231 research outputs found
The physical mechanisms that initiate and drive solar eruptions
Solar eruptions are due to a sudden destabilization of force-free coronal
magnetic fields. But the detailed mechanisms which can bring the corona towards
an eruptive stage, then trigger and drive the eruption, and finally make it
explosive, are not fully understood. A large variety of storage-and-release
models have been developed and opposed to each other since 40 years. For
example, photospheric flux emergence vs. flux cancellation, localized coronal
reconnection vs. large-scale ideal instabilities and loss of equilibria,
tether-cutting vs. breakout reconnection, and so on. The competition between
all these approaches has led to a tremendous drive in developing and testing
all these concepts, by coupling state-of-the-art models and observations.
Thanks to these developments, it now becomes possible to compare all these
models with one another, and to revisit their interpretation in light of their
common and their different behaviors. This approach leads me to argue that no
more than two distinct physical mechanisms can actually initiate and drive
prominence eruptions: the magnetic breakout and the torus instability. In this
view, all other processes (including flux emergence, flux cancellation, flare
reconnection and long-range couplings) should be considered as various ways
that lead to, or that strengthen, one of the aforementioned driving mechanisms.Comment: 13 pages, 0 figure, to appear in proceedings of the IAUS300 meetin
X-ray and UV investigation into the magnetic connectivity of a solar flare
We investigate the X-ray and UV emission detected by RHESSI and TRACE in the
context of a solar flare on the 16th November 2002 with the goal of better
understanding the evolution of the flare. We analysed the characteristics of
the X-ray emission in the 12-25 and 25-50 keV energy range while we looked at
the UV emission at 1600 {\AA}. The flare appears to have two distinct phases of
emission separated by a 25-second time delay, with the first phase being
energetically more important. We found good temporal and spatial agreement
between the 25-50 keV X-rays and the most intense areas of the 1600 {\AA} UV
emission. We also observed an extended 100-arcsecond < 25 keV source that
appears coronal in nature and connects two separated UV ribbons later in the
flare. Using the observational properties in X-ray and UV wavelengths, we
propose two explanations for the flare evolution in relation to the spine/fan
magnetic field topology and the accelerated electrons. We find that a
combination of quasi separatrix layer reconnection and null-point reconnection
is required to account for the observed properties of the X-ray and UV
emission.Comment: 8 pages, 8 figures, published in Astronomy and Astrophysic
The Apparent Critical Decay Index at the Onset of Solar Prominence Eruptions
International audienceA magnetic flux rope (MFR) embedded in a line-tied external magnetic field that decreases with height as-z n is unstable to perturbations if the decay index of the field n is larger than a critical value. The onset of this instability, called torus instability, is one of the main mechanisms that can initiate coronal mass ejections. Since flux ropes often possess magnetic dips that can support prominence plasma, this is also a valuable mechanism to trigger prominence eruptions. Magnetohydrodynamic (MHD) simulations of the formation and/or emergence of MFRs suggest a critical value for the onset of the instability in the range [1.4−2]. However, detailed observations of prominences suggest a value in the range [0.9−1.1]. In this Letter, by using a set of MHD simulations, we show why the large discrepancy between models and observations is only apparent. Our simulations indeed show that the critical decay index at the onset of the eruption is = n 1.4 0.1 when computed at the apex of the flux rope axis, while it is = n 1.1 0.1 when it is computed at the altitude of the topmost part of the distribution of magnetic dips. The discrepancy only arises because weakly twisted curved flux ropes do not have dips up to the altitude of their axis
Chain Reconnections observed in Sympathetic Eruptions
The nature of various plausible causal links between sympathetic events is
still a controversial issue. In this work, we present multi-wavelength
observations of sympathetic eruptions, associated flares and coronal mass
ejections (CMEs) occurring on 2013 November 17 in two close-by active regions.
Two filaments i.e., F1 and F2 are observed in between the active regions.
Successive magnetic reconnections, caused by different reasons (flux
cancellation, shear and expansion) have been identified during the whole event.
The first reconnection occurred during the first eruption via flux cancellation
between the sheared arcades overlying filament F2, creating a flux rope and
leading to the first double ribbon solar flare. During this phase we observed
the eruption of overlaying arcades and coronal loops, which leads to the first
CME. The second reconnection is believed to occur between the expanding flux
rope of F2 and the overlying arcades of the filament F1. We suggest that this
reconnection destabilized the equilibrium of filament F1, which further
facilitated its eruption. The third stage of reconnection occurred in the wake
of the erupting filament F1 between the legs of overlying arcades. This may
create a flux rope and the second double ribbon flare and a second CME. The
fourth reconnection was between the expanding arcades of the erupting filament
F1 and the nearby ambient field, which produced the bi-directional plasma flows
towards both upward and downward. Observations and a nonlinear force-free field
extrapolation confirm the possibility of reconnection and the causal link
between the magnetic systems.Comment: 37 pages, 17 figures, Accepted for publication in Ap
Saddle-shaped solar flare arcades
Arcades of flare loops form as a consequence of magnetic reconnection
powering solar flares and eruptions. We analyse the morphology and evolution of
flare arcades that formed during five well-known eruptive flares. We show that
the arcades have a common saddle-like shape. The saddles occur despite the fact
that the flares were of different classes (C to X), occurred in different
magnetic environments, and were observed in various projections. The saddles
are related to the presence of longer, relatively-higher, and inclined flare
loops, consistently observed at the ends of the arcades, which we term
`cantles'. Our observations indicate that cantles typically join straight
portions of flare ribbons with hooked extensions of the conjugate ribbons. The
origin of the cantles is investigated in stereoscopic observations of the 2011
May 9 eruptive flare carried out by the Atmospheric Imaging Assembly (AIA) and
Extreme Ultraviolet Imager (EUVI). The mutual separation of the instruments led
to ideal observational conditions allowing for simultaneous analysis of the
evolving cantle and the underlying ribbon hook. Based on our analysis we
suggest that the formation of one of the cantles can be explained by magnetic
reconnection between the erupting structure and its overlying arcades. We
propose that the morphology of flare arcades can provide information about the
reconnection geometries in which the individual flare loops originate.Comment: 9 pages, 5 figure
Hooked flare ribbons and flux-rope related QSL footprints
We studied the magnetic topology of active region 12158 on 2014 September 10
and compared it with the observations before and early in the flare which
begins at 17:21 UT (SOL2014-09-10T17:45:00). Our results show that the
sigmoidal structure and flare ribbons of this active region observed by SDO/AIA
can be well reproduced from a Grad-Rubin non linear force free field
extrapolation method. Various inverse-S and -J shaped magnetic field lines,
that surround a coronal flux rope, coincide with the sigmoid as observed in
different extreme ultraviolet wavelengths, including its multi-threaded curved
ends. Also, the observed distribution of surface currents in the magnetic
polarity where it was not prescribed is well reproduced. This validates our
numerical implementation and set-up of the Grad-Rubin method. The modeled
double inverse-J shaped Quasi-Separatrix Layer (QSL) footprints match the
observed flare ribbons during the rising phase of the flare, including their
hooked parts. The spiral-like shape of the latter may be related to a complex
pre-eruptive flux rope with more than one turn of twist, as obtained in the
model. These ribbon-associated flux-rope QSL-footprints are consistent with the
new standard flare model in 3D, with the presence of a hyperbolic flux tube
located below an inverse tear drop shaped coronal QSL. This is a new step
forward forecasting the locations of reconnection and ribbons in solar flares,
and the geometrical properties of eruptive flux ropes.Comment: Accepted for publication in Ap
Hot Spine Loops and the Nature of a Late-Phase Solar Flare
The fan-spine magnetic topology is believed to be responsible for many
curious features in solar explosive events. A spine field line links distinct
flux domains, but direct observation of such feature has been rare. Here we
report a unique event observed by the Solar Dynamic Observatory where a set of
hot coronal loops (over 10 MK) connected to a quasi-circular chromospheric
ribbon at one end and a remote brightening at the other. Magnetic field
extrapolation suggests these loops are partly tracer of the evolving spine
field line. Continuous slipping- and null-point-type reconnections were likely
at work, energizing the loop plasma and transferring magnetic flux within and
across the fan quasi-separatrix layer. We argue that the initial reconnection
is of the "breakout" type, which then transitioned to a more violent flare
reconnection with an eruption from the fan dome. Significant magnetic field
changes are expected and indeed ensued. This event also features an
extreme-ultraviolet (EUV) late phase, i.e. a delayed secondary emission peak in
warm EUV lines (about 2-7 MK). We show that this peak comes from the cooling of
large post-reconnection loops beside and above the compact fan, a direct
product of eruption in such topological settings. The long cooling time of the
large arcades contributes to the long delay; additional heating may also be
required. Our result demonstrates the critical nature of cross-scale magnetic
coupling - topological change in a sub-system may lead to explosions on a much
larger scale.Comment: Accepted for publication in ApJ. Animations linked from pd
Distribution of Electric Currents in Solar Active Regions
There has been a long-lasting debate on the question of whether or not
electric currents in solar active regions are neutralized. That is, whether or
not the main (or direct) coronal currents connecting the active region
polarities are surrounded by shielding (or return) currents of equal total
value and opposite direction. Both theory and observations are not yet fully
conclusive regarding this question, and numerical simulations have,
surprisingly, barely been used to address it. Here we quantify the evolution of
electric currents during the formation of a bipolar active region by
considering a three-dimensional magnetohydrodynamic simulation of the emergence
of a sub-photospheric, current-neutralized magnetic flux rope into the solar
atmosphere. We find that a strong deviation from current neutralization
develops simultaneously with the onset of significant flux emergence into the
corona, accompanied by the development of substantial magnetic shear along the
active region's polarity inversion line. After the region has formed and flux
emergence has ceased, the strong magnetic fields in the region's center are
connected solely by direct currents, and the total direct current is several
times larger than the total return current. These results suggest that active
regions, the main sources of coronal mass ejections and flares, are born with
substantial net currents, in agreement with recent observations. Furthermore,
they support eruption models that employ pre-eruption magnetic fields
containing such currents.Comment: 6 pages, 5 figures, to appear in Astrophysical Journal Letter
SOLAR PROMINENCE INTERACTIONS
ABSTRACT We report numerical simulations of the formation, interaction, and magnetic reconnection between pairs of solar prominences within the sheared-arcade model. Our experiments consider the four possible basic combinations of chiralities (identical or opposite) and axial magnetic fields (aligned or opposed) between the participating prominences. When the topology of the global flux system comprising the prominences and arcades is bipolar, so that a single polarity inversion line is shared by the two structures, then identical chiralities necessarily imply aligned axial fields, while opposite chiralities imply opposed axial fields. In the former case, external magnetic reconnections forming field lines linking the two prominences occur; in the latter, such reconnections are disfavored, and no linkage takes place. These results concur with empirical rules for prominence interactions. When the topology instead is quadrupolar, so that a second polarity inversion line crossing the first lies between the prominences, then the converse relation holds between chirality and axial-field alignment. External reconnections forming linking field lines now occur between prominences with opposite chiralities; they also occur, but result only in footpoint exchanges, between prominences with identical chiralities. These findings conflict with the accepted empirical rules but may not have been tested in observations to date. All of our model prominences, especially those that undergo linking reconnections, contain substantial magnetic shear and twist. Nevertheless, none exhibits any sign of onset of instability or loss of equilibrium that might culminate in an eruption
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