# Observations of the Formation and Disappearance of a Funnel Prominence

Bo Yang  Yunnan Observatories, Chinese Academy of Sciences, 396 Yangfangwang,
Guandu District, Kunming, 650216, People’s Republic of China  Yunnan Key
Laboratory of Solar Physics and Space Science, Kunming 650011, People¡¯s
Republic of China  Jiayan Yang  Yunnan Observatories, Chinese Academy of
Sciences, 396 Yangfangwang, Guandu District, Kunming, 650216, People’s
Republic of China  Yunnan Key Laboratory of Solar Physics and Space Science,
Kunming 650011, People¡¯s Republic of China  Yi Bi  Yunnan Observatories,
Chinese Academy of Sciences, 396 Yangfangwang, Guandu District, Kunming,
650216, People’s Republic of China  Yunnan Key Laboratory of Solar Physics and
Space Science, Kunming 650011, People¡¯s Republic of China  Junchao Hong
Yunnan Observatories, Chinese Academy of Sciences, 396 Yangfangwang, Guandu
District, Kunming, 650216, People’s Republic of China  Yunnan Key Laboratory
of Solar Physics and Space Science, Kunming 650011, People¡¯s Republic of
China

###### Abstract

We present an observational study of the formation and disappearance of a
funnel prominence. Before the funnel prominence formed, cool materials from
the top of a preexisting polar crown prominence flowed along saddle-shaped
coronal loops to their base, forming a smaller prominence. Meanwhile, the
saddle-shaped coronal loops gradually rose, and U-shaped coronal loops, termed
prominence horns, began to appear along with a coronal cavity. Afterwards, a
cool column emerged from the chromosphere, rose vertically into the corona,
and then moved laterally to be transported into the U-shaped coronal loops.
The formed prominence slid into the chromosphere, while the U-shaped coronal
loops and the coronal cavity became more pronounced. As cool materials
accumulated at the base of the U-shaped coronal loops, these loops underwent a
significant descent and a V-shaped structure appeared at the base of the cool
materials, indicating that the U-shaped coronal loops may be dragged down to
sag. Subsequently, cool materials from the V-shaped structure continued to
flow almost vertically toward the chromosphere, forming the funnel prominence.
The vertical downflows might be produced by magnetic reconnection within or
between the sagging field lines. Due to persistent vertical downflows, the
U-shaped coronal loops were lifted up and prominence materials followed along
inclined coronal loops towards the chromosphere, causing the funnel prominence
to disappear. Our observations suggest that chromospheric plasma transported
into a coronal cavity and then drained out via vertical downflows can form a
funnel prominence.

Solar filaments(1495) — Solar prominences(1519) — Solar magnetic fields(1503)
— Solar coronal loops(1485)

##  1 Introduction

Solar prominences are relatively cool, dense, plasma structures suspended in
the much hotter, tenuous corona. They are commonly found in the solar
atmosphere and are distributed from the active latitudes all the way to the
polar crown, implying that prominences are a manifestation of a common
physical process occurring in the solar atmosphere. To understand how
prominences are supported, where and why they form, as well as how the
overdense prominence materials and magnetic fields dynamically interact have
long been the central problems in prominence physics (see reviews by Martin,
1998; Mackay et al., 2010; Berger, 2014; Parenti, 2014; Gibson, 2018; Chen et
al., 2020). The study of prominence formation provides an opportunity to
investigate these issues and may aid in a better understanding of the
evolution of magnetic fields and the mass cycles in the solar atmosphere.

Prominences are often surrounded by a low-density coronal structure known as a
coronal cavity (Gibson et al., 2010; Gibson, 2018). This is especially true
with polar crown prominences. Coronal cavities are observed as circular, half-
circular, or elliptical darkened regions above the solar limb in white light
(Gibson et al., 2006) and extreme ultraviolet (EUV) coronal images (Karna et
al., 2015a, b), particularly visible in the 193 Å passband of the Atmospheric
Imaging Assembly (AIA; Lemen et al., 2012) on board the _Solar Dynamics
Observatory_ (_SDO_ ; Pesnell et al., 2012). They are the limb counterpart to
the filament channel on the disk (Gibson et al., 2006; Karna et al., 2017) and
are in general believed to be representative of magnetic flux ropes made up of
twisted magnetic field lines (Gibson & Fan, 2006; Bak-Stȩślicka et al., 2013;
Gibson, 2018). When viewed down the axis of the magnetic flux ropes, the
helical field lines on the interior of the magnetic flux ropes form an
elliptical structure. The magnetic fields in the magnetic flux ropes on the
Sun are under-dense due to the force balance with surrounding coronal regions
and thus appear darker than the surrounding corona plasma (Gibson & Fan, 2006;
Gibson, 2015). In the _SDO_ /AIA 171 Å observation, coronal plasma is usually
shaped by magnetic field lines with a negative curvature, thus appearing as
U-shaped structures sitting at the bottom of a coronal cavity (Régnier et al.,
2011; Wang et al., 2016). The U-shaped structures, dubbed prominence horns
(Berger et al., 2012; Schmit et al., 2013; Wang et al., 2016), emanate from
the prominence and extend into the coronal cavity.

It is widely accepted that prominence magnetic fields support the cold dense
prominence materials against gravity and insulate them from the surrounding
hot corona. The coronal cavity reflects the fact that prominences have a flux
rope topology with dips in the magnetic field lines (Gibson et al., 2006;
Gibson, 2015). Direct measurement of prominence magnetic fields (Leroy, 1989;
Orozco Suárez et al., 2014; Schmieder et al., 2014), theoretical models
(Kippenhahn & Schlüter, 1957; Kuperus & Raadu, 1974; van Ballegooijen &
Martens, 1989; Aulanier & Demoulin, 1998; DeVore & Antiochos, 2000), and
observations (Cheng et al., 2014; Yan et al., 2014; Yang et al., 2014;
Filippov et al., 2015; Wang et al., 2015; Ouyang et al., 2017) have indicated
that magnetic dips of twisted magnetic flux ropes or sheared arcades are
believed to be suitable for supporting channel prominences, which have spines
and are usually formed above the polarity inversion lines (PILs), a line
separating two large regions of opposite vertical components of the
photospheric magnetic fields (Martin, 1998). The _SDO_ /AIA observations
revealed a potentially new type of prominence, termed “funnel prominences”
(Berger, 2014; Liu et al., 2014). They feature a characteristic inverted cone
structure (Kleczek, 1972) and share some morphological similarities to
quiescent prominences, such as the conical shapes at the tops of funnels and
the prominence horns (Berger et al., 2012; Liu et al., 2014). However, whether
funnel prominences are surrounded by a coronal cavity and must be associated
with PILs remain heretofore unknown (Berger, 2014; Liu et al., 2014). In
earlier ground-based observations, funnel prominences and the so called
“coronal spiders” (Allen et al., 1998) were categorized as coronal cloud
prominences (Martin, 2015). Coronal spiders appear closer in character to
coronal rain events, in which plasma condenses high in the corona and then
falls under gravitational attraction along magnetic field lines to the
chromosphere (Antolin & Rouppe van der Voort, 2012). In contrast, downflows in
funnel prominences are constrained to narrow conical regions, and they flow
along meandering paths down to the chromosphere at much lower speeds than
free-fall speeds, suggestive of cross-field mass transport (Liu et al., 2012,
2014). A basic question of funnel prominences is what type of magnetic field
configuration supports them. Some previous observational studies have provided
clues that magnetic dips of coronal arcades (Kleczek, 1972; Ivanov & Platov,
1977; Liu et al., 2012; Keppens & Xia, 2014; Schad et al., 2016; Li et al.,
2018b; Filippov, 2023), separatrix surfaces between adjoining coronal loop
systems (Lin et al., 2006; Martin, 2015), and the funnel-like open coronal
magnetic fields (Panasenco et al., 2019) may be the sites for the formation
and accumulation of funnel prominence mass.

At present, it is generally believed that the material of channel prominences
originates from the solar photosphere or chromosphere (Spicer et al., 1998;
Song et al., 2017). Much progress has been made in revealing the mechanisms
that account for the transport of the photospheric or chromospheric material
into the corona to form channel prominences. Chromospheric material can be
directly injected into a filament channel through intermittent magnetic
reconnection in the form of surges or jets (Wang, 1999; Li & Zhang, 2016; Wang
et al., 2018; Shen et al., 2019; Yang & Chen, 2019; Yang et al., 2019; Li et
al., 2023). Besides, photospheric or chromospheric materials could be lifted
into the corona by the emergence of a helical flux rope (Okamoto et al., 2008,
2010; Yan et al., 2017). Numerical simulations have illustrated that
chromospheric material can be heated and evaporated into the corona via
localized heating concentrated exclusively at the chromospheric footpoints of
the prominence, and then condensed as cool prominence material (Antiochos &
Klimchuk, 1991; Karpen & Antiochos, 2008; Luna et al., 2012; Kaneko &
Yokoyama, 2015; Xia & Keppens, 2016; Zhou et al., 2020; Huang et al., 2021).
This is the so-called evaporation-condensation model and has recently been
confirmed by our observations (Yang et al., 2021), as well as those of Li et
al. (2021). More recently, observations have demonstrated that the formation
of prominences may not be a single mechanism, but may be under the combined
action of several mechanisms (Huang et al., 2023; Li et al., 2023).

The mass source and transport mechanisms of funnel prominences still remain
questionable. A handful of observations have presented that in situ
condensation of coronal hot plasmas in magnetic dips can result in the
formation of funnel prominences (Liu et al., 2012; Schad et al., 2016;
Filippov, 2023). By implementing a 2.5D numerical simulation, Keppens & Xia
(2014) found that continuous condensation of hot plasma in magnetic dips of
arcaded field lines through ongoing evaporation-condensation process can lead
to the formation of funnel prominences. In addition to the chromospheric
evaporation, the injected hot jets (Li et al., 2023), emerging magnetic
bubbles and plumes (Berger et al., 2011) might all be potential sources of the
hot plasma. Furthermore, several authors have proposed that the mass source of
funnel prominences could even come from the mass falling back to the Sun in
the wake of the eruption in the surrounding atmosphere (Lin et al., 2006;
Panasenco et al., 2019).

High spatiotemporal images of the prominences have revealed a wealth of fine,
highly dynamic phenomena within the prominences, such as rising bubbles and
plumes (Berger et al., 2011; Shen et al., 2015), vortexes (Li et al., 2018a),
descending knots (Chae et al., 2008; Chae, 2010; Bi et al., 2020), and dynamic
vertical threads (Chae et al., 2008; Chae, 2010), etc. It is not fully force-
free inside the prominence (Anzer & Heinzel, 2007). Gravity forces play an
essential role in the dynamic evolution of the prominence (Gibson, 2018). The
descending knots and the dynamic vertical threads, which follow along the
direction of gravity and may imply the prominence plasma to slip across
magnetic field lines towards the solar surface, are gravity-driven flows. Chae
et al. (2008) found that a pattern of persistent horizontal flows originated
from a chromospheric region and rose to corona to form descending knots and
vertical threads. They suggested that the horizontal flows provide mass to sag
the initially horizontal magnetic field lines, resulting in a vertical stack
of magnetic dips suitable for the support and development of vertical threads
or bright descending knots. Liu et al. (2012) observed that continuous
condensed prominence mass gradually formed a V-shaped structure, and sustained
mass drainage originated from the V-shaped structure and orientated along
vertical direction as dynamic vertical threads. A number of theories and
simulations have managed to investigate how the addition and accumulation of
prominence material alters the structure of the coronal magnetic field and
then produces the descending knots and the dynamic vertical threads. They
found that as the prominence material continuous to be added and accumulated
in the magnetic dips during the prominence formation, the accumulated
prominence material can weigh down the dipped field lines, causing them to sag
with its weight and compress them at the base of the prominence (Hillier & van
Ballegooijen, 2013; Terradas et al., 2015). As a result, magnetic reconnection
may take place within or between the sagging, dipped field lines to form the
descending knots and the dynamic vertical threads (Lerche & Low, 1980; Petrie
& Low, 2005; Chae, 2010; Hillier et al., 2012; Low et al., 2012b; Keppens &
Xia, 2014; Jenkins & Keppens, 2021). It is noteworthy that the initial
compression of the magnetic field by the heavy prominence should create a
large but finite current between the flux surfaces, which would result in the
spontaneous breakdown in the frozen-in condition, allowing material to pass
from one field line to another in the direction of gravity (Low et al.,
2012a). Nevertheless, the cross-field diffusion of neutral matter should also
play an important role in the mass loss and dynamics of the prominence
(Gilbert et al., 2002). Although previous studies have highlighted the
important role of gravity in the dynamic evolution of prominences, the
detailed process by which heavy prominence material interacts with magnetic
fields has not yet been directly observed.

In this paper, we present observations of a funnel prominence formed by
transporting chromospheric materials into a coronal cavity. This provides us
with a good opportunity to investigate in detail the dynamical interaction
process between the heavy prominence materials and the magnetic fields
supporting them. In particular, this observation has the potential to enhance
our understanding of the formation, support, and disappearance of funnel
prominences and dynamic vertical threads.

##  2 Observations

The event was well-observed by the _SDO_ /AIA, which takes the full-disk
images of the Sun in seven extreme ultraviolet (EUV) and two ultraviolet (UV)
wavelengths with a pixel size of 0$\arcsec$.6 at a high cadence of up to 12s.
Here, the Level 1.5 images centered at 304 Å (He II, 0.05 MK) were adopted to
study the transport of chromospheric materials into the coronal cavity and the
subsequent formation of the funnel prominence. The Level 1.5 images centered
at 171 Å (Fe ix, 0.6 MK ) and 193 Å (Fe xii, 1.3 MK and Fe xxiv, 20 MK) were
used to investigate the magnetic field configuration of the funnel prominence
and the evolution of the horn-like structures associated with the funnel
prominence. In addition, this event was also monitored with routine
observations from the Global Oscillation Network Group (GONG; Harvey et al.,
1996) in $H_{\alpha}$. The GONG $H_{\alpha}$ images have a pixel size of
1$\arcsec$.0 and a cadence of 1 minute. The AIA data used in this study were
taken between 19:59 UT on 2015 January 2 and 20:00 UT on 2015 January 3, and
the GONG data were taken between 07:30 UT and 20:00 UT on 2015 January 3. They
were processed by using standard software programs in the Solar Software
(SSW), and aligned by differentially rotating to the reference time of 18:00
UT on 2015 January 3.

##  3 Results

###  3.1 Disappearance of a Preexisting Prominence and Formation of a New
Smaller One

The funnel prominence in question was formed on 2015 January 3. Before the
funnel prominence formed, there was a small polar crown prominence seen beyond
the northwest limb of the Sun. The funnel prominence formed to the north of
the preexisting prominence. Figure 1(a) illustrates the general appearance of
the preexisting prominence at 20:00 UT on 2015 January 2. From about 20:00 UT
on 2015 January 2 to 05:00 UT on 2015 January 3, the preexisting prominence is
disturbed. It underwent a dynamic evolution process and its main body
disappeared. Persistent prominence materials drained from its top down to the
solar surface along curved paths (Figure 1(b −d)), reminiscent of the coronal
rain caused by the falling of the prominence material (Schad et al., 2016;
Antolin & Froment, 2022; Chen et al., 2022). By about 00:00 UT on January 3,
the falling prominence materials split into two branches, the lower one
sliding along the original path, and the upper one moving along a curved path
that did not fall back to the solar surface (Figure 1(d)). In the meantime,
saddle-shaped coronal loops, which are highly consistent with the path of the
upper branch, appeared (Figure 1(e) and Figure 1(h)). The draining prominence
materials traced out of these saddle-shaped coronal loops, along which they
traveled and accumulated to their base to form a new smaller prominence
(Figure 1(e −g)). With the draining of the prominence materials and the
formation of the new prominence, the left half of the saddle-shaped coronal
loops, which correspond to the top part of the preexisting prominence,
gradually rose and moved closer to the newly formed prominence (see the
associated animation). Subsequently, U-shaped coronal loops began to appear,
which emanate from the newly formed prominence and extend into the corona
(Figure 1(g)). The preexisting prominence finally disappeared, leaving only a
small fraction of the prominence materials suspended in the corona. We
speculate that the accumulated prominence materials may weigh down and drag
the saddle-shaped coronal loops, causing them to sag. Moreover, as the
prominence materials are drained, the force balance on the saddle-shaped loops
changes and this may cause them to appear to rise. The U-shaped coronal loops
are reminiscent of the prominence horns (Berger et al., 2012; Schmit et al.,
2013; Wang et al., 2016), which sit on top of a prominence and usually depict
the boundary of a coronal cavity. The rise of the saddle-shaped loops could
also be due to rotation of a coronal cavity flux rope over the limb, and the
appearance of the U-shaped coronal loops might be the result of the rotation
of the coronal cavity into a favorable viewing angle. These observations
indicate that the prominence materials are supported in the magnetic dips of
the sagged coronal magnetic field lines, and they can be transferred from one
part of the prominence to another to form a new prominence.

###  3.2 Chromospheric Materials Transported into a Coronal Cavity and
Formation of a Funnel Prominence

Figure 2 shows the transport of chromospheric materials into a coronal cavity
to form the funnel prominence. Immediately after the formation of the small
prominence, a cool column emerged from the solar chromosphere below the
suspended residual prominence materials and rose into the corona ( see Figure
1(g) and Figure 2(a −b)). The cool column rotated clockwise as it rose (see
the associated animation), which shows some similarities to the untwisting
motion of the erupted minifilaments. With the emergence and rise of the cool
column, the residual prominence materials gradually rose and moved laterally
towards the newly formed small prominence. The rising of the cool column and
the residual prominence materials are clearly shown on a spacetime plot
(Figure 3(c)), which along a dotted line,“AB,” in Figure 2(b), was constructed
from AIA 304 Å images. The top of the cool column rose with a mean velocity of
about 13.2 km s-1, which is higher than the rising motion of the emerging flux
observed by Okamoto et al. (2010). The rising cool column might be a rising
minifilament or a cool magnetic flux rope with a negative, left-hand twist.

The rise of the cool column lasted approximately 1.5 hr. By about 06:40 UT on
2015 January 3, the cool column reached a projected height of about 32 Mm. It
then moved laterally towards the previously formed prominence. At the same
time, the U-shaped coronal loops, more precisely termed prominence horns,
gradually became more pronounced (Figure 2(c −e)). Of particular note is that
a coronal cavity also gradually appeared above the horn-like structure (see
Figure 2(c −i) and Figure 3(a −b)). These observations may further support our
conjecture that the appearance of the horn-like structure and the coronal
cavity might be caused by the rotation of a coronal cavity flux rope structure
into a favorable viewing angle. Eventually, most of the material in the cool
column was transferred into the coronal cavity (Figure 2(d)). The process of
the cool column moving laterally and being transported into the coronal cavity
can be clearly shown on a spacetime plot (Figure 3(d)), which along a dotted
line,“CD,” in Figure 2(c), was constructed from AIA 304 Å images. The cool
column moved laterally and was transferred into the coronal cavity at a speed
of about 20.4 km s-1. It is possible to be disturbed by the mass transport,
the previously formed prominence slid to the solar surface and disappeared
(Figure 2(b −d)). Subsequently, it is clear that the cool materials at the
bottom of the coronal cavity underwent a to-and-fro movement during 08:00
UT-09:30 UT on 2015 January 3 (see Figure 2(d −f) and associated animation).
Thereafter, cool materials accumulated at the bottom of the coronal cavity and
then started to extend almost vertically toward the solar surface, forming a
funnel prominence with a typical inverted cone structure pointing sunward and
a distinct horn-like structure attached to its top and connected it to a
coronal cavity (Figure 2(g −i)).

It is evident from the AIA 171 Å difference images (Figure 3(a −b)) that the
U-shaped coronal loops gradually sunk down during the to-and-fro movement of
the cool materials. This suggests that the cool materials transported into the
coronal cavity may significantly deform the magnetic fields of the coronal
cavity. It is likely that the cool materials in the coronal cavity may drag
the magnetic fields of the coronal cavity down and compress those magnetic
fields that lie directly below the cool materials, causing the associated
magnetic fields to sink.

The most notable characteristic observed during the formation of the funnel
prominence was the development of a V-shaped structure at the base of the
accumulated cool materials after the to-and-fro movement of the cool materials
(Figure 4(a1 −a2)). The V-shaped structure, which hints at the presence of
magnetic dips, may be created by the sagging of the magnetic fields of the
coronal cavity by the accumulated cool materials. Subsequently, persistent
cool materials stemmed from this structure and slid towards the solar
chromosphere (Figure 4(a2 −a4) and Figure 4(b2 −b4)). To track the dynamic
evolution of the sliding cool materials, a spacetime plot was constructed
along slice “EF” in Figure 4(a5) using AIA 304 Å images, and the result was
provided in Figure 4(e). We find that the projected velocity of the sliding
cool materials reached up to about 40.7 km s-1, which is in line with previous
observations (Chae et al., 2008; Chae, 2010; Liu et al., 2012) and simulations
(Keppens & Xia, 2014; Jenkins & Keppens, 2021). Accompanied by the persistent
cool material sliding towards the solar chromosphere, the inverted cone
structure of the funnel prominence gradually formed (Figure 4(a4 −a5)).
Furthermore, the GONG $H_{\alpha}$ observations show that the intensity at the
central part of the prominence also gradually increased towards the solar
chromosphere (Figure 4(b1 −b5)). The formed funnel prominence showed up as a
vertical absorption thread on the AIA 193 Å image (Figure 4(c)). The formed
V-shaped structure, the sustained sliding of cool materials from its
underside, and the formation of the vertical absorption thread might be
consistent with the scenario of cross-field mass transport caused by magnetic
reconnection within or between the sagging, dipped field lines (Lerche & Low,
1980; Petrie & Low, 2005; Chae, 2010; Liu et al., 2012; Low et al., 2012b;
Keppens & Xia, 2014; Jenkins & Keppens, 2021). It is worth mentioning that the
U-shaped coronal loops rose up significantly in the course of the cool
material sliding toward the chromosphere (Figure 4(d)). Magnetic reconnection
within or between the sagging, dipped field lines may alter the local magnetic
field topology of the prominence and lead to significant mass loss due to the
vertical downflows. This can result in the upward magnetic tension force that
exceeds the gravity force, causing the U-shaped coronal loops to rise.

###  3.3 Disappearance of the Funnel Prominence by Persistent Mass Drainage

The funnel prominence lived for about seven hours and eventually disappeared.
Figure 5 shows the detailed process of the disappearance of the funnel
prominence. As the cool materials continued to slide from the bottom of the
prominence towards the chromosphere, from about 12:20 UT on 2015 January 3, a
large amount of cool materials flowed from the prominence along well-defined
curved paths towards the solar surface (Figure 5(a −c)). By about 18:40 UT on
2015 January 3, the funnel prominence completely disappeared (Figure 5(d)).
Scrutinizing the AIA 171 Å negative images, it is found that the U-shaped
coronal loops continued to rise, gradually became flatter, and finally evolved
into inclined coronal loops (Figure 5(e −h)). Comparing the AIA 304 Å and 171
Å observations, it is obvious that the prominence materials are drained
towards the solar chromosphere along these inclined coronal loops. On the
basis of our observations, we conjecture that the disappearance of the funnel
prominence may be caused by the drainage of the prominence materials along
well-defined curved coronal loops, which may be initiated by the persistent
sliding of cool materials from the base of the prominence towards the
chromosphere. Owing to the persistent sliding of cool materials from the base
of the prominence towards the chromosphere, the balance between magnetic
tension and gravitational forces is broken down. As a result, the surplus
magnetic tension force may cause the U-shaped coronal loops and the cool
materials trapped in them to be lifted, then the U-shaped coronal loops become
flatter and eventually involve into inclined coronal loops. At the same time,
the prominence materials drain along the inclined coronal loops towards the
solar surface, leading it to disappear completely.

##  4 Conclusion and Discussion

Employing _SDO_ /AIA EUV and GONG $H_{\alpha}$ observations, the formation and
disappearance of a funnel prominence is investigated in detail. Prior to the
formation of the funnel prominence, we find that cool materials from the top
of a preexisting polar crown prominence flowed partly along a curved path down
to the solar chromosphere and partly along saddle-shaped coronal loops to
their base. This results in the disappearance of the preexisting prominence
and the simultaneous formation of a new, smaller one. The saddle-shaped
coronal loops slowly raised and U-shaped coronal loops associated with a
coronal cavity gradually began to appear. Immediately after the formation of
the small prominence, a cool column emerged from the solar chromosphere and
was lifted into the corona. Upon reaching a projected height of approximately
32 Mm above the solar surface, the cool column moved laterally. Cool materials
carried by the cool column then were transported into the U-shaped coronal
loops and accumulated at their base. The formed prominence slid into the solar
chromosphere and disappeared. In the meantime, the U-shaped coronal loops and
the coronal cavity became more pronounced. The U-shaped coronal loops are in
line with prominence horns, which depict the boundary of the coronal cavity.
The rise of the saddle-shaped loops could be due to rotation of a coronal
cavity flux rope over the limb, and the appearance of the U-shaped coronal
loops associated with the coronal cavity might be the result of the rotation
of the coronal cavity flux rope into a favorable viewing angle. Subsequently,
a V-shaped structure formed at the base of the accumulated cool materials,
from which persistent cool materials slid almost vertically towards the solar
chromosphere, forming a funnel prominence with a typical inverted cone
structure and a distinct horn-like structure emanating from the prominence and
extending into a coronal cavity. Due to the vertical drainage of the
prominence materials, a large amount of prominence materials followed along
inclined coronal loops towards the chromosphere, and the funnel prominence
disappeared. The AIA 171 Å difference images reveal that the U-shaped coronal
loops sunk down significantly as cool materials accumulated at the base of
these loops and rose up significantly as persistent cool materials draining
from the base of the prominence. The descent of the U-shaped coronal loops and
the formation of the V-shaped structure may suggest that cool materials
transported into the coronal cavity dragged the magnetic field lines down,
causing them to sag. Magnetic reconnection within or between the sagging field
lines may lead to the vertical drainage of prominence materials. This
observation demonstrates that chromospheric plasma transported into a coronal
cavity and then drained out through vertical downflows can result in the
formation of a funnel prominence.

The formed funnel prominence have a distinct horn-like structure (the U-shaped
coronal loops) emanating from the prominence and extending into a coronal
cavity (see Figure 2(e −i) and Figure 3(a −b)). Gibson et al. (2010) and Karna
et al. (2015b) have indicated that the overall three-dimensional morphology of
coronal prominence cavities are long tubes with an elliptical cross-section.
The long tubes may correspond to magnetic flux ropes made up of twisted
magnetic field lines (Gibson & Fan, 2006; Bak-Stȩślicka et al., 2013). Our
observation suggests that the formed funnel prominence may be supported in a
coronal cavity flux rope. Therefore, the slow rise of the saddle-shaped
coronal loops, and the gradually appearance of the U-shaped coronal loops and
the coronal cavity could be well explained by slowly rotating a long flux rope
whose axis distributed roughly along the solar latitude around the limb. This
observation fits very well with such a picture that chromospheric materials
are transported into a coronal cavity by a rising cool column to form a funnel
prominence. The rising cool column, which might be a rising minifilament or an
emerging magnetic flux rope carrying chromospheric materials, could interact
with the coronal cavity to transport its materials into the coronal cavity.
The scenario of rising minifilament is similar to the observations of Shen et
al. (2019) and Yang et al. (2019), while the scenario of magnetic flux rope
emergence is consistent with the observations of Okamoto et al. (2010) and
Yang & Chen (2019). However, the observations presented here are insufficient
for a discussion of the carriers of chromospheric materials and the detailed
interaction processes. The chromospheric materials transported into the
coronal cavity may drag the magnetic field lines downward and compress them at
their base, causing the magnetic field lines to sag and further to form the
V-shaped structure. Afterwards, magnetic reconnection may occur within or
between the sagging magnetic field lines, resulting in the continued slide of
cool materials from the V-shaped structure into the solar chromosphere,
followed by the rise of the U-shaped coronal loops and finally the formation
of the funnel prominence. To a certain extent, our observations reveal the
interaction process between the transported prominence materials and the
prominence magnetic fields. In our observations, the persistent vertical
drainage of cool materials from the base of the prominence to the solar
chromosphere may be indicative of cross-field slippage of cool, poorly ionized
prominence material, perhaps involving magnetic reconnection (Low et al.,
2012a, b). Furthermore, these observations support the previous idea (Chae et
al., 2008; Low et al., 2012a) that the formed funnel prominence, appearing as
a vertical absorption thread on the AIA 193 Å observation, might be supported
in a vertical stack of magnetic dips of sagged coronal field lines.

Horn-like structures extend from the cool and dense prominence, connecting it
to the hot and tenuous corona. It is widely accepted that horns are a part of
the formation process of prominences from cooling and condensing coronal
plasmas (Berger et al., 2012; Liu et al., 2012; Luna et al., 2012; Schmit et
al., 2013; Xia et al., 2014). An opposite scenario was proposed by Wang et al.
(2016) that horns are formed due to the prominence evaporation, likely
involving some heating and diluting process of the central prominence mass. In
the present study, even though the appearance of the horn-like structures (the
U-shaped coronal loops) could be attributed to the rotation of a coronal
cavity flux rope into a favorable viewing angle, the interplay of prominence
materials and magnetic fields may also play a role in the evolution of the
horn-like structures. The chromospheric materials were transported into the
coronal cavity and then they dragged the magnetic field lines downward,
leading them to be compressed (see Figure 3(a −b)). As a result, the density
at the base of the dragged and compressed U-shaped coronal loops may rise,
intensifying the local emission and making the horn-like structures more
visible (see Figure 2(e −i) and Figure 3(a −b)).

Direct observational evidence for magnetic reconnection within or between the
sagging magnetic field lines has not yet been found. In the observations
reported here, the descent of the U-shaped coronal loops and the formed
V-shaped structure are direct observational evidence that the U-shaped coronal
loops are dragged by the cool materials accumulated in the coronal cavity to
sag and generate deeper magnetic dips. The projected velocities of the
vertical downflows from the V-shaped structure to the chromosphere reach up to
tens of kilometers per second, which are commensurate with the velocities of
the vertical downflows created in the numerical simulations by the magnetic
reconnection between the sagging magnetic field lines (Keppens & Xia, 2014;
Jenkins & Keppens, 2021). It is worthwhile to mention that the cross-field
diffusion of neutral atoms in the solar prominence along the direction of
gravity can also create vertical downflows. However, the velocities of the
vertical downflows created by the cross-field diffusion of neutral atoms are
substantially lower than those found in the present study. Gilbert et al.
(2002) calculated the diffusion of neutral atoms in a simple prominence model
and found that the magnitudes of the H and He downflow velocities have values
of about 370 cm s-1 and 8.1 $\times$ 103 cm s-1, respectively, for a density
of 1010 cm-3. In the course of the prominence materials sliding vertically
into the chromosphere, the U-shaped coronal loops were evidently raised (see
Figure 4(d)). The vertical downflows, which might be caused by magnetic
reconnection in or between sagging magnetic field lines, can lead to
prominence material loss, resulting in an upward magnetic tension force that
exceeds gravitational force, forcing the U-shaped coronal loops to rise. It is
very likely that the persistent vertical downflows and the associated rise of
the U-shaped coronal loops are indirect evidence of magnetic reconnection in
or between the sagging magnetic field lines.

This observation has far-reaching consequences for our understanding of
prominence disappearance and eruptions. Theoretical models of mass loading in
eruptions (Fong et al., 2002; Low et al., 2003) have predicted that prominence
weight may play a role in storing magnetic energy for driving coronal mass
ejections, and significant mass-loss to a prominence may facilitate its
eruption. Some previous observations have shown that a sizable volume of
filament material drains along the filament axis toward their endpoints prior
to its violent eruption (Bi et al., 2014; Jenkins et al., 2018; Dai et al.,
2021). They suggested that the material drainage may be able to influence the
local and global properties of the filament magnetic fields and thus
facilitate its eruption. However, what causes the filament material to drain
along the filament axis towards its endpoints is still unknown. The filament
may be disturbed by its nearby eruptive activities and thus drive material
drainage (Dai et al., 2021). Song et al. (2023) found that the interaction
between the filament and its nearby network magnetic fields could also cause
material drainage. In prominences, it is possible that the heavy prominence
material in the magnetic dips may drag the prominence magnetic fields to sag
and drive the magnetic reconnection within or between these sagging magnetic
field lines to form vertical downflows that appear as dynamic vertical threads
(Chae, 2010; Low et al., 2012b). On the one hand, the magnetic reconnection
may modify the local magnetic field topology of the prominence. On the other
hand, material in the magnetic dips is considerably lost by the vertical
downflows, thereby reducing the gravity of the prominence material and
accordingly increasing the net upward force, leading the prominence material
to be lifted and drained along its magnetic field lines. As a result,
prominences may be facilitated to erupt. Further investigations are needed to
determine whether gravity-driven vertical downflows inside prominences can
cause prominence material to drain along the prominence magnetic fields,
leading to the disappearance or eruption of the prominence.

The authors sincerely thank the anonymous referee for detailed comments and
useful suggestions for improving this manuscript. The authors thank the
scientific/engineering team of _SDO_ and GONG for providing the excellent
data. This work is supported by the National Key R&D Program of China
(2019YFA0405000); the Natural Science Foundation of China, under grants
12073072, 12173084, 12273108, 12273106, 12203097, and 11933009; the Yunnan
Province XingDian Talent Support Program; the “Yunnan Revitalization Talent
Support Program” Innovation Team Project (202405AS350012); the CAS grant
“QYZDJ-SSW-SLH012”; and the Yunnan Key Laboratory of Solar Physics and Space
Science (202205AG070009).

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Figure 1: Sequence of AIA 304 Å (panels (a −d)) and composite of the AIA 304 Å
(red) and 171 Å (blue) (panels (e −g)) images showing the disappearance of a
preexisting prominence and the formation of a new one. (h) An AIA 171 Å
difference image displaying the saddle-shaped coronal loops. The white arrows
in panels (d) and (e) point to the prominence material that moves along the
saddle-shaped loops and sinks to their base. The dashed box outlines the FOV
of panels (a −d). An animation of panels (e) −(g) is available. The animation
has a 7 s cadence, covering 19:59 UT on 2015 January 2 to 05:02 UT on 2015
January 3. Figure 2: (a −i) Composite of the _SDO_ /AIA 304 Å (red) and 171 Å
(blue) images display the formation of the funnel prominence and associated
horn-cavity structures. The white arrow in panel (a) points to a rising cool
column. The cool column is raised in the direction of the dotted line, “AB”,
and then is transported into the U-shaped coronal loops in the direction of
the dotted line, “CD”. The dotted lines, AB and CD also mark two slits and are
used for making the distance-time plots as shown in Figure 3. The inserted
image is a GONG $H_{\alpha}$ image. The formed funnel prominence, with bright
horn-like structures emanating from it and extending into a coronal cavity,
shows an inverted cone structure pointing sunward. An animation of this figure
is available. The animation has an 8 s cadence, covering 05:00 UT to 11:00 UT
on 2015 January 3. Figure 3: (a −b) AIA 171 Å difference images displaying the
prominence materials drag the U-shaped coronal loops down, leading to the
compression of the U-shaped coronal loops at their base. (c −d) Distance-time
plots from AIA 304 Å images for the two slits ( AB and CD) as shown in Figure
2(b) and (c). The white dashed lines outline the motions of the rising cool
column. Figure 4: Sequence of AIA 304 Å (panels (a1 −a5)) and GONG
$H_{\alpha}$ negative (panels (b1 −b5)) images presenting the detailed process
that cool materials drain from a V-shaped structure down to the solar
chromosphere to form the funnel prominence. The funnel prominence corresponds
to a vertical absorption thread in AIA 193 Å (panel (c)) observation. An AIA
171 Å difference image (panel (d)) showing the rise of the U-shaped loops
during the prominence material drainage. The white arrow points to a V-shaped
structure. The dashed line “EF” indicates the slit position of the distance-
time plot as shown in panel (e). The plus signs outline the motions of the
draining material. An animation of panels (a1) −(a5) is available. The
animation has a 4 s cadence, covering 09:29 UT to 11:20 UT on 2015 January 3.
Figure 5: AIA 304 Å (panels (a −d)) and the negative 171 Å (panels (e −h))
images exhibiting the disappearance of the funnel prominence and the
associated horn-like structure by the drainage of prominence material along
well-defined curved coronal loops.