324 research outputs found
Global Dynamics of Subsurface Solar Active Regions
We present three-dimensional numerical simulations of a magnetic loop
evolving in either a convectively stable or unstable rotating shell. The
magnetic loop is introduced in the shell in such a way that it is buoyant only
in a certain portion in longitude, thus creating an \Omega-loop. Due to the
action of magnetic buoyancy, the loop rises and develops asymmetries between
its leading and following legs, creating emerging bipolar regions whose
characteristics are similar to the ones of observed spots at the solar surface.
In particular, we self-consistently reproduce the creation of tongues around
the spot polarities, which can be strongly affected by convection. We moreover
emphasize the presence of ring-shaped magnetic structures around our simulated
emerging regions, which we call "magnetic necklace" and which were seen in a
number of observations without being reported as of today. We show that those
necklaces are markers of vorticity generation at the periphery and below the
rising magnetic loop. We also find that the asymmetry between the two legs of
the loop is crucially dependent on the initial magnetic field strength. The
tilt angle of the emerging regions is also studied in the stable and unstable
cases and seems to be affected both by the convective motions and the presence
of a differential rotation in the convective cases.Comment: 23 pages (ApJ 2-column format), 19 figures, accepted for publication
in Ap
Critical decay index at the onset of solar eruptions
Magnetic flux ropes are topological structures consisting of twisted magnetic
field lines that globally wrap around an axis. The torus instability model
predicts that a magnetic flux rope of major radius undergoes an eruption
when its axis reaches a location where the decay index of the ambient magnetic field is larger than a critical value. In
the current-wire model, the critical value depends on the thickness and
time-evolution of the current channel. We use magneto-hydrodynamic (MHD)
simulations to investigate if the critical value of the decay index at the
onset of the eruption is affected by the magnetic flux rope's internal current
profile and/or by the particular pre-eruptive photospheric dynamics. The
evolution of an asymmetric, bipolar active region is driven by applying
different classes of photospheric motions. We find that the critical value of
the decay index at the onset of the eruption is not significantly affected by
either the pre-eruptive photospheric evolution of the active region or by the
resulting different magnetic flux ropes. As in the case of the current-wire
model, we find that there is a `critical range' , rather than a
`critical value' for the onset of the torus instability. This range is in good
agreement with the predictions of the current-wire model, despite the inclusion
of line-tying effects and the occurrence of tether-cutting magnetic
reconnection.Comment: 15 pages, 9 figures. To appear in The Astrophysical Journa
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
Relationship between photospheric currents and coronal magnetic helicity for force-free bipolar fields
Aims.
The origin and evolution of the magnetic helicity in the solar corona are not well understood. For instance, the magnetic helicity of an active region is often about 1042 Mx2 (1026 Wb2), but the observed processes whereby it is thought to be injected into the corona do not yet provide an accurate estimate of the resulting magnetic helicity budget or time evolution. The variation in magnetic helicity is important for understanding the physics of flares, coronal mass ejections, and their associated magnetic clouds. To shed light on this topic, we investigate here the changes in magnetic helicity due to electric currents in the corona for a single twisted flux tube that may model characteristic coronal structures such as active region filaments, sigmoids, or coronal loops.
Methods.
For a bipolar photospheric magnetic field and several distributions of current, we extrapolated the coronal field as a nonlinear force-free field. We then computed the relative magnetic helicity, as well as the self and mutual helicities.
Results.
Starting from a magnetic configuration with a moderate amount of current, the amount of magnetic helicity can increase by 2 orders of magnitude when the maximum current strength is increased by a factor of 2. The high sensitivity of magnetic helicity to the current density can partially explain discrepancies between measured values on the photosphere, in the corona, and in magnetic clouds. Our conclusion is that the magnetic helicity strongly depends on both the strength of the current density and also on its distribution.
Conclusions.
Only improved measurements of current density at the photospheric level will advance our knowledge of the magnetic helicity content in the solar atmosphere
Formation of a rotating jet during the filament eruption on 10-11 April 2013
We analyze multi-wavelength and multi-viewpoint observations of a helically
twisted plasma jet formed during a confined filament eruption on 10-11 April
2013. Given a rather large scale event with its high spatial and temporal
resolution observations, it allows us to clearly understand some new physical
details about the formation and triggering mechanism of twisting jet. We
identify a pre-existing flux rope associated with a sinistral filament, which
was observed several days before the event. The confined eruption of the
filament within a null point topology, also known as an Eiffel tower (or
inverted-Y) magnetic field configuration results in the formation of a twisted
jet after the magnetic reconnection near a null point. The sign of helicity in
the jet is found to be the same as that of the sign of helicity in the
filament. Untwisting motion of the reconnected magnetic field lines gives rise
to the accelerating plasma along the jet axis. The event clearly shows the
twist injection from the pre-eruptive magnetic field to the jet.Comment: 14 pages, 12 figures, to appear in MNRA
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
Interchange Slip-Running Reconnection and Sweeping SEP Beams
We present a new model to explain how particles (solar energetic particles;
SEPs), accelerated at a reconnection site that is not magnetically connected to
the Earth, could eventually propagate along the well-connected open flux tube.
Our model is based on the results of a low-beta resistive magnetohydrodynamics
simulation of a three-dimensional line-tied and initially current-free bipole,
that is embedded in a non-uniform open potential field. The topology of this
configuration is that of an asymmetric coronal null-point, with a closed fan
surface and an open outer spine. When driven by slow photospheric shearing
motions, field lines, initially fully anchored below the fan dome, reconnect at
the null point, and jump to the open magnetic domain. This is the standard
interchange mode as sketched and calculated in 2D. The key result in 3D is
that, reconnected open field lines located in the vicinity of the outer spine,
keep reconnecting continuously, across an open quasi-separatrix layer, as
previously identified for non-open-null-point reconnection. The apparent
slipping motion of these field lines leads to form an extended narrow magnetic
flux tube at high altitude. Because of the slip-running reconnection, we
conjecture that if energetic particles would be traveling through, or be
accelerated inside, the diffusion region, they would be successively injected
along continuously reconnecting field lines that are connected farther and
farther from the spine. At the scale of the full Sun, owing to the super-radial
expansion of field lines below 3 solar radii, such energetic particles could
easily be injected in field lines slipping over significant distances, and
could eventually reach the distant flux tube that is well-connected to the
Earth
Magnetohydrostatic solar prominences in near-potential coronal magnetic fields
We present numerical magnetohydrostatic solutions describing the
gravitationally stratified, bulk equilibrium of cool, dense prominence plasma
embedded in a near-potential coronal field. These solutions are calculated
using the FINESSE magnetohydrodynamics equilibrium solver and describe the
morphologies of magnetic field distributions in and around prominences and the
cool prominence plasma that these fields support. The equilibrium condition for
this class of problem is usually different in distinct subdomains, separated by
free boundaries, across which solutions are matched by suitable continuity or
jump conditions describing force balance. We employ our precise finite element
elliptic solver to calculate solutions not accessible by previous analytical
techniques with temperature or entropy prescribed as free functions of the
magnetic flux function, including a range of values of the polytropic index,
temperature variations mainly across magnetic field lines and photospheric
field profiles sheared close to the polarity inversion line. Out of the many
examples computed here, perhaps the most noteworthy is one which reproduces
precisely the three-part structure often encountered in observations: a cool
dense prominence within a cavity/flux rope embedded in a hot corona. The
stability properties of these new equilibria, which may be relevant to solar
eruptions, can be determined in the form of a full resistive MHD spectrum using
a companion hyperbolic stability solver.Comment: To appear in ApJ August 200
Twisting solar coronal jet launched at the boundary of an active region
A broad jet was observed in a weak magnetic field area at the edge of active
region NOAA 11106. The peculiar shape and magnetic environment of the broad jet
raised the question of whether it was created by the same physical processes of
previously studied jets with reconnection occurring high in the corona. We
carried out a multi-wavelength analysis using the EUV images from the
Atmospheric Imaging Assembly (AIA) and magnetic fields from the Helioseismic
and Magnetic Imager (HMI) both on-board the SDO satellite. The jet consisted of
many different threads that expanded in around 10 minutes to about 100 Mm in
length, with the bright features in later threads moving faster than in the
early ones, reaching a maximum speed of about 200 km s^{-1}. Time-slice
analysis revealed a striped pattern of dark and bright strands propagating
along the jet, along with apparent damped oscillations across the jet. This is
suggestive of a (un)twisting motion in the jet, possibly an Alfven wave. A
topological analysis of an extrapolated field was performed. Bald patches in
field lines, low-altitude flux ropes, diverging flow patterns, and a null point
were identified at the basis of the jet. Unlike classical lambda or
Eiffel-tower shaped jets that appear to be caused by reconnection in current
sheets containing null points, reconnection in regions containing bald patches
seems to be crucial in triggering the present jet. There is no observational
evidence that the flux ropes detected in the topological analysis were actually
being ejected themselves, as occurs in the violent phase of blowout jets;
instead, the jet itself may have gained the twist of the flux rope(s) through
reconnection. This event may represent a class of jets different from the
classical quiescent or blowout jets, but to reach that conclusion, more
observational and theoretical work is necessary.Comment: 12 pages, 9 figures, accepted for publication in A&
Hyperdiffusion as a Mechanism for Solar Coronal Heating
A theory for the heating of coronal magnetic flux ropes is developed. The
dissipated magnetic energy has two distinct contributions: (1) energy injected
into the corona as a result of granule-scale, random footpoint motions, and (2)
energy from the large-scale, nonpotential magnetic field of the flux rope. The
second type of dissipation can be described in term of hyperdiffusion, a type
of magnetic diffusion in which the helicity of the mean magnetic field is
conserved. The associated heating rate depends on the gradient of the torsion
parameter of the mean magnetic field. A simple model of an active region
containing a coronal flux rope is constructed. We find that the temperature and
density on the axis of the flux rope are lower than in the local surroundings,
consistent with observations of coronal cavities. The model requires that the
magnetic field in the flux rope is stochastic in nature, with a perpendicular
length scale of the magnetic fluctuations of order 1000 km.Comment: 9 pages (emulateapj style), 4 figures, ApJ, in press (v. 679; June 1,
2008
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