1,575 research outputs found
Estimation of width and inclination of a filament sheet using He II 304 A observations by STEREO/EUVI
The STEREO mission has been providing stereoscopic view of the filament
eruptions in EUV wavelengths. The most extended view during filament eruptions
is seen in He II 304 \AA observations, as the filament spine appears darker and
sharper. The projected filament width appears differently when viewed from
different angles by STEREO satellites. Here, we present a method for estimating
the width and inclination of the filament sheet using He II 304 \AA\
observations by STEREO-A and B satellites from the two viewpoints. The width of
the filament sheet, when measured from its feet to its apex, gives estimate of
filament height above the chromosphere.Comment: 9 pages, 2 figures, in Annales Geophysica
Solar filament eruptions and their physical role in triggering Coronal Mass Ejections
Solar filament eruptions play a crucial role in triggering coronal mass
ejections (CMEs). More than 80 % of eruptions lead to a CME. This correlation
has been studied extensively during the past solar cycles and the last long
solar minimum. The statistics made on events occurring during the rising phase
of the new solar cycle 24 is in agreement with this finding. Both filaments and
CMEs have been related to twisted magnetic fields. Therefore, nearly all the
MHD CME models include a twisted flux tube, called a flux rope. Either the flux
rope is present long before the eruption, or it is built up by reconnection of
a sheared arcade from the beginning of the eruption. In order to initiate
eruptions, different mechanisms have been proposed: new emergence of flux,
and/or dispersion of the external magnetic field, and/or reconnection of field
lines below or above the flux rope. These mechanisms reduce the downward
magnetic tension and favor the rise of the flux rope. Another mechanism is the
kink instability when the configuration is twisted too much. In this paper we
open a forum of discussions revisiting observational and theoretical papers to
understand which mechanisms trigger the eruption. We conclude that all the
above quoted mechanisms could bring the flux rope to an unstable state.
However, the most efficient mechanism for CMEs is the loss-of-equilibrium or
torus instability, when the flux rope has reached an unstable threshold
determined by a decay index of the external magnetic field.Comment: 23 pages, 13 figures, revie
Two Step Filament Eruption During 14-15 March 2015
We present here an interesting two-step filament eruption during 14-15 March
2015. The filament was located in NOAA AR 12297 and associated with a halo
Coronal Mass Ejection (CME). We use observations from the Atmospheric Imaging
Assembly (AIA) and Heliospheric Magnetic Imager (HMI) instruments onboard the
Solar Dynamics Observatory (SDO), and from the Solar and Heliospheric
Observatory (SOHO) Large Angle and Spectrometric Coronagraph (LASCO). We also
use H-alpha data from the Global Oscillation Network Group (GONG) telescope and
the Kanzelhoehe Solar Observatory. The filament shows a first step eruption on
14 March 2015 and it stops its rise at a projected altitude ~ 125 Mm on the
solar disk. It remains at this height for ~ 12 hrs. Finally it eruptes on 15
March 2015 and produced a halo CME. We also find jet activity in the active
region during both days, which could help the filament de-stabilization and
eruption. The decay index is calculated to understand this two-step eruption.
The eruption could be due to the presence of successive
instability-stability-instability zones as the filament is rising.Comment: 11 pages, 7 figures, accepted for the publication in Solar Physic
Velocity vectors of a quiescent prominence observed by Hinode/SOT and the MSDP (Meudon)
The dynamics of prominence fine structures is a challenge to understand the
formation of cool plasma prominence embedded in the hot corona. Recent
observations from the high resolution Hinode/SOT telescope allow us to compute
velocities perpendicularly to the line-of-sight or transverse velocities.
Combining simultaneous observations obtained in H-alpha with Hinode/SOT and the
MSDP spectrograph operating in the Meudon solar tower we derive the velocity
vectors of a quiescent prominence. The velocities perpendicular to the
line-of-sight are measured by time slice technique, the Dopplershifts by the
bisector method. The Dopplershifts of bright threads derived from the MSDP
reach 15 km/s at the edges of the prominence and are between +/- 5 km/s in the
center of the prominence. Even though they are minimum values due to seeing
effect, they are of the same order as the transverse velocities. These
measurements are very important because they suggest that the
verticalstructures shown in SOT may not be real vertical magnetic structures in
the sky plane. The vertical structures could be a pile up of dips in more or
less horizontal magnetic field lines in a 3D perspective, as it was proposed by
many MHD modelers. In our analysis we also calibrate the Hinode H-alpha data
using MSDP observations obtained simultaneously.Comment: 7 pages, 7 figures, submitted to A &
Filament eruption connected to protospheric activity
Two cases of activation of filaments that occured in regions of intense magnetic activity was studied. The simultaneous observations from Debrecen Observatory (white light and H alpha filtergram), and from Meudon Observatory (magnetogram, MSDP dopplergram and intensity maps in H alpha) gave a complementary set of data from which can be produced evidence of the influence of the photospheric magnetic field on the destabilization process of the filaments. On June 22, 1980, the eruption of the filament is associated with the motion of pores, which are manifestations of emerging flux knots. On September 3, 1980, the twisting motions in the filament are associated to the birth of a pore in its neighborhood. These observations are discussed
Structure of prominence legs: Plasma and magnetic field
We investigate the properties of a `solar tornado' observed on 15 July 2014,
and aim to link the behaviour of the plasma to the internal magnetic field
structure of the associated prominence. We made multi-wavelength observations
with high spatial resolution and high cadence using SDO/AIA, the IRIS
spectrograph and the Hinode/SOT instrument. Along with spectropolarimetry
provided by the THEMIS telescope we have coverage of both optically thick
emission lines and magnetic field information. AIA reveals that the two legs of
the prominence are strongly absorbing structures which look like they are
rotating, or oscillating in the plane of the sky. The two prominence legs,
which are both very bright in Ca II (SOT), are not visible in the IRIS Mg II
slit-jaw images. This is explained by the large optical thickness of the
structures in Mg II which leads to reversed profiles, and hence to lower
integrated intensities at these locations than in the surroundings. Using lines
formed at temperatures lower than 1 MK, we measure relatively low Doppler
shifts on the order of +/- 10 km/s in the tornado-like structure. Between the
two legs we see loops in Mg II, with material flowing from one leg to the
other, as well as counterstreaming. It is difficult to interpret our data as
showing two rotating, vertical structures which are unrelated to the loops.
This kind of `tornado' scenario does not fit with our observations. The
magnetic field in the two legs of the prominence is found to be preferentially
horizontal.Comment: 13 pages, 14 figures, one tabl
Coronal mass ejections from the same active region cluster: Two different perspectives
The cluster formed by active regions (ARs) NOAA 11121 and 11123,
approximately located on the solar central meridian on 11 November 2010, is of
great scientific interest. This complex was the site of violent flux emergence
and the source of a series of Earth-directed events on the same day. The onset
of the events was nearly simultaneously observed by the Atmospheric Imaging
Assembly (AIA) telescope aboard the Solar Dynamics Observatory (SDO) and the
Extreme-Ultraviolet Imagers (EUVI) on the Sun-Earth Connection Coronal and
Heliospheric Investigation (SECCHI) suite of telescopes onboard the
Solar-Terrestrial Relations Observatory (STEREO) twin spacecraft. The
progression of these events in the low corona was tracked by the Large Angle
Spectroscopic Coronagraphs (LASCO) onboard the Solar and Heliospheric
Observatory (SOHO) and the SECCHI/COR coronagraphs on STEREO. SDO and SOHO
imagers provided data from the Earth's perspective, whilst the STEREO twin
instruments procured images from the orthogonal directions. This spatial
configuration of spacecraft allowed optimum simultaneous observations of the AR
cluster and the coronal mass ejections that originated in it. Quadrature
coronal observations provided by STEREO revealed a notably large amount of
ejective events compared to those detected from Earth's perspective.
Furthermore, joint observations by SDO/AIA and STEREO/SECCHI EUVI of the source
region indicate that all events classified by GOES as X-ray flares had an
ejective coronal counterpart in quadrature observations. These results have
direct impact on current space weather forecasting because of the probable
missing alarms when there is a lack of solar observations in a view direction
perpendicular to the Sun-Earth line.Comment: Solar Physics - Accepted for publication 2015-Apr-25 v2: Corrected
metadat
Fan-spine topology formation through two-step reconnection driven by twisted flux emergence
We address the formation of 3D nullpoint topologies in the solar corona by
combining Hinode/XRT observations of a small dynamic limb event, which occurred
beside a non-erupting prominence cavity, with a 3D zero-beta MHD simulation. To
this end, we model the boundary-driven kinematic emergence of a compact,
intense, and uniformly twisted flux tube into a potential field arcade that
overlies a weakly twisted coronal flux rope. The expansion of the emerging flux
in the corona gives rise to the formation of a nullpoint at the interface of
the emerging and the pre-existing fields. We unveil a two-step reconnection
process at the nullpoint that eventually yields the formation of a broad 3D
fan-spine configuration above the emerging bipole. The first reconnection
involves emerging fields and a set of large-scale arcade field lines. It
results in the launch of a torsional MHD wave that propagates along the
arcades, and in the formation of a sheared loop system on one side of the
emerging flux. The second reconnection occurs between these newly formed loops
and remote arcade fields, and yields the formation of a second loop system on
the opposite side of the emerging flux. The two loop systems collectively
display an anenome pattern that is located below the fan surface. The flux that
surrounds the inner spine field line of the nullpoint retains a fraction of the
emerged twist, while the remaining twist is evacuated along the reconnected
arcades. The nature and timing of the features which occur in the simulation do
qualititatively reproduce those observed by XRT in the particular event studied
in this paper. Moreover, the two-step reconnection process suggests a new
consistent and generic model for the formation of anemone regions in the solar
corona.Comment: Accepted for publication in ApJ, 11 pages and 5 figure
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