# The Solar Origin of an Intense Geomagnetic Storm on 2023 December 1st:
Successive Slipping and Eruption of Multiple Magnetic Flux Ropes

Z.Zheng Sun0000-0001-5657-7587  T.Ting Li0000-0001-6655-1743  Y.Yijun
Hou0000-0002-9534-1638  H.Hui Tian0000-0002-1369-1758  Z.Ziqi
Wu0000-0002-1349-8720  K.Ke Li  Y.Yining Zhang0000-0001-5933-5794  Z.Zhentong
Li0000-0002-4230-2520  X.Xianyong Bai0000-0003-2686-9153  L.Li
Feng0000-0003-4655-6939  C.Chuan Li0000-0001-7693-4908  Z.Zhenyong
Hou0000-0003-4804-5673  Q.Qiao Song0000-0003-3568-445X  J.Jingsong Wang
G.Guiping Zhou0000-0001-8228-565X  School of Earth and Space Sciences, Peking
University, Beijing 100871, People’s Republic of China  National Astronomical
Observatories, Chinese Academy of Sciences, Beijing 100101, People’s Republic
of China  School of Astronomy and Space Science, University of Chinese Academy
of Sciences, Beijing 100049, People’s Republic of China  Key Laboratory of
solar activity and space weather, National Space Science Center, Chinese
Academy of Sciences, Beijing 100190, People’s Republic of China  Key
Laboratory of Dark Matter and Space Astronomy, Purple Mountain Observatory,
Chinese Academy of Sciences, Nanjing 210023, People’s Republic of China
School of Astronomy and Space Science, University of Science and Technology of
China, Hefei 230026, People’s Republic of China  School of Astronomy and Space
Science, Nanjing University, Nanjing 210023, People’s Republic of China  Key
Laboratory for Modern Astronomy and Astrophysics (Nanjing University),
Ministry of Education, Nanjing 210023, People’s Republic of China  National
Satellite Meteorological Center (National Centre for Space Weather), China
Meteorological Administration, Beijing 100081, People’s Republic of China
Innovation Center for FengYun Meteorological Satellite (FYSIC), Beijing
100081, People’s Republic of China  Key Laboratory of Space Weather, National
Center for Space Weather, China Meteorological Administration, Beijing 100081,
People’s Republic of China

###### Abstract

The solar eruption that occurred on 2023 November 28 (SOL2023-11-28) triggered
an intense geomagnetic storm on Earth on 2023 December 1. The associated
Earth’s auroras manifested at the most southern latitudes in the northern
hemisphere observed in the past two decades. In order to explore the profound
geoeffectiveness of this event, we conducted a comprehensive analysis of its
solar origin to offer potential factors contributing to its impact. Magnetic
flux ropes (MFRs) are twisted magnetic structures recognized as significant
contributors to coronal mass ejections (CMEs), thereby impacting space weather
greatly. In this event, we identified multiple MFRs in the solar active region
and observed distinct slipping processes of the three MFRs: MFR1, MFR2, and
MFR3. All three MFRs exhibit slipping motions at a speed of 40–137 km s-1,
extending beyond their original locations. Notably, the slipping of MFR2
extends to $\sim$30 Mm and initiate the eruption of MFR3. Ultimately, MFR1’s
eruption results in an M3.4-class flare and a CME, while MFR2 and MFR3
collectively produce an M9.8-class flare and another halo CME. This study
shows the slipping process in a multi-MFR system, showing how one MFR’s
slipping can trigger the eruption of another MFR. We propose that the CME–CME
interactions caused by multiple MFR eruptions may contribute to the
significant geoeffectiveness.

###### keywords:

Coronal Mass Ejections, Initiation and Propagation; Magnetic Reconnection,
Observational Signatures; Magnetosphere, Geomagnetic Disturbances

##  1 Introduction

Magnetic flux ropes (MFRs) are constituted by a collection of helical magnetic
field lines winding around a common axis (Titov and Démoulin, 1999; Krall et
al., 2001; Liu, 2020). Extensive observations and simulations highlight the
significant role of MFRs in driving solar flares and coronal mass ejections
(CMEs), which have important impacts on the Earth and planetary space
environments (Inoue and Kusano, 2006; Cheng et al., 2011; SONG et al., 2015;
Jiang et al., 2019). Statistical analysis reveals that approximately half of
the CMEs are accompanied by the presence of MFRs (Nindos et al., 2015).
Therefore, investigating the detailed mechanisms of the MFR’s formation,
initiation, and evolution processes is essential for a better understanding of
their effects on space weather.

Two prevailing assumptions regarding the formation of an MFR exist. Firstly,
MFRs can be pre-existing before an eruption, originated from the convection
zone through flux emergence (Leka et al., 1996; Manchester et al., 2004; Fan,
2009; Török et al., 2014) or from magnetic reconnection preceding the eruption
(Patsourakos, Vourlidas, and Stenborg, 2013; Chintzoglou, Patsourakos, and
Vourlidas, 2015; Cheng et al., 2023). Secondly, MFRs may be formed during the
eruption through magnetic reconnection (Cheng et al., 2011; Wang et al., 2017;
Liu et al., 2018). An additional scenario involves a pre-existing seed MFR,
which continues its buildup as the eruption progresses (Cheng, Guo, and Ding,
2017; Veronig et al., 2018; Chen et al., 2019).

Coronal sigmoids, S-shaped magnetic structures above polarity-inversion lines
(PILs) observed in soft X-ray and EUV, are often considered MFR progenitors
(Chen, 2011; Jiang et al., 2013; Archontis, Hood, and Tsinganos, 2014). They
could either be the MFR itself or sheared arcades providing magnetic
conditions for MFR buildup (Gibson et al., 2006; Liu et al., 2010; Savcheva et
al., 2012). Solar filaments, dense and cold plasma suspended in the tenuous
and hot corona, can also be regarded as MFRs or sheared arcades (Canou and
Amari, 2010; Li et al., 2018b; Sun et al., 2023). Observations indicate that
they can act as seed MFRs and increase poloidal magnetic flux during eruptions
(Liu et al., 2019; Xie et al., 2023). There can be simultaneous multiple MFRs
in one eruption, which may cause a complicated eruption process (Ding, Hu, and
Wang, 2006; Hou et al., 2018, 2023; Awasthi et al., 2018).

Upon formation, the MFR exhibits distinct magnetic connectivities compared to
the surrounding field lines due to their twisted nature. Consequently, the
twisted MFR are wrapped by quasi-separatrix layers (QSLs), serving as a
boundary between the MFR and the surrounding magnetic field (Demoulin et al.,
1996, 1997; Titov and Démoulin, 1999; Titov, Hornig, and Démoulin, 2002). The
cross-section between the QSLs and the photosphere, known as the QSL
footprints, often manifests a double J shape, with the hook part encircling
the MFR footpoints. This characteristic has been validated through numerous
simulations and observations (Aulanier, Janvier, and Schmieder, 2012; Janvier
et al., 2013; Zhao et al., 2016). Notably, several studies indicate a
correspondence between the flare ribbons and QSL footprints (Savcheva et al.,
2015; Zhao et al., 2016; Jiang et al., 2018).

Three-dimensional (3D) magnetic reconnection occuring in the QSLs is often
associated with apparent slipping motion of field lines, known as the
“slipping reconnection” (Priest and Forbes, 1992; Priest and Démoulin, 1995;
Li, Priest, and Guo, 2021). In the 3D MFR eruption models, overlying sheared
arcades can configure into hyperbolic flux tube (HFT) structures beneath the
MFR during or before the eruption (Aulanier, Janvier, and Schmieder, 2012).
The slipping reconnection within the HFT typically results in continuous
brightenings observed at the footprints, often at sub-Alfvénic speeds
(Aulanier et al., 2006; Janvier et al., 2013; Dudík et al., 2014). This
reconnection process enhances the poloidal flux of the MFR, thereby
facilitating its buildup. Previous observations and simulations have shown the
slipping reconnection in the process of the buildup of an MFR (Janvier et al.,
2013; Dudík et al., 2014, 2016; Li and Zhang, 2014; Li et al., 2016, 2018a).

The eruption of MFRs forms the CMEs, which propagate into interplanetary space
and are typically referred to as interplanetary CMEs (ICMEs). Some ICMEs
directed towards Earth can be geoeffective, leading to geomagnetic storms and
aurora on Earth. The disturbance storm time (Dst) and Kp indices are used to
quantify the geomagnetic effect. In this study, our focus centers on a solar
eruption characterized by two successive solar flares, first an M3.4-class
flare followed by an M9.8-class flare, occurring on 2023 November 28. This
particular eruption leads to an intense geomagnetic storm on 2023 December
1st, with a Dst index of $-105$ nT. This event stands out as one of the six
intense geomagnetic storms (Dst $\leq$ $-100$ nT) recorded since Solar Cycle
25. Particularly notable is that the auroras associated with this event
manifest at the most southern latitudes in the northern hemisphere for the
past two decades. Given the exceptional geoeffectiveness of this event, it is
necessary to carry out a thorough analysis of the solar source of the
eruption. Our objective is to unravel the possible reasons contributing to its
profound geomagnetic effects from the solar observations. The remainder of
this paper is organized as follows. Section 2 describes the observations and
data analysis used in our study. In Section 3, we present our observational
results in detail. The major findings are thoroughly discussed and summarized
in Section 4. Finally, we draw our conclusions in Section 5.

##  2 Observations and data analysis

The Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory
(SDO) can provide both extreme ultraviolet (EUV) and ultraviolet (UV) images
of the Sun since 2010, capturing atmospheric dynamics in a wide range of
temperatures from $\log(T)\approx 4.0$ K to $\log(T)\approx 7.0$ K (Lemen et
al., 2012). We analyzed 94 Å, 131 Å, 171 Å, 193 Å, and 304 Å data with a
spatial resolution of 0.6′′/pixel and a temporal resolution of 12 seconds in
our study. UV 1600 Å observations with a resolution of 0.6′′ pixel-1 and 24
seconds are also used. The Advanced Space-based Solar Observatory (ASO-S; Gan
et al. 2019) is the first comprehensive Chinese dedicated solar observatory in
space, proposed for the 25th solar maximum. Three payloads are deployed: the
Hard X-ray Imager (HXI; Zhang et al. 2019), the Full-disk vector MagnetoGraph
(FMG; Deng et al. 2019) and the Lyman-$\alpha$ Solar Telescope (LST; Li et al.
2019). The time cadence varies from 4 seconds in regular observation mode to
approximately 0.125 seconds in burst mode. Our study utilizes the full-disk
light curve from three total flux monitors (D92, D93, D94). The data is
subtracted by the summed count rates of the corresponding three background
monitors (D95, D96, D99), focusing on the energy range of 10–50 keV. The
White-light Solar Telescope (WST) onboard LST works in the 360±2 nm waveband
and has a field of view of 1.2 solar radii. The images taken by WST have a
size of 4608 $\times$ 4608 pixels and the pixel size is $\sim$0.5′′. FMG can
measure the solar photospheric magnetic fields through the Fe I 532.42 nm line
with high spatial and temporal resolutions, providing line-of-sight (LOS)
magnetograms. Solar X-ray Extreme Ultraviolet Imager (X-EUVI; Chen et al.
2022) onboard Fengyun-3E meteorological satellite (FY-3E) can provide EUV
images in 195.5 Å, with a cadence of $\sim$14 s and a pixel size of 2′′.5. The
Solar Upper Transition Region Imager (SUTRI; Bai et al. 2023) contributes to
our study by providing EUV images at the wavelength of Ne VII 465 Å,
corresponding to a temperature of approximately 0.5 MK (Tian, 2017). SUTRI’s
capabilities include a spatial resolution of approximately 8′′ and a time
resolution of about 30 seconds. Moreover, H$\alpha$ images from the New Vacuum
Solar Telescope (NVST; Liu et al. 2014b) and the Chinese H$\alpha$ Solar
Explorer (CHASE; Li et al. 2022) are used to show the filaments involved in
the event. NVST offers H$\alpha$ images with a field of view (FOV) of 150′′
$\times$ 150′′, a cadence of 45 seconds, and a spatial resolution of 0.165′′
per pixel. Calibration of the data follows the methodology outlined by Cai et
al. (2022). CHASE’s H$\alpha$ Imaging Spectrograph (HIS) has a pixel spectral
resolution of 0.024 Å and a pixel spatial resolution of 0.52′′. The
Geostationary Operational Environmental Satellite (GOES) can provided soft
X-ray (SXR) flux data in 1-8 Å and 0.5-4 Å, with a cadence of 1 second. The
white-light observations of CMEs are acquired from the Large Angle
Spectrometric Coronagraph (LASCO; Brueckner et al. 1995) instrument aboard the
Solar and Heliospheric Observatory (SOHO) spacecraft (Brueckner et al., 1995).
We utilize LASCO-C3, which offers a FOV spanning from 3 to 32 solar radii,
enabling us to observe the CME within the near-Sun corona. In addition, the
Wind satellite (Harten and Clark, 1995) provides magnetic and plasma
observations from the L1 point, allowing for detailed monitoring of solar wind
properties and magnetic field dynamics in the vicinity of the Earth.

To derive the 3D coronal magnetic field, we utilized the photospheric vector
magnetic field data provided by SDO/HMI with a cadence of 720 seconds. A non-
linear force-free field (NLFFF) extrapolation was performed using an
optimization approach method (Wheatland, Sturrock, and Roumeliotis, 2000;
Wiegelmann, 2004; Wiegelmann et al., 2012). To ensure the force-free
assumption is satisfied in our input data, we applied the preprocessing method
introduced by Wiegelmann, Inhester, and Sakurai (2006). The extrapolation box
was defined as 740 ${\times}$ 480 ${\times}$ 256, with a grid resolution of
0.5′′. Utilizing the extrapolated field, we computed the twist number $T_{w}$
employing the procedure developed by Liu et al. (2016). The twist number
$T_{w}$ serves as an indicator of the degree of twisting in magnetic field
lines, which can help us to identify the MFRs (Berger and Prior, 2006).

##  3 Results

On 2023 November 28, NOAA active region (AR) 13500 was positioned at S20∘W04∘,
slightly south of the center of the solar disk (Figure 1(b)). Figure 1
presents the overview of this AR. The AR exhibited a significant solar
eruption, first an M3.4-class flare (SOL2023-11-28T19:07) starting at
$\sim$19:07 UT, reaching its peak at $\sim$19:32 UT. Following this event, a
subsequent M9.8-class flare (SOL2023-11-28T19:35) occurred, beginning at
$\sim$19:35 UT and peaking at $\sim$19:50 UT. Figure 1(a) illustrates the SXR
and HXR light curves corresponding to these associated flares. In panel (c),
zoom-in images of the EUV 94 Å wavelength reveal the presence of a sigmoid
locating around the PIL, suggesting the existence of a MFR or sheared magnetic
arcades.

Figure 2 presents the NLFFF extrapolation results of the AR 13500 at 18:36 UT,
which is approximately half an hour before the eruption. This AR exhibits
three distinct MFRs: MFR1 and MFR2 are near each other, with their west ends
anchoring the northwest negative-polarity sunspot. The magnetic twist of MFR1
is calculated in the range of 1.0–2.1, and the twist of MFR2 is smaller than
MFR1 with its maximum twist of about 1.25. MFR3 is approximately 30 Mm away
from MFR2, anchoring in the south negative-polarity sunspot. The twist of the
MFR3 is in the range of 1.0–1.63. Pre-eruptive observations of the AR are
shown in the first row of Figure 3. H$\alpha$ observations in panels (a) and
(b) reveal multiple filaments, with particular emphasis on two (outlined by
yellow and red dashed lines) located in the northwest. Combining the
magnetogram in panel (c), it is found that the locations of two filament are
close to those of MFR1 and MFR2 extrapolated in Figure 2. The east end of the
extrapolated MFR2 is somewhat north to the observed filament (red dashed lines
in Figures 3((a)-(c)), which may imply that the observed filament is composed
of both twist magnetic fields and sheared arcades (Guo et al., 2010). These
filaments can be considered as the counterparts of the extrapolated MFRs
(Canou and Amari, 2010; Li et al., 2018b). Though there is no associated
filament for the extrapolated MFR3, the EUV observations show the presence of
high-temperature sheared or twisted loop bundles at the location of MFR3
(Figures 3(d1)-(d3)), which probably correspond to the twisted loop bundles of
MFR3 (Cheng et al., 2011; Liu, 2020). Panel (c) reveals that the AR possesses
a $\beta\gamma\delta$ magnetic configuration, indicative of a complicated
magnetic field configuration that tend to erupt as obtained through
statistical works (Hale and Nicholson, 1938; Zhongxian and Jingxiu, 1994;
Toriumi and Wang, 2019). Beneath F1 and near the western footpoint of F2, the
emergence of numerous discrete magnetic poles of opposite polarity are
observed. Motions in the photosphere may lead to magnetic cancellation, making
the system unstable and initiating the eruption.

The second to fourth rows of Figure 3 illustrate the initial eruption of MFR1.
Brightenings near the location of F1 occur at $\sim$19:13 UT as shown in
panels (d1) and (e1). Then the east end of F1 exhibited the apparent slipping
motion towards the southeast (yellow arrows in panels (d2)–(f2)). Anchored at
the newly-drifted eastern footpoint, a loop bundle (LB1) marked as yellow dots
can be observed. This observed feature in 131 Å has been investigated by
previous studies and often considered as part of the hot channel, which is one
of the indicators for MFRs (Cheng et al., 2011; Chintzoglou, Patsourakos, and
Vourlidas, 2015; Gou et al., 2019; Zheng et al., 2021). At $\sim$19:19 UT, the
slipping motion of the east end of LB1 continued with the extension of about
10 Mm (panels (d3)–(f3)). At the same time, another loop bundle LB2 near LB1
occurred, marked as the red dotted line. After that, LB1 and F1 showed an
ascending and then erupted (seen in Figure 3(f3) and the online animation),
however, LB2 and F2 remained. It can be speculated that the LB1 are associated
with F1, which represents an indicator of twisted magnetic field lines of
MFR1. On the other hand, LB2 and F2 are probably both associated with MFR2.
This slipping motion of MFR during eruptions has been reported by many
previous studies (Dudík et al., 2014; Li and Zhang, 2014; Li et al., 2016;
Wang et al., 2017), indicating the buildup of MFRs.

After the eruption of F1, continual eastward slipping motion started from
about 19:21 UT (red arrows in Figure 4). The east end of the slippage extended
towards the southeast and passed through the positive-polarity sunspot at the
east of the AR (panels (c1)-(c3)). The slippage lasted for about 10 min and
showed a sequence of brightening of approximately 30 Mm from the initial
eastern end of seed MFR2. Moreover, the AIA observations showed the presence
of dense jet-like structures at the base of the east end of the slippage.
Combining the observations, we thought that these jet-like structures are
generated from the loop bundles of MFR2. In this way, the slipping process is
likely to be associated with buildup of MFR2. Finally, the slippage halted
near the location of LB3 (marked as green dotted line in panel (b3)), followed
by LB3’s ascending (Figure 5), indicating that the slippage might initiate the
eruption of MFR3. The slipping motion near LB3 was observed to begin from the
eastern footpoint of LB3 (the first row of Figure 5), which depicts a hook-
shaped part of the positive-polarity flare ribbon (PR). At the same time, the
west negative-polarity flare ribbon (NR) marked by blue arrows in panel (b2)
appears. Noteworthy, the slipping of NR was bi-directional, as shown in panels
(b1)-(b3). The slippage continued with the flare ribbon elongating southward
approximately 70 Mm (panels (a1)-(a4) and (b1)-(b4)). Finally, the LB3 kept
ascending and erupted, followed by a flare shown in panel (c4). From the
online animation, the bi-directional slipping of NR corresponds to the
slipping of PR and hook positive-polarity ribbon (HPR), respectively, which
may indicate that this might be associated with both MFR2 and MFR3.
Furthermore, the flare region covers the region of both MFR2 and MFR3, which
is another evidence of the simultaneous slippage. Ultimately, the simultaneous
eruptions of MFR2 and MFR3 contribute to an M9.8-class flare, as depicted in
Figure 5(c4).

To characterize the kinetic dynamics of the slipping process and flare ribbon
motions, we employed selected slices on the EUV images. Slice A-B in Figure
4(a2) illustrates the slipping process of MFR1 and MFR2, while the
corresponding time-distance diagram is presented in Figure 6(a). Initially,
MFR1 exhibits slipping motion at a velocity of 106 km s-1, shown in the
distance from 60 to 30 Mm in panel (a). In this process, the slipping of MFR2
is covered by that of MFR1, making it difficult to distinguish. Following
MFR1’s slipping and subsequent eruption, MFR2 continues slipping southeastward
at 40 km s-1 from 30 to 10 Mm in panel (a). This slipping extends toward the
vicinity of MFR3, potentially initiating the slippage of MFR3. Next, slice C-D
in Figure 5(a4) is placed to observe the southward slipping of MFR3, and the
associated time-distance diagram is depicted in Figure 6(b). The starting time
of MFR3’s slipping process is corresponding to when the sequential brightening
of MFR2 reaches near MFR3, suggesting that MFR3’s slippage may be triggered by
MFR2. Then, the east end of MFR3 moves southward at the speed of $\sim$137 km
s-1 to a distance of $\sim$70 Mm. Slice E-F in Figure 5(b4) elucidates the
slipping direction of the negative-polarity ribbon of the double flare ribbon,
and the corresponding time-distance diagram is shown in Figure 6(c). Bi-
directional brightenings indicate simultaneous eruptions of MFR2 and MFR3. The
brightening of MFR2 occurs earlier than that of MFR3, indicating that the
buildup process of MFR2 is earlier. The slipping speeds of the PR are measured
at 26 km s-1 towards the south, which probably corresponds to the west end of
MFR2, and 51 km s-1 towards the north corresponding to the west end of the
MFR3. Further examination of the time-distance diagram of Slice G-H in Figure
5(b4), represented in Figure 6(d), shows the separation of the double flare
ribbons. Initially, around 19:15 UT, PR begins to separate due to the
M3.4-class flare at the speed of 16 km s-1. Subsequently, around 19:35 UT, the
negative-polarity ribbon starts to separate following the M9.8-class flare at
a velocity of 18 km s-1. Notably, during this period, the northern ribbon
remains relatively stable, exhibiting minimal movement perpendicular to the
PIL.

Fine structures at the newly-formed eastern footpoint of MFR3 are highlighted
within the white square in Figure 5(b4). The temporal evolution of this
zoomed-in area is detailed in Figure 7. Analysis of AIA 304 Å and SUTRI 465 Å
observations reveals that the elongation of the HPR halts at $\sim$19:42 UT.
Subsequently, the end of the flare ribbon undergoes a transformation to a
curved shape, as observed in panels (a3) and (d3). Approximately 7 minutes
later, it evolves into a pentagram shape (panels (a4),(a5),(b4), and (b5)).
This observed evolution process may be attributed to the reconnection between
the MFR and surrounding magnetic field lines, leading to the drifting of
footpoints as predicted in Aulanier and Dudík (2019).

Large-scale EUV waves generated by the eruptions are shown in Figure 8. Panel
(a) presents the EUV waves accompanying the M3.4-class flare, which is
triggered by the eruption of MFR1. The wave propagates northward at a speed of
980 km s-1 with an angular width of approximately 120∘. Panels (b) and (c)
depict the waves generated by the M9.8-class flare, which results from the
eruptions of MFR2 and MFR3. Notably, we observe the expansion of loops from
MFR3, indicated by pink cross signs. The expansion of loops serves as an
indicator for the ascent of MFR3, with observations revealing a slow rise at
45 km s-1 followed by a rapid increase to 390 km s-1. This slow-to-fast
transition aligns with previous observations of MFR dynamics (Zhang, Cheng,
and Ding, 2012; Kliem et al., 2014; Cheng et al., 2023). In front of the loop
expansion, an EUV wave propagates southeastward at a speed of 1080 km s-1.
Additionally, another wavefront propagates northward at a speed of 1050 km s-1
(panel (c)), which is associated with the eruption of MFR2. Such a speed of
these EUV waves should be explained as fast magnetoacoustic waves or shock
waves (Patsourakos and Vourlidas, 2012; Warmuth, 2015; Sun et al., 2022).

Figure 9 displays white-light LASCO observations of the CMEs. LASCO captured
two CMEs (CME1 and CME2) during this event, both documented by the SEEDS
catalog111http://spaceweather.gmu.edu/seeds/. At $\sim$21:30 UT, CME1 is
observed propagating westward at a speed projected in the plane of sky of 535
km s-1. This CME is associated with the M3.4-class flare triggered by the
eruption of MFR1. Approximately 24 minutes later, CME2 originates initially at
the south limb of the Sun (panel (b)), evolving subsequently into a halo-CME
(panels (c) and (d)), with a projected speed of 2227 km s-1 as recorded by
SEEDS.

The Wind observations are depicted in Figure 10, where data is averaged over
one-minute intervals in the Geocentric Solar Ecliptic (GSE) coordinate system.
Analysis of the radial velocity data (panel (c)) reveals a background solar
wind speed of approximately 350 km s-1. A notable sudden increase at
$\sim$23:29 UT on November 30th (yellow dashed lines) indicates the arrival of
a fast shock, followed by an ICME. After carefully checking of the EUV and
coronagraph data, we found that there are two significant halo CMEs on
November 27th and 28th, with the first one being a quiescent filament eruption
on November 27th and the second one being the eruption in this study. The in-
situ data on November 30th and December 1st only show two significant shocks,
which are presented in Figure 10. So we could infer that these two significant
shocks correspond to the two halo CMEs, respectively. As a result, the first
shock should originate from the halo CME caused by a quiescent filament
eruption on November 27th, the day prior to the analyzed eruption. After
approximately 9 hours, another fast shock (orange dashed lines) arrived at
$\sim$08:48 UT on December 1st. This ICME corresponds to the eruption under
analysis. The main body of this ICME likely corresponds to the halo CME2 in
Figure 9, however, we cannot exclude the possibility that CME2 caught up with
CME1 and merged (Li et al., 2012; Lugaz et al., 2017). Following the ICME
shock-sheath region, the magnetic cloud (MC) passes, identified by systematic
variations in the magnetic field components, indicative of a strong magnetic
field associated with an MFR-like structure. The Bz component changes from
negative to positive sign. Based on these observations, we identified the time
intervals for the MC as spanning from $\sim$19:48 UT on December 1st to
$\sim$01:24 UT on December 2nd. Besides the clear MFR structure, there is a
complex structure after December 3rd 00:00 UT, also with a magnetic component
reversal. This might be a possible signal of another MFR structure.

Figure 11 presents hourly-averaged in-situ observations of geomagnetic
conditions. Upon encountering the first ICME shock (yellow dashed lines), the
Bz component increases to approximately $-8$ nT (where minus indicates
southward). Subsequently, upon the arrival of the second shock (orange dashed
lines) produced by the eruption under analysis, the Bz component further
increases to around $-23$ nT. Such a significant southward magnetic field
induces a rapid decrease in the Dst index, with the minimum reaching $-105$
nT, classifying the event as an intense geomagnetic storm (Dst $\leq$ $-100$
nT; Gonzalez et al. 1994; Echer et al. 2008; Lakhina and Tsurutani 2016).
Following the main phase of the geomagnetic storm, there is a long-duration
recovery phase. Additionally, this event leads to an increase in the Kp index
(panel (c)) to 6.7, categorized as G2 according to NOAA Space Weather Scales.
Furthermore, this event triggers low-latitude auroras in the northern
hemisphere, which falls beyond the scope of this paper.

##  4 Discussion

This paper investigates a solar eruption that resulted in an intense
geomagnetic storm and a low-latitude aurora. The eruption involved three
erupting MFRs, two flares and two CMEs. Throughout this event, we observed
three slipping process during the buildup of the three MFRs. The possible
scenario is as follows: MFR1 initiates its ascent, accompanied by eastward
slipping of the its east end at the speed of 106 km s-1. Subsequently, MFR1
erupts, leading to the formation of the M3.4-class flare and CME1. Meanwhile,
MFR2 undergoes slipping towards the southeast at 40 km s-1. As MFR2 slips
approximately 30 Mm, it approaches the vicinity of MFR3, triggering the
initiation of MFR3. MFR3 then begins slipping, with its flare ribbon
elongating southward at the speed of 137 km s-1. Ultimately, the eruptions of
MFR2 and MFR3 contribute both to the M9.8-class flare and CME2.

Slipping motions in the MFR eruptions have been simulated and observed in many
works (Janvier et al., 2013; Dudík et al., 2014, 2016; Li and Zhang, 2014; Li
et al., 2016). In the 3D extensions to the standard CSHKP model, MFRs are
typically anchored at the hooks of double-J QSLs. There are numerous sheared
arcades overlying the MFR, with one end anchored at the hook part of one
J-shaped QSL and the other end at the straight part of the other J-shaped QSL
(see in Fig.5 in Aulanier, Janvier, and Schmieder 2012). During the pre-
eruption or eruption phases of MFRs, the sheared arcades can form a HFT
structure beneath the MFR and reconnect with each other. This reconnection
process converts two sheared field lines into a magnetic flux rope line and a
post-flare loop, thereby increasing the poloidal magnetic flux of the MFR and
contributing to its buildup. The continuous slip-running reconnection of these
field lines leads to observable slipping motions in the flare ribbons during
the buildup of the MFR (Janvier et al., 2013). The buildup process corresponds
to the scenario where a pre-existing seed MFR accumulates poloidal magnetic
flux during the eruption phase. Our results reveal extrapolated MFR structures
prior to the eruption and the subsequent buildup process, in agreement with
the seed MFR eruption scenario.

Numerous observational case studies have confirmed the presence of slipping
motions before and during MFR eruptions (Dudík et al., 2014, 2016; Li and
Zhang, 2014; Li et al., 2016, 2018a). These studies have identified continuous
brightening along the flare ribbons or QSLs during the buildup process of MFR
eruptions. Despite many case studies demonstrating the slipping motions of
individual MFR eruptions, few studies have investigated the slipping motions
in multi-MFR systems. In our event, we observed the slipping motions of each
MFR, providing observational supplements for the three-dimensional
reconnection in the MFR eruption. Additionally, the multi-MFR eruption is
commonly explained as the reduction of the restraining forces above
(Schmieder, Démoulin, and Aulanier, 2013; Dhakal, Chintzoglou, and Zhang,
2018; Green et al., 2018). However, in our event, the end of MFR2 was
elongated to the vicinity of another flux rope MFR3 through the slipping
process, which plausibly triggered the eruption of MFR3. Our observations
imply that the slipping reconnection plays a key role in the multi-MFR
eruptions.

During the slipping process, we observed that brightenings not only occurred
in the immediate vicinity of the MFRs, but extends in length to a distance of
up to 70 Mm. This phenomenon can also be clearly identified in Dudík et al.
(2014), where an elongation of the flare ribbon is observed. The sequential
brightening should occur along the hook part of the QSLs (i.e. the footpoints
of the MFR), and thus, the direction and distance of the slipping should be
associated with the specific configuretion of MFRs (Aulanier et al., 2013; Li
and Zhang, 2014). Priest and Longcope (2017) proposed a scenario for the
buildup of MFR suggesting a two-step process: first, the 3D zipper
reconnection of sheared arcades forms an MFR along the PIL, followed by
quasi-2D main phase reconnection of unsheared flux. The former corresponds to
the parallel extension of conjugate flare ribbons in the same direction, while
the latter corresponds to the separation of double flare ribbons. However, in
our event, we observed that the extension of conjugate flare ribbons HPR and
NR occurs in opposite directions (see Figure 5), suggesting that slipping
reconnection rather than zipper reconnection is involved in this event.

Numerous observations indicate that some intense geomagnetic storms are linked
with multiple CMEs (Farrugia et al., 2006; Zhang et al., 2007; Liu et al.,
2014a). CMEs in the near-Sun corona or interplanetary space can interact with
each other, termed the CME–CME interaction. Studies have highlighted the
significant geoeffectiveness of CME–CME interactions (Lugaz et al., 2017;
Joshi et al., 2018; Scolini et al., 2020). The long-duration driving and
compressed magnetic fields resulting from CME–CME interactions can contribute
to intense geomagnetic storms (Lugaz et al., 2017). In our event, the halo
CME2 includes eruptions possibly from both MFR2 and MFR3, indicating that CME2
itself is a product of CME–CME interaction. Moreover, CME1 preceding CME2 also
suggests the possibility of another CME–CME interaction. Consequently, CME–CME
interactions among these three MFRs’ eruptions may significantly contribute to
the observed geomagnetic effects. Additionally, when the ICME associated with
this eruption reaches Earth, it encounters the ICME produced on the day before
(Figure 10), potentially compressing the prior ICME and further enhancing
geoeffectiveness. Ongoing investigations with a focus on understanding how
these CME–CME interactions influence magnetosphere is crucial for the observed
profound geoeffectiveness, which is beyond the scope of this paper.

##  5 Conclusion

This study investigated a multi-MFR eruption that occurred on 2023 November
28th, involving MFR1, MFR2, and MFR3. MFR1 led to the formation of an
M3.4-class flare and CME1, while the eruptions of MFR2 and MFR3 collectively
contributed to an M9.8-class flare and CME2. We observed distinct slipping
processes during the buildup of the three MFRs. All three MFRs exhibited
slipping motions, extending beyond their original locations. Notably, the
slipping of MFR2 extends $\sim$30 Mm and plausibly initiate the eruption of
MFR3. To our knowledge, this is the first study to display successive slipping
process in a multi-MFR system, showing how one MFR’s slipping can trigger the
slippage and eruption of another MFR.

The eruption resulted in an intense geomagnetic storm, with a Dst index
reaching $-105$ nT on December 1st, 2023. This event stands out as one of the
six intense geomagnetic storms recorded since Solar Cycle 25. Notably, the
associated aurora reached the most southern latitudes in the northern
hemisphere observed in the past two decades. Through analysis of the solar
origin of the eruption, we consider that CME–CME interactions, triggered by
multiple MFR eruptions, may be significant contributors to the observed
geomagnetic effects. Further studies focusing on ICMEs and Earth’s
magnetosphere are necessary to elucidate the detailed mechanisms behind the
intense geomagnetic storm and the low-altitude aurora associated with this
event.

We thank the referee and the editor for their careful work and thoughtful
suggestions. We also thank Prof. Shuo Yao and Dr. Wensi Wang for fruitful
discussions. The ASO-S mission is supported by the Strategic Priority Research
Program on Space Science, Chinese Academy of Sciences. SUTRI is a
collaborative project conducted by the National Astronomical Observatories of
CAS, Peking University, Tongji University, Xi’an Institute of Optics and
Precision Mechanics of CAS and the Innovation Academy for Microsatellites of
CAS. The CHASE mission is supported by China National Space Administration
(CNSA). AIA is a payload onboard _SDO_ , a mission of NASA’s Living With a
Star Program. We thank the SDO, Wind, SOHO/LASCO, FY-3E/X-EUVI, SUTRI, CHASE,
ASO-S, and NVST for providing data. This work is supported by the Strategic
Priority Research Program of the Chinese Academy of Sciences (Grant No.
XDB0560000 and XDB41000000), the National Key R&D Program of China
(2019YFA0405000,2021YFA1600500,2022YFF0503800), the National Natural Science
Foundations of China (11973056, 12222306, and 12273060, 12333009), and the
Youth Innovation Promotion Association CAS (2023063).

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Figure 1: Overview of the eruption. panel (a): temporal evolution of the GOES
XRS 1-8 Å (black), ASO-S HXI 10-20 keV (red), and 20-50 keV (blue). The blue
and yellow rectangles indicate the time range of the M3.4-class flare and the
M9.8-class flare, respectively. panel (b): overview of the AR 13501 and AR
13500 in FY-3E/X-EUVI 195 Å. The white rectangle denotes the field of view
(FOV) of panels (c)-(d). Panels (c)-(d): zoom-in area of AR 13500 and AR 13501
in AIA 94 Å (c) and in LST/ WST 3600 Å (d). The orange rectangle shows the FOV
of Figure 2. Panel (c) is superimposed by contours of the magnetic field at
$\pm$ 400 Gauss levels, with yellow indicating positive values and blue
representing negative values.  Figure 2: Multi-MFR configuration of the AR
13500 revealed by NLFFF extrapolation at 18:36 UT. Panel (a) and (b) shows the
top view and side view of the extrapolated results, respectively. Three
identified MFRs are marked as MFR1, MFR2 and MFR3. The extrapolation is
performed within the area with a twist number above 1.0. Figure 3: Pre-
eruption observations and slipping process of MFR1. Panels (a) and (b) shows
the H$\alpha$ observations from NVST and CHASE, respectively. Panels (c)
displays the FMG magnetogram. The red and yellow dashes lines depict the
location of Filament 1 (F1) and Filament 2 (F2), respectively. Panels (d)-(f)
shows the EUV observations of the slipping process of MFR1 in 94 Å (panel
(d)), 131 Å (panel (e)), and 131 Å running difference with a 48-second
cadence. The yellow arrow shows the slipping process of MFR1, while the yellow
and red dotted lines represent the loop bundles of LB1 and LB2, respectively.
The LB1 and LB2 might be associated with MFR1 and MFR2, respectively. (An
animation of this figure is available online.)  Figure 4: Slipping process of
MFR2. Panels (a)-(c) display the observations in AIA 171 Å, 131 Å, and 1600 Å,
respectively. The pink arrow denotes the location of the outflows resulting
from the reconnection beneath the MFR, thereby aiding in the identification of
MFR2. The red arrow shows the slipping process of MFR2. Additionally, the
green dots indicate the loop bundle LB3, which might be associated with MFR3.
Slice A-B in panel (a2) is specifically selected to illustrate the slipping
process of MFR1 and MFR2 and the result is shown in Figure 6(a). The observed
solar flare is the M3.4-class flare produced by the eruption of MFR1.  Figure
5: Slipping process of MFR3. Panels (a)-(c) depict the observations in AIA 171
Å, 304 Å, and 131 Å, respectively. The green arrow illustrates the slipping
process of LB3, while the blue arrows indicate the bi-directional slipping of
the second flare ribbon. Green dots in panels (c1)-(c3) mark the location of
MFR3’s loop bundle. The white rectangle in panel (b4) outlines the FOV
depicted in Figure 7. Slices C-D, E-F, and G-H in the bottom row are
specifically chosen to elucidate the slipping process of MFR3, bi-directional
slipping of the second ribbon, and the separation of the double flare ribbons,
respectively. The observed flare corresponds to the M9.8-class flare resulting
from the eruption of MFR2 and MFR3.  Figure 6: Time-distance diagrams. Slice
A-B depicts the slipping process of MFR1 and MFR2, while slice C-D illustrates
the slipping process of MFR3. Slice E-F shows the bi-directional slipping of
the second ribbon, and slice G-H illustrates the separation of the double
flare ribbons. Slice A-B and C-D are shown in the Figure 4(a2) and Figure
5(a4), respectively, while Slice E-F and G-H are displayed in Figure 5(b4).
The calculated velocity is denoted in green and all velocities are projected
velocities. Figure 7: Fine structures of the end of the flare ribbon in the
slipping process of MFR3, observed in AIA 304 Å (panels (a1)-(a5)) and SUTRI
465 Å (panels (b1)-(b5)). Panel (c) is superimposed by contours of the
magnetic field at $\pm$ 400 Gauss levels, with yellow indicating positive
values and blue representing negative values.  Figure 8: EUV waves produced by
the eruption. Panels (a)-(c) shows the EUV wavefronts propagating outward the
AR 13500 in AIA 193 Å running difference with a 48-second cadence. The green
and cyan cross signs indicate the observed wavefront, with the green one
associated with Flare 1 (M3.4-class flare) and the others associated with
Flare 2 (M9.8-class flare). The pink cross signs indicate the loops of MFR3.
Yellow dashed lines are placed to generate of time-distance diagrams, as
illustrated in panels (d)-(e). These slices are chosen to illustrate the
kinetics of the EUV waves and MFR loops. Panels (d)-(e) shows the time-
distance diagrams of AIA 193 Å running difference. The green, cyan and pink
dashed lines are corresponding to the fronts depicted in the same color in
panels (a)-(c). Panel (d) is overlaid with the GOES SXR 1-8 Å curve in red.
Figure 9: LASCO white-light observations within the near-Sun corona.  Figure
10: Wind in-situ observations of the ICME. The panels from top to bottom,
display plots of magnetic field strength ($\left|B\right|$) and its components
(Bx, By, Bz), as well as solar wind speed (Vr) as a function of time,
respectively. The cyan and orange dashed lines indicate two instances of fast
shock arrival, with the cyan line corresponding to the ICME produced one day
before the eruption under analysis, and the orange line corresponding to the
ICME being analyzed. The time period highlighted in pink denotes the duration
of MFR passage.  Figure 11: In-situ observations of the CME encountering
Earth’s atmosphere. Panel (a) presents the Bz in GSE coordinates averaged over
one hour. Panel (b) displays the Dst index, reaching a minimum of -105 nT,
indicating an intense geomagnetic storm. The cyan and orange dashed lines
indicate the shock arrival times as in Fig. 10. The purple dashed line
indicates the MFR arrival time, also shown in Fig. 10. The red and pink arrows
denote the main phase and recovery phase of the geomagnetic storm,
respectively. Panel (c) shows the histogram of the Kp index, with green,
yellow, and red bars representing index values 0-3, 3-4.3, and 4.3-7,
respectively. The Kp index values reach up to 6.7 during the storm’s main
phase, categorized as G2 according to NOAA Space Weather Scales.