Kinematics and helicity evolution of a loop-like eruptive prominence

Abstract

We aim at investigating the morphology, kinematic and helicity evolution of a
loop-like prominence during its eruption. We use multi-instrument observations
from AIA/SDO, EUVI/STEREO and LASCO/SoHO. The kinematic, morphological,
geometrical, and helicity evolution of a loop-like eruptive prominence are
studied in the context of the magnetic flux rope model of solar prominences.
The prominence eruption evolved as a height expanding twisted loop with both
legs anchored in the chromosphere of a plage area. The eruption process
consists of a prominence activation, acceleration, and a phase of constant
velocity. The prominence body was composed of left-hand (counter-clockwise)
twisted threads around the main prominence axis. The twist during the eruption
was estimated at 6pi (3 turns). The prominence reached a maximum height of 526
Mm before contracting to its primary location and partially reformed in the
same place two days after the eruption. This ejection, however, triggered a CME
seen in LASCO C2. The prominence was located in the northern periphery of the
CME magnetic field configuration and, therefore, the background magnetic field
was asymmetric with respect to the filament position. The physical conditions
of the falling plasma blobs were analysed with respect to the prominence
kinematics. The same sign of the prominence body twist and writhe, as well as
the amount of twisting above the critical value of 2pi after the activation
phase indicate that possibly conditions for kink instability were present. No
signature of magnetic reconnection was observed anywhere in the prominence body
and its surroundings. The filament/prominence descent following the eruption
and its partial reformation at the same place two days later suggest a confined
type of eruption. The asymmetric background magnetic field possibly played an
important role in the failed eruption.

Full text

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arXiv:1202.4541v1 [astro-ph.SR] 21 Feb 2012
Astronomy & Astrophysics manuscript no. 18588
February 22, 2012
c ? ESO 2012
Kinematics and helicity evolution of a loop-like eruptive
prominence
K. Koleva1, M.S. Madjarska2, P. Duchlev1, C. J. Schrijver5, J.-C. Vial3,4, E. Buchlin3,4, and M. Dechev1
1Institute of Astronomy and National Astronomical Observatory,
Bulgarian Academy of Sciences, 72 Tsarigradsko Chaussee Blvd., 1784 Sofia, Bulgaria
2Armagh Observatory, College Hill, Armagh BT61 9DG, N. Ireland
3CNRS, Institut d’Astrophysique Spatiale, UMR8617, 91405 Orsay, France
4Univ Paris-Sud, Institut d’Astrophysique Spatiale, UMR8617, 91405 Orsay, France
5Solar and Astrophysics Lab., Lockheed Martin Advanced Techn. Ctr., 3251 Hanover St., Bldg. 252, Palo Alto, CA
94304-1191, USA
Received date, accepted date
ABSTRACT
Aims. We aim at investigating the morphology, kinematic and helicity evolution of a loop-like prominence during its
eruption.
Methods. We use multi-instrument observations from AIA/SDO, EUVI/STEREO and LASCO/SoHO. The kinematic,
morphological, geometrical, and helicity evolution of a loop-like eruptive prominence are studied in the context of the
magnetic flux rope model of solar prominences.
Results. The prominence eruption evolved as a height expanding twisted loop with both legs anchored in the chromo-
sphere of a plage area. The eruption process consists of a prominence activation, acceleration, and a phase of constant
velocity. The prominence body was composed of left-hand (counter-clockwise) twisted threads around the main promi-
nence axis. The twist during the eruption was estimated at 6π (3 turns). The prominence reached a maximum height of
526 Mm before contracting to its primary location and partially reformed in the same place two days after the eruption.
This ejection, however, triggered a CME seen in LASCO C2. The prominence was located in the northern periphery of
the CME magnetic field configuration and, therefore, the background magnetic field was asymmetric with respect to
the filament position. The physical conditions of the falling plasma blobs were analysed with respect to the prominence
kinematics.
Conclusions. The same sign of the prominence body twist and writhe, as well as the amount of twisting above the critical
value of 2π after the activation phase indicate that possibly conditions for kink instability were present. No signature of
magnetic reconnection was observed anywhere in the prominence body and its surroundings. The filament/prominence
descent following the eruption and its partial reformation at the same place two days later suggest a confined type of
eruption. The asymmetric background magnetic field possibly played an important role in the failed eruption.
Key words. Sun: activity – Sun: prominences – Sun: magnetic fields
1. Introduction
Prominence eruptions are large-scale eruptive phenomena
which occur in the low solar atmosphere. Observations show
that prominences display a wide range of eruptive activity.
There are three types of prominence (filament) eruptions
according to the observational definitions of Gilbert et al.
(2007) based on the relation between the filament mass and
corresponding supporting magnetic structure: full, partial,
and failed (confined), of which the partial ones are the most
complex. A full eruption occurs when the entire magnetic
structure and the pre-eruptive prominence material are ex-
pelled into the heliosphere. The case when neither the fil-
ament mass, nor the supporting magnetic structure escape
the solar gravitational field is a failed eruption. Partial
eruptions can be divided into two subcategories: i) when
the entire magnetic structure erupts, with the eruption con-
taining either part or none of its supported pre-eruptive
prominence material, and ii) when the magnetic structure
Send offprint requests to: koleva@astro.bas.bg
itself partially escapes with either some or none of the fil-
ament mass (Gilbert et al. 2007). One important observa-
tional consequence concerning partial and failed eruptions
is the re-formation of the filament at the pre-eruptive loca-
tion.
Sterling & Moore (2004a,b) unveiled a common pattern
of prominence eruptions: an initial slow-rise phase (with a
very small acceleration), during which the filament gradu-
ally ascends, followed by a sharp change to a phase of fast
acceleration. There exist three types of prominence erup-
tion after the fast rise phase: i) an eruptive prominence can
continue to rise with acceleration, ii) the fast rise can be
followed by a constant velocity phase, or iii) the constant
velocity phase of an eruptive prominence can be followed
by a deceleration phase (Vrˇ snak 1998).
Eruptive prominences (EPs) (or filaments if observed
on the solar disk) are frequently associated and physi-
cally related to coronal mass ejections (CMEs) and flares
(Tandberg-Hanssen 1995; Webb et al. 1976; Munro et al.
1979; Webb & Hundhausen 1987; St. Cyr & Webb 1991).
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K. Koleva et al.: Kinematics and helicity evolution
Usually, all three eruptive events occur in the same large-
scale coronal magnetic field, in which the EP only occupies
a limited volume at its base. Such an association suggests
that possibly the same physical process drives these erup-
tive phenomena. Filament-CME relationship studies often
do not take into account the relation between filament-
related magnetic field configurations, like for instance, an
empty filament channel, and a CME initiation (Alexander
2006). Therefore, the filament-CME relationship can be
even stronger than is known so far. An EP and/or a CME
are considered as the eruption of a preexisting magnetic
flux rope (MFR) or an initial magnetic arcade that evolves
into a magnetic flux rope during the eruption process via
magnetic reconnection (Chen et al. 2006; Antiochos et al.
1999). However, the question whether an MFR exists prior
to the eruption or it is formed during the eruption remains
controversial, with the MFR topology preceding the erup-
tion being favoured in many studies during the last decade.
During activation, the slow rise motion of an EP is
usually accompanied by a gradual morphological evolu-
tion from an initially intricate structure into an appar-
ently toroidal shape, exposing sometimes a twisted pattern,
that is most often prominent in the legs of the prominence
(Vrˇ snak et al. 1991). EPs very often develop a clearly heli-
cal shape in the course of the eruption, which is the char-
acteristic signature of a MagnetoHydroDynamic (MHD)
kink instability of a twisted MFR (e.g. Rust & LaBonte
2005). A MFR becomes kink-unstable if the twist exceeds
a critical value of 2π (e.g. Hood & Priest 1981; Fan 2005;
T¨ or¨ ok & Kliem 2005). The axis of an MFR then under-
goes writhing (kinking) motions and part of the twist of
the field is transformed into helical writhe of the axis,
since the magnetic helicity is conserved (Rust & LaBonte
2005). The conservation of helicity in ideal MHD (Berger
1984) requires the resulting writhe to be of the same sign
as the transformed twist. The kink instability has long
been investigated as a possible triggering mechanism for
solar eruptive phenomena, especially in flux rope models
(Liu & Alexander 2009).
Kink and torus instabilities are suggested to be
two mechanisms for triggering solar flares and coro-
nal mass ejections (Sakurai 1976; T¨ or¨ ok & Kliem 2005;
Kliem & T¨ or¨ ok 2006; Schrijver et al. 2008). Kink instabil-
ity was first suggested as the trigger of prominence erup-
tions (confined and ejective) by Sakurai (1976) but has been
generally regarded as a possible explanation only for con-
fined events (e.g. Gerrard & Hood 2003). T¨ or¨ ok & Kliem
(2005), and Fan (2005) succeeded in modelling full ejection
of a coronal flux rope from the Sun driven by kink insta-
bility. A kinking may be an important factor in the erupt-
ing process, but the type of eruption may strongly depend
on the role played by magnetic reconnection and its loca-
tion with regards to the prominence body (Antiochos et al.
1999). As discussed in Gilbert et al. (2007, and the ref-
erences therein) the kink instability as a main trigger of
prominence destabilization and eruption is challenging to
be proven observationally because the helicity can force a
flux rope to writhe, without any instability occurring.
Kliem & T¨ or¨ ok (2006) studied the expansion instability
of a toroidal current ring embedded in a low-beta magne-
tised plasma. Schrijver et al. (2008) analysed two near-limb
filament destabilisations involved in CMEs. Numerical sim-
ulation of a torus instability showed a relatively close quan-
titative match of the observations, implying that these two
cases are likely to be torus instability eruptions.
Observations of filament eruptions that strongly sug-
gest a helical kink occurring in flux rope topology were
presented by Williams et al. (2005) for a full eruption, by
Zhou et al. (2006) and by Liu et al. (2007) for partial erup-
tions, and Ji et al. (2003) and Alexander et al. (2006) for
confined (failed) eruption. An observational definition of
kinking related to the different types of filament eruptions
is given by Gilbert et al. (2007) and Green et al. (2007).
In this paper, we investigate the phenomenology of
an eruptive loop-shaped helically twisted prominence with
fixed footpoints using state-of-art observations from the
Atmospheric Imaging Assembly (AIA) aboard the Solar
Dynamics Observatory (SDO) in the 304˚ A EUV passband.
We have the unique opportunity to combine limb with on-
disk observations of an eruptive prominence thanks to the
EUVI/STEREO B observations which at the time of the
observations was at an angular distance of 71 degrees with
the Earth. We aim at investigating the morphology, kine-
matic and helicity evolution of a loop-like prominence dur-
ing its eruption. In Section 2 we describe the set of obser-
vations used in this study. In Section 3 we present the main
results, which are discussed in Section 4. The conclusions
are drawn in Section 5.
Fig.1. Mauna Loa Hα images before and during the start
of the prominence activation. The arrow points at the re-
gion revealing twisted structure.
2. Observations and data analysis
The prominence eruption occurred at the North-East solar
limb between 17:30 UT and 19:30 UT on 2010 March 30.
The EP was centered at mean heliographic co-ordinates
N22.63◦; E78.80◦and mean position angle 66◦.
For the present study we used images taken with 1 min
cadence in the He ii 304˚ A passband of AIA/SDO (AIA;
Lemen et al. 2011). The AIA consists of seven Extreme
Ultra-Violet (EUV) and three Ultra-Violet (UV) channels
which provide an unprecedented view of the solar corona
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K. Koleva et al.: Kinematics and helicity evolution
Fig.2. He ii 304˚ A AIA images (in reversed colour table) showing the morphology and, in particular, the helicity evolution
of the erupting prominence. AIA He ii 304˚ A image animation of the prominence eruption can be seen online.
with an average cadence of ∼12 s. The AIA image field-
of-view reaches 1.3 solar radii with a spatial resolution of
∼1.5′′. We used level 1 reduced data, i.e. with the dark
current removed and the flat-field correction applied. The
images were further stabilised for the satellite movements
by applying intensity cross-correlation analysis using the
SolarSoftware procedure get correl offsets.pro. For the pur-
pose of our analysis we defined the visible limb from the
AIA He ii 304˚ A images. Note that the jitter correction was
only needed this early in the mission as the image stabili-
sation was still subject to calibration in the commissioning
phase when these observations were taken.
We also analysed observations from the Extreme
Ultraviolet Imager (EUVI) aboard STEREO Behind (B)
spacecraft. EUVI has a field-of-view of 1.7R⊙and observes
in four spectral channels (He ii 304˚ A, Fe ix/x 171˚ A,
Fe xii 195˚ A and Fe XIV 284˚ A) that cover the 0.1 to 20 MK
temperature range (Wuelser et al. 2004). The EUVI detec-
tor has 2048 × 2048 pixels2size and a pixel size of 1.6′′.
In the present study we used images in the He ii 304˚ A
channel with an average cadence of 10 minutes. Images ob-
tained by the Large Angle and Spectrometric Coronagraph
(LASCO)/C2 on board SOHO, whose field-of-view extends
from 2 to 6 solar radii (Brueckner et al. 1995) were also
analysed in the present study.
3. Results
3.1. Morphology and helicity evolution
The prominence eruption evolved as a height expanding
twisted loop with both legs anchored in the chromosphere
of a plage area. The prominence before the activation can
already be seen in Hα at 17:19 UT (Fig. 1). It shows as a
few bright clouds above the limb with no distinguishable
fine structure. In the AIA He ii 304˚ A images at 17:32 UT
and 17:40 UT in Fig. 2 (see also the online material), the
fine structure of the prominence appears very dense, the
prominence lies very close to the limb and, therefore, it
is impossible to distinguish its small-scale structure. Note
that in Hα we see the cold core of the prominence, while
in He ii 304˚ A it is the envelope at higher temperatures
(Labrosse et al. 2010). The Hα image at 17:49 UT shows
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K. Koleva et al.: Kinematics and helicity evolution
Fig.3. Running difference images from EUVI B in the He ii 304˚ A channel. EUVI/STEREO B He ii 304˚ A image
difference animation of the prominence eruption on 2010 March 30 can be seen online.
Table 1. The kinematics of the eruptive prominence derived from the AIA images.
Phase
Activation
Acceleration
Constant velocity
Time (UT)
17:33 – 18:00
18:00 – 18:12
18:12 – 18:44
Height (Mm)
18 - 34
34 - 121
121 – 295
Velocity (km s−1)
10
15 – 166
91
Acceleration (m s−2)
0
46 – 430
0
the first signature of a twisted prominence fine structure.
From part of the prominence where the fine structure is vis-
ible, we could estimate a 2π twist, i.e. one turn of the promi-
nence rope around its main axis. This time coincides with
the beginning of the prominence slow rise during the acti-
vation phase. The full amount of prominence twist is first
seen at around 18:20 UT (Fig. 2) in the AIA He ii 304˚ A im-
ages. We estimated from the AIA 304˚ A images at 18:20 UT,
18:26 UT and 18:32 UT, a total twist of about 6π (3 turns)
of the eruptive prominence body.
We also searched for a signature of magnetic reconnec-
tion in all available data including AIA and EUVI He ii
304˚ A and Fe xii 195˚ A. Both the off-limb AIA and on-disk
EUVI data do not show any brightening in the footpoints
of the prominence, their surroundings as well as along the
prominence body. This suggests that magnetic reconnection
may not have had contributed to the prominence destabi-
lization and eruption.
After 18:25 UT the eruption can also be followed on
the EUVI/STEREO image differences shown in Fig. 3 (see
also the online material). Here, thanks to the different view
point (see the AIA view translated on the EUVI B image
in Fig. 4) we could see that the filament upper body under-
went a right-hand writhe. The deformation of the promi-
nence is still not visible in the AIA images at this time
(Fig. 2) due to the line-of-sight effect. At 18:56 UT the
crossing of the prominence body axis due to the writhe as
seen in projection on the disk, in the STEREO difference
images (Fig. 3) is visible at approximately half of the ris-
ing filament height. At 19:06 UT, this crossing is visible at
approximately one third from the feet of the prominence.
The writhe increased with time, reaching π/2 at 19:26 UT
(Fig. 3).
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K. Koleva et al.: Kinematics and helicity evolution
Fig.4. EUVI difference image (top left) and EUVI image (top right). AIA image (bottom left) and the same image
transformed to the STEREO FOV (bottom right). The filament’s footpoints and a reference feature ‘R’ above the
northern leg are marked with circles.
The AIA He ii 304˚ A images after 18:26 UT reveal best
the helical twist of the prominence and its evolution in time
(last row of Fig. 2). We can clearly see that during the
prominence uplift the twist transfers from the lower (legs)
to the upper prominence body causing the prominence
to evolve from a loop-shaped into a ribbon-like structure.
With the help of the EUVI B difference images (Fig. 3), we
established the footpoint location noted with black circles
in Fig. 4 (top left). The so defined foot positions are also
shown on the EUVI image at 18:36 UT (Fig. 4, top right). A
feature above the northern leg is also encircled and labelled
with ‘R’. This feature is used as a reference for the transla-
tion of the AIA images into the STEREO B view. In Fig. 4
(bottom right) we show the AIA 304˚ A image at 18:32 UT
with the expanding loop-like prominence. This image was
transformed to the STEREO B view and the legs of promi-
nence identified in the EUVI B images were transposed
(shown with black circles) as well as the reference feature.
We paid special attention to the localisation of the feet of
the EP in order to determine the type of twist and writhe,
and their evolution in time. Based on the magnetic field po-
larity configuration obtained from the magnetograms, the
Hα images and the prominence drawings (Fig. 5), we could
determine in which magnetic polarity the southern and the
northern legs of the prominence were located. From all the
above information we established that the prominence un-
derwent a writhe as a result of counter-clockwise rotation
with respect to the two polarity fluxes.
3.2. Kinematics
The prominence height was determined as the height of the
main axis of the prominence above the visible limb as ob-
served in the He ii 304˚ A channel of the AIA/SDO images
(Fig. 6). The time evolution of the height reveals three dis-
tinctive phases of the prominence eruption: a prominence
activation, an eruption with acceleration, and an eruption
with a constant velocity (Fig. 7). From the first and second
derivatives of the polynomial fit of the height-time curve,
we defined the speed and the acceleration of the prominence
eruption. The prominence activation is already in progress
at the begining of the AIA observations around 17:33 UT.
It is defined as the time period of a slow rise of the loop
system with a velocity of 10 km s−1. Until 18:00 UT the
height of the prominence changed from 18 Mm to 34 Mm.
The eruption onset was registered at 18:00 UT and it was
determined from the AIA images as a sudden increase of
the prominence height. It lasted until around 18:12 UT
(Fig. 7). During this phase the prominence height changed
from 34 Mm to 121 Mm and the speed of the prominence
rise increased from 15 km s−1to a maximum of 166 km s−1
with an acceleration from 46 to 430 m s−2. The constant
velocity phase was measured until 18:44 UT. After this time
the top of the looped prominence is outside the AIA field-of-
view. The kinematics of the different phases of the eruptive
process is summarised in Table 1.
It took 80 min the prominence to reach its maximum
height (Fig. 3) at 19:16 UT as seen in the EUVI B im-
5