
and 2). Figure 6 shows time variations of these parameters related to two
large plumes. For the first large plume, at the early phase, the AIA/SDO EUV
emissions in the bubble display a slow enhancement trend at 171, 193, and 211
$\AA$. It is worth noting that the radiation emissions at 171 $\rm\AA$ in the
bubble are slightly smaller than the background but at 193 and 211 $\rm\AA$
these strongly exceed those of the background. The radiation emissions in 94
$\rm\AA$ remain invariable. Meanwhile, EMs at log T= 5.7- 6.0 and log T
=6.0-6.3 also exhibit a slight enhancement. Simultaneously, the projected
areas and the height of the bubble also experience a slow increase. It is
noted that several small plumes like the ones discovered by SOT on board
Hinode also occurred at the bubble–prominence interface during this time. When
the projected area and height of the bubble reached about 44 Mm2 and 6.5 Mm at
06:27 UT, the bubble became unstable. We find that the height of the bubble
was comparable to half of its width (see Fig. 2 (b2)) at that moment. After
that, the height of the bubble increased rapidly, meaning that the plume began
to come out from the bubble. At around 06:35 UT, the first plume separated
from the bubble, with its projected area of about 10 Mm2. The blue asterisks
plot the variation of the projected area of the plume. The projected area of
the plume experienced a linear-like decay. With a linear fit, the decaying
rate of the projected area is about -15.95 $\pm$ 1.37 m2/s. For the second
large plume, unlike the first plume, the EUV emission at the different
wavelengths other than 94 $\rm\AA$ increased rapidly during the early phase.
This may be related to the eruption of the small-scale filament occurring in
the bottom part of the bubble. Both EMs at log T=5.7 - 6.0 and log T=6.0 - 6.3
also enhanced. The EMs at log T=6.0 - 6.3 increased more sharply. Meanwhile,
the projected area of the bubble also increased sharply from about 10 Mm2 to
33 Mm2. The height of the bubble also increased from 4 Mm to 6 Mm. At around
08:03 UT, the bubble became unstable and began to generate the second plume.
We also find that the height of the bubble was also comparable to half of the
width of the bubble (see Fig.2 (b2)) at that moment. At that point, the height
of the bubble increased sharply and the plume came out of the bubble. At
around 08:11 UT, the second large plume separated from the bubble, with a
projected area of about 17 Mm2. As the plume rose up, the plume became smaller
and smaller. The projected area of the plume is plotted as the blue asterisks
in panel (f). The projected area of this plume also decreased with a linear
trend. The linear fit to the projected area decay curve for the second plume
in panel (f) shows an initially linear decaying rate of about -23.70 $\pm$
1.70 m2/s. Altogether, we find enhanced emission of several EUV bands and EMs
in the bubble during its growth, which is consistent with the brightenings in
Figs. 2 (a2) and 3 (a3). More importantly, as the height of the bubble becomes
comparable to half the width of the bubble, the bubble becomes unstable and
generates the large plumes.

##  4 Summary and Discussion

In this paper, we investigate the successive formation of two large plumes in
a solar prominence on 2021 April 14. Using high-resolution observations from
NVST and data observed by SDO and the STEREO satellite, we study the kinetic
and thermal characteristics of two large plumes and their formation process.
Our main results are as follows:

(1) Two large plumes originate from the same bubble, and are associated with
the disruption of the bubble. Before the occurrence of each large plume, the
bubble expanded and grew into a quasi-semicircular shape. Simultaneously, the
emission intensity of EUV bands (including wavelengths of 171, 193, and 211
$\rm\AA$) and EMs also increased in the bubble. The second large plume was
associated with an eruption of the small-scale filament in the bubble. When
the height of the bubble reached half of its width, the bubble became unstable
and began to generate the large plumes.

(2) A novel phenomenon was discovered during the formation of each plume,
namely a slender void thread in H$\alpha$ connecting the bubble and plume. The
width of this void thread is about 1 Mm. Furthermore, the left and right sides
of this slender void thread are dominated by blue- and redshifted Doppler
signals, respectively.

(3) The large plumes rose up in the body of the prominence with a constant
ascent speed of about 13-15 km/s. During their development, the projected area
of the plumes decreased with a linear trend. The decay rates of these two
plumes are about -15.95 $\pm$ 1.37 Mm2/s and -23.70 $\pm$ 1.70 Mm2/s,
respectively.

(4) The temperature and electron number density of these two large plumes are
estimated to be about 1.3 $\times$ 106 Kelvin and 2.0 $\times$ 109 cm-3,
respectively. Compared to the typical prominence, the large plume is two
orders of magnitude hotter and one order thinner.

The nature of prominence bubble —whether it is a void volume or filled with
plasma— is a controversial problem. We provide extremely powerful evidence
supporting the hypothesis that the bubble below the prominence is filled with
hot and low-density plasma. Because of a lack of stereoscopic observation of
prominence bubbles, some authors considered the prominence bubbles to be open
windows that are no obstacle to the background coronal radiation (Dudík et
al., 2012; Gunár et al., 2014). Here, thanks to two viewing angles, the bubble
was seen to exhibit bright structure in the 195 $\AA$ band viewed from the top
and a void region in H$\alpha$ line viewed from the side, which shows that the
emission seen inside bubbles by SDO/AIA at 171, 193, and 211 $\AA$ originates
from the internal plasma rather than from behind or in front of the bubble.
Furthermore, the bubble has a density of about 1.7 $\times$ 109 cm-3 and a
temperature of about 1.2 $\times$ 106 K, similar to the typical quiet Sun
(e.g., Doschek et al., 1997; Landi & Feldman, 2003; Warren & Brooks, 2009;
Landi & Young, 2010), which is an order of magnitude thinner and two orders
hotter than the prominence (Parenti et al., 2005; Bommier et al., 1986). This
demonstrates that the bubble is a hot, low-density volume beneath the cold and
dense prominence instead of an open window.

The two large plumes studied here are quite different from the small-scale
dark upflow plumes discovered with Hinode (Berger et al., 2008, 2010, 2011,
2017), which are more like the large plumes reported previously (Stellmacher &
Wiehr, 1973; de Toma et al., 2008). The plumes studied here are larger in size
($\sim$10-20 Mm2) and have a bubble-related structure. We discovered a novel
phenomenon during the formation of each large plume, that is, a slender void
thread with a width of about 1 Mm in H$\alpha$ observations. The plumes show
strong emission measures at log T = 6.0 - 6.3, meaning that their temperature
might be around 1-2 MK. Interestingly, a similar structure was seen in the
numerical simulations (Hillier et al., 2012). In the simulations, a low-
density, high-temperature region involved in the bubble, which is consistent
with our observation, was considered to yield the plume. The narrow dark
region was thought to correspond to the channel of transport of hot plasma
from the bubble to the upflow plume. During the formation of the plume in the
simulations, magnetic reconnection between the rising and falling magnetic
field resulted in strong downflow in addition to this narrow dark channel
(Hillier et al., 2012c). In our observations, some downflows could also be
identified during the formation of the large plumes (see Figs.2 (b4) and 3
(b5) and animations 1 and 2). In addition, the two sides of the plume–bubble
connection were found to approach each other during the formation of the large
plumes. If the bubble is considered to be an arc-like magnetic field structure
(Dudík et al., 2012; Gunár et al., 2014; Guo et al., 2021), the magnetic
fields on the two sides of the slender void thread should be of opposite
direction. We found the left and right sides of the slender void thread to be
dominated by blue- and redshifted Doppler signals, respectively. This could be
explained as the result of the approaching motion in the plane of bubble arc-
like magnetic field that is not parallel to the plane of the solar disk, which
is also consistent with the location of the small-scale filament (see Fig.3
(a1) and (b1)). Therefore, we consider these slender void threads to
correspond to the channel of bubble-plasma upflow with opposite magnetic field
edges (strong electric current).

The formation mechanism of plumes is the key problem in understanding
prominence plumes. Previous studies considered the magnetic Rayleigh–Taylor
(RT) instability to be responsible for the dark turbulent small-scale upflow
plumes (Berger et al., 2008, 2010, 2011; Hillier et al., 2012; Berger et al.,
2017). This type of small-scale upflow plume, triggered by small
perturbations, is often initiated irregularly over any part of the bubble-
prominence interface. The magnetic field transverse to gravity has a strong
effect on the RT instability and suppresses the RT instability (Ryutova et
al., 2010; Chandrasekhar, 1961). However, these two large plumes were formed
at the top of the bubble, and are obviously related to its disruption. We find
that the bubble becomes unstable and generates the plumes at the point at
which the height of the bubble is comparable to half of its width. The
dynamics of the bubble should be subject to the Rayleigh-Plesset equation (see
Eq.(5) in the Appendix A: Rayleigh, 1917; Plesset, 1949). When the bubble
expands over its capacity, the bubble should collapse and release its internal
gas. With the modeling we present in Appendix A, the bubble is not able to
keep its shape and begins to collapse as the bubble grows and its height
exceeds half of its width. This is consistent with our observations.
Therefore, we suggest that these large plumes are the result of the disruption
of the bubble. On the other hand, it is found that the two side edges of the
top of the bubble approach each other after the bubble has become unstable.
The large plume then forms. Furthermore, the left and right side edges are
dominated by blue- and redshifts (see the breakup region in Figs.2 (d4) and 3
(d5)), respectively. These observational signals strongly suggest that the
escape of the large plumes from the bubble is likely by way of magnetic
reconnection (Parker, 1957; Sweet, 1958; Gonzalez & Parker, 2016; Xue et al.,
2020, 2021b) at the boundary of the bubble. However, we do not find any strong
enhancements in SDO/AIA high-temperature bands (such as 94, 131 $\rm\AA$)
around the slender void thread corresponding to the magnetic reconnection
site. A possible reason for this is that the magnetic reconnection occurring
at the boundary is too weak to enhance the intensities of SDO/AIA high-
temperature lines.

Another problem that interests us is the driving force behind the disruption
of the bubble. From the Rayleigh-Plesset equation (see Eq.(5)), the driving
parameter for the bubble dynamics is pressure (Franc & Michel, 2004). In our
study, we show that the radiation of different EUV wavebands and EMs in the
bubble become enhanced before the breakup of the bubble, which urges us to
believe that the enhancement of thermal pressure in the bubble is the main
driver of the disruption of the bubble. When the pressure in the bubble
enhances, the bubbles expand its volume. As the bubble expands enough or the
magnetic field cannot maintain its integrity, the bubble becomes unstable and
begins to break up. When the bubble breaks up, the top half of the bubble is
the first part to become distorted because of the weaker magnetic field there.
As the bubble breaks up, some internal gas escapes the bubble and forms the
plumes which continue to ascend because of their low density. For the second
large plume, the main originating source enhancing thermal pressure in the
bubble is related to the eruption of a small-scale filament. As the small-
scale filament erupts in the bubble, it releases magnetic energy in the form
of thermal energy, momentum, and radiative energy (Lin & Forbes, 2000).
However, for the first large plume, the source of the enhancement of thermal
pressure is uncertain. It seems to be a slow process, unlike the second plume.
There are several disturbances occurring at the bubble–prominence interface
before the appearance of the large plumes. This type of disturbance could
result from small–scale energy release under the bubble–prominence interface
(Shen et al., 2015). Therefore, we suspect that another likely source of the
enhancement of thermal pressure is related to small-scale magnetic
reconnections (Samanta et al., 2019).

These two large plumes ascend with an approximately constant speed of about 15
km/s, which is comparable to that of the small-scale plumes (Berger et al.,
2008, 2010, 2011). The ascent speed of plumes may be related to the intrinsic
attributes of the prominences instead of the scale of the upflowing plumes. In
the course of the upflow of plumes through the main body of the prominence in
a constant motion, ignoring the effect of the magnetic nature of the
prominence and its accompanying forces, the plume maintains equilibrium by
gravitational forces, a buoyant force, and a viscous resistance force:

${(\rho}_{pro}-\rho_{plu})gV\ -\ \frac{1}{2}cS\rho_{pro}v^{2}=0,$ (4)

where $\rho_{pro}$ and $\rho_{plu}$ are the densities of the prominence and
plume, respectively, $V$ and $S$ are the volume and cross-sectional area of
the plume, respectively, and $c$ is the coefficient of viscous resistance in
the prominence, related to the dynamic viscosity. As
$\rho_{pro}\gg\rho_{plu}$, we obtain that $c\approx 2gV/(Sv^{2})$. In the
rising plumes, if we assume that the ratio of the volume to cross-sectional
area in plumes is a constant, the coefficient of viscous resistance in the
prominence is inversely proportional to the square of the ascent speed of
plumes. It is worth noting that Eq.4 disregards the magnetic nature of the
prominence or assumes that the upflow plasma goes along the vertical magnetic
field of the prominence. Therefore, considering the above situations, the
comparable derived ascent speed of plumes may reveal the approximate
coefficient of viscous resistance in different prominences.

###### Acknowledgements.

The authors are indebted to the NVST, SDO/AIA and STEREO teams for data
support. This work is supported by the National Science Foundation of China
(NSFC) under grant numbers 12003064, 11873087, 11803085, Yunnan Science
Foundation of China (2019FD085), the National Key R&D Program of China
(2019YFA0405000), the CAS “Light of West China” Program under number
Y9XB018001, the Open Research Program of the Key Laboratory of Solar Activity
of Chinese Academy of Sciences (grant No. KLSA202014),the Yunnan Talent
Science Foundation of China (2018FA001), the Yunnan Science Foundation for
Distinguished Young Scholars under No. 202001AV070004, and the Key Research
and Development Project of Yunnan Province under number 202003AD150019. Dr.
Yang Su and Dr. Mark Cheung are acknowledged for the DEM code used to derive
the thermodynamical properties of the plasma from the EUV images.

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##  Appendix A Bubble evolution: Rayleigh-Plesset equation

To describe the dynamic evolution of the bubble, we must consider the gas
pressure, nonlinearity, and dissipative effects. In the semi-spherical shape
of the bubble, the bubble radius (R(t)) satisfies the following second-order
differential equation known as the Rayleigh-Plesset equation:

$\rm\rho\left[R\ddot{R}+\frac{3}{2}\dot{R}^{2}\right]=[P_{v}-P_{\infty}(t)]-\frac{2\sigma}{R}-4\mu\frac{\dot{R}}{R},$
(5)

in which $\rho$ is the density of prominence plasma, and $\dot{R}$ and
$\ddot{R}$ are the first- and second-order derivatives of the bubble radius
with respect to time.

On the right-hand side of the equation, the first term
${[P}_{v}-P_{\infty}(t)]$, which measures the closeness of the applied
pressure to the bubble internal pressure, is the driving term for the bubble
evolution. The second term is the surface tension dominated by the magnetic
field, $\rm\sigma\sim\frac{B^{2}}{4\pi}$, and the last term is contributed by
effect of dynamic viscosity $\mu$ in the prominence. If we assume that the
bubble is a quasi-static evolution, the bubble remains in equilibrium at
different heights in the bubble. Then, by setting all time derivatives
($\ddot{R}$, $\dot{R}$) to zero, we get the following equilibrium condition:

$\rm[P_{v}-P_{\infty}(t)]=\frac{2\sigma}{R}\sim\frac{B^{2}}{2\pi R}.$ (6)

If the magnetic field remains invariant during the bubble evolution, the
pressure in the bubble should be inversely related to the bubble radius. If a
bubble keeps an arcade shape with a constant width of $W$ and a height of $h$,
we obtain that:

${\rm R}=\frac{1}{2}h+\frac{W^{2}}{8h}.$ (7)

As the bubble expands, the height of the bubble increases while the radius of
the bubble decreases (see Fig.7). According to the above equation, we obtain
that:

${\rm R}_{min}=\frac{1}{2}W=h_{{\rm R}_{min}},$ (8)

which suggests that as the bubble height exceeds half its width, the bubble
cannot keep its shape and begins to collapse. This scenario is consistent with
our observations.

Figure 7: Variations of bubble radius and height during the growth of the
bubble. The bubble is shown by the orange region.