1,750 research outputs found
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
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 &
Expanding and Contracting Coronal Loops as Evidence of Vortex Flows Induced by Solar Eruptions
Eruptive solar flares were predicted to generate large-scale vortex flows at
both sides of the erupting magnetic flux rope. This process is analogous to a
well-known hydrodynamic process creating vortex rings. The vortices lead to
advection of closed coronal loops located at peripheries of the flaring active
region. Outward flows are expected in the upper part and returning flows in the
lower part of the vortex. Here, we examine two eruptive solar flares, an
X1.1-class flare SOL2012-03-05T03:20 and a C3.5-class SOL2013-06-19T07:29. In
both flares, we find that the coronal loops observed by the Atmospheric Imaging
Assembly in its 171\,\AA, 193\,\AA, or 211\,\AA~passbands show coexistence of
expanding and contracting motions, in accordance with the model prediction. In
the X-class flare, multiple expanding/contracting loops coexist for more than
35 minutes, while in the C-class flare, an expanding loop in 193\,\AA~appears
to be close-by and co-temporal with an apparently imploding loop arcade seen in
171\,\AA. Later, the 193\,\AA~loop also switches to contraction. These
observations are naturally explained by vortex flows present in a model of
eruptive solar flares.Comment: The Astrophysical Journal, accepte
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
Hα Doppler shifts in a tornado in the solar corona
Context. High resolution movies in 193 Å from the Atmospheric Imaging Assembly (AIA) on the Solar Dynamic Observatory (SDO) show apparent rotation in the leg of a prominence observed during a coordinated campaign. Such structures are commonly referred to as tornadoes. Time-distance intensity diagrams of the AIA data show the existence of oscillations suggesting that the structure is rotating.
Aims. The aim of this paper is to understand if the cool plasma at chromospheric temperatures inside the tornado is rotating around its central axis.
Methods. The tornado was also observed in Hα with a cadence of 30 s by the MSDP spectrograph, operating at the Solar Tower in Meudon. The MSDP provides sequences of simultaneous spectra in a 2D field of view from which a cube of Doppler velocity maps is retrieved.
Results. The Hα Doppler maps show a pattern with alternatively blueshifted and redshifted areas of 5 to 10′′ wide. Over time the blueshifted areas become redshifted and vice versa, with a quasi-periodicity of 40 to 60 min. Weaker amplitude oscillations with periods of 4 to 6 min are superimposed onto these large period oscillations.
Conclusions. The Doppler pattern observed in Hα cannot be interpreted as rotation of the cool plasma inside the tornado. The Hα velocity observations give strong constraints on the possible interpretations of the AIA tornado
Topological Analysis of Emerging Bipole Clusters Producing Violent Solar Events
During the rising phase of Solar Cycle 24 tremendous activity occurred on the
Sun with fast and compact emergence of magnetic flux leading to bursts of
flares (C to M and even X-class). We investigate the violent events occurring
in the cluster of two active regions (ARs), NOAA numbers 11121 and 11123,
observed in November 2010 with instruments onboard the {\it Solar Dynamics
Observatory} and from Earth. Within one day the total magnetic flux increased
by with the emergence of new groups of bipoles in AR 11123. From all the
events on 11 November, we study, in particular, the ones starting at around
07:16 UT in GOES soft X-ray data and the brightenings preceding them. A
magnetic-field topological analysis indicates the presence of null points,
associated separatrices and quasi-separatrix layers (QSLs) where magnetic
reconnection is prone to occur. The presence of null points is confirmed by a
linear and a non-linear force-free magnetic-field model. Their locations and
general characteristics are similar in both modelling approaches, which
supports their robustness. However, in order to explain the full extension of
the analysed event brightenings, which are not restricted to the photospheric
traces of the null separatrices, we compute the locations of QSLs. Based on
this more complete topological analysis, we propose a scenario to explain the
origin of a low-energy event preceding a filament eruption, which is
accompanied by a two-ribbon flare, and a consecutive confined flare in AR
11123. The results of our topology computation can also explain the locations
of flare ribbons in two other events, one preceding and one following the ones
at 07:16 UT. Finally, this study provides further examples where flare-ribbon
locations can be explained when compared to QSLs and only, partially, when
using separatrices.Comment: 42 pages, 15 figure
Electric current in flares ribbons: observations and 3D standard model
We present for the first time the evolution of the photospheric electric
currents during an eruptive X-class flare, accurately predicted by the standard
3D flare model. We analyze this evolution for the February 15, 2011 flare using
HMI/SDO magnetic observations and find that localized currents in \J-shaped
ribbons increase to double their pre-flare intensity. Our 3D flare model,
developed with the OHM code, suggests that these current ribbons, which develop
at the location of EUV brightenings seen with AIA imagery, are driven by the
collapse of the flare's coronal current layer. These findings of increased
currents restricted in localized ribbons are consistent with the overall free
energy decrease during a flare, and the shape of these ribbons also give an
indication on how much twisted the erupting flux rope is. Finally, this study
further enhances the close correspondence obtained between the theoretical
predictions of the standard 3D model and flare observations indicating that the
main key physical elements are incorporated in the model.Comment: 12 pages, 7 figure
Major Surge Activity of Super-Active Region NOAA 10484
We observed two surges in H-alpha from the super-active region NOAA 10484.
The first surge was associated with an SF/C4.3 class flare. The second one was
a major surge associated with a SF/C3.9 flare. This surge was also observed
with SOHO/EIT in 195 angstrom and NoRh in 17 GHz, and showed similar evolution
in these wavelengths. The major surge had an ejective funnel-shaped spray
structure with fast expansion in linear (about 1.2 x 10^5 km) and angular
(about 65 deg) size during its maximum phase. The mass motion of the surge was
along open magnetic field lines, with average velocity about 100 km/s. The
de-twisting motion of the surge reveals relaxation of sheared and twisted
magnetic flux. The SOHO/MDI magnetograms reveal that the surges occurred at the
site of companion sunspots where positive flux emerged, converged, and canceled
against surrounding field of opposite polarity. Our observations support
magnetic reconnection models for the surges and jets.Comment: 4 pages, 3 figures; To appear in "Magnetic Coupling between the
Interior and the Atmosphere of the Sun", eds. S.S. Hasan and R.J. Rutten,
Astrophysics and Space Science Series, Springer-Verlag, Heidelberg, Berlin,
200
Magnetic field in atypical prominence structures: Bubble, tornado and eruption
Spectropolarimetric observations of prominences have been obtained with the
THEMIS telescope during four years of coordinated campaigns. Our aim is now to
understand the conditions of the cool plasma and magnetism in `atypical'
prominences, namely when the measured inclination of the magnetic field
departs, to some extent, from the predominantly horizontal field found in
`typical' prominences. What is the role of the magnetic field in these
prominence types? Are plasma dynamics more important in these cases than the
magnetic support? We focus our study on three types of `atypical' prominences
(tornadoes, bubbles and jet-like prominence eruptions) that have all been
observed by THEMIS in the He I D_3 line, from which the Stokes parameters can
be derived. The magnetic field strength, inclination and azimuth in each pixel
are obtained by using the Principal Component Analysis inversion method on a
model of single scattering in the presence of the Hanle effect. The magnetic
field in tornadoes is found to be more or less horizontal, whereas for the
eruptive prominence it is mostly vertical. We estimate a tendency towards
higher values of magnetic field strength inside the bubbles than outside in the
surrounding prominence. In all of the models in our database, only one magnetic
field orientation is considered for each pixel. While sufficient for most of
the main prominence body, this assumption appears to be oversimplified in
atypical prominence structures. We should consider these observations as the
result of superposition of multiple magnetic fields, possibly even with a
turbulent field component.Comment: 13 pages, 9 figure
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