608 research outputs found
Plasma Jets and Eruptions in Solar Coronal Holes: a 3D flux emergence experiment
A three-dimensional numerical experiment of the launching of a hot and fast
coronal jet followed by several violent eruptions is analyzed in detail. These
events are initiated through the emergence of a magnetic flux rope from the
solar interior into a coronal hole. We explore the evolution of the emerging
magnetically-dominated plasma dome surmounted by a current sheet and the
ensuing pattern of reconnection. A hot and fast coronal jet with inverted-Y
shape is produced that shows properties comparable to those frequently observed
with EUV and X-Ray detectors. We analyze its 3D shape, its inhomogeneous
internal structure, and its rise and decay phases, lasting for some 15-20 min
each. Particular attention is devoted to the field-line connectivities and the
reconnection pattern. We also study the cool and high-density volume that
appears encircling the emerged dome. The decay of the jet is followed by a
violent phase with a total of five eruptions. The first of them seems to follow
the general pattern of tether-cutting reconnection in a sheared arcade,
although modified by the field topology created by the preceding reconnection
evolution. The two following eruptions take place near and above the strong
field-concentrations at the surface. They show a twisted, \Omega-loop like rope
expanding in height, with twist being turned into writhe, thus hinting at a
kink instability (perhaps combined with a torus-instability) as the cause of
the eruption. The succession of a main jet ejection and a number of violent
eruptions that resemble mini-CME's and their physical properties suggest that
this experiment may provide a model for the blowout jets recently proposed in
the literature.Comment: Accepted for publication in The Astrophysical Journal (vol 770, June
2013
Test particle acceleration in a numerical MHD experiment of an anemone jet
To use a 3D numerical MHD experiment representing magnetic flux emerging into
an open field region as a background field for tracing charged particles. The
interaction between the two flux systems generates a localised current sheet
where MHD reconnection takes place. We investigate how efficiently the
reconnection region accelerates charged particles and what kind of energy
distribution they acquire. The particle tracing is done numerically using the
Guiding Center Approximation on individual data sets from the numerical MHD
experiment. We derive particle and implied photon distribution functions having
power law forms, and look at the impact patterns of particles hitting the
photosphere. We find that particles reach energies far in excess of those seen
in observations of solar flares. However the structure of the impact region in
the photosphere gives a good representation of the topological structure of the
magnetic field.Comment: 9 pages, 7 figures, accepted for publication in A&
MHD Wave Propagation in the Neighbourhood of Two Null Points
The nature of fast magnetoacoustic and Alfv\'en waves is investigated in a
zero plasma in the neighbourhood of a pair of two-dimensional null
points. This gives an indication of wave propagation in the low solar
corona, for a more complicated magnetic configuration than that looked at by
McLaughlin & Hood (2004). It is found that the fast wave is attracted to the
null points and that the front of the wave slows down as it approaches the null
point pair, with the wave splitting and part of the wave accumulating at one
null and the rest at the other. Current density will then accumulate at these
points and ohmic dissipation will then extract the energy in the wave at these
points. This suggests locations where wave heating will occur in the corona.
The Alfv\'en wave behaves in a different manner in that the wave accumulates
along the separatrices. Hence, the current density will accumulate at this part
of the topology and this is where wave heating will occur. However, the
phenomenon of wave accumulation at a specific place is a feature of both wave
types, and illustrates the importance of studying the topology of the corona
when considering MHD wave propagation.Comment: 11 pages, 14 figure
Current sheet formation and nonideal behavior at three-dimensional magnetic null points
The nature of the evolution of the magnetic field, and of current sheet
formation, at three-dimensional (3D) magnetic null points is investigated. A
kinematic example is presented which demonstrates that for certain evolutions
of a 3D null (specifically those for which the ratios of the null point
eigenvalues are time-dependent) there is no possible choice of boundary
conditions which renders the evolution of the field at the null ideal.
Resistive MHD simulations are described which demonstrate that such evolutions
are generic. A 3D null is subjected to boundary driving by shearing motions,
and it is shown that a current sheet localised at the null is formed. The
qualitative and quantitative properties of the current sheet are discussed.
Accompanying the sheet development is the growth of a localised parallel
electric field, one of the signatures of magnetic reconnection. Finally, the
relevance of the results to a recent theory of turbulent reconnection is
discussed.Comment: to appear in Phys. Plasmas. A version with higher quality figures can
be found at http://www.maths.dundee.ac.uk/~dpontin/ In this replacement
version, typos have been corrected, and in addition references and some
further discussion adde
On the nature of reconnection at a solar coronal null point above a separatrix dome
Three-dimensional magnetic null points are ubiquitous in the solar corona,
and in any generic mixed-polarity magnetic field. We consider magnetic
reconnection at an isolated coronal null point, whose fan field lines form a
dome structure. We demonstrate using analytical and computational models
several features of spine-fan reconnection at such a null, including the fact
that substantial magnetic flux transfer from one region of field line
connectivity to another can occur. The flux transfer occurs across the current
sheet that forms around the null point during spine-fan reconnection, and there
is no separator present. Also, flipping of magnetic field lines takes place in
a manner similar to that observed in quasi-separatrix layer or slip-running
reconnection.Comment: Accepted for publication in the Astrophysical Journa
Magnetic reconnection in flux-tubes undergoing spinning footpoint motions
Aims. Photospheric motions acting on the coronal magnetic field have the potential to build up huge amounts of magnetic energy. The energy may be released through magnetic reconnection, and so a detailed understanding of the 3D process is crucial if its implications for coronal heating are to be fully addressed. Methods. A 3D MHD experiment is described in which misaligned magnetic flux tubes are subjected to simple spinning boundary motions. Results. The resulting shear between adjacent flux systems generates a twisted central separator current sheet that extends vertically throughout the domain. Current density is amplified to a sufficient extent that reconnection begins, and occurs everywhere along the separator current sheet, while the separatrix current sheets that exist in the early stages of the experiment are found to be unimportant in the systems dynamical evolution. In 2D cross-sections, the reconnection process exhibits many similarities to the regime of flux pile-up reconnection
HINODE Observations of Chromospheric Brightenings in the Ca II H Line during small-scale Flux Emergence Events
\ion{Ca}{2} H emission is a well-known indicator of magnetic activity in the
Sun and other stars. It is also viewed as an important signature of
chromospheric heating. However, the \ion{Ca}{2} H line has not been used as a
diagnostic of magnetic flux emergence from the solar interior. Here we report
on Hinode observations of chromospheric \ion{Ca}{2} H brightenings associated
with a repeated, small-scale flux emergence event. We describe this process and
investigate the evolution of the magnetic flux, G-band brightness, and
\ion{Ca}{2} H intensity in the emerging region. Our results suggest that energy
is released in the chromosphere as a consequence of interactions between the
emerging flux and the pre-existing magnetic field, in agreement with recent 3D
numerical simulations.Comment: 12 Pages, 6 Figures, Accepted for publication in ApJ Letter
Fragment Driven Magnetic Reconnection
In this paper, we investigate a simple model where two, initially
unconnected, flux systems are forced to interact in response to the imposed
boundary driving by solving the non-ideal 3D MHD equations numerically. The
reconnection rate of the dynamical process is determined and compared with the
corresponding rate for the potential evolution of the magnetic field. This
shows that the dynamic reconnection rate is about a factor of two smaller than
the potential (perfect, instantaneous) rate for realistic solar driving
velocities demonstrating that this three-dimensional magnetic reconnection
process is fast.
The energy input for a fixed advection distance is found to be independent of
the driving velocity. The Joule dissipation associated with the reconnection
process is also found to be basically dependent on the advection distance
rather than driving velocity. This implies that the timescale for the event
determines the effect the heating has on the temperature increase.
Finally, the numerical experiments indicate that the observational structure
of the reconnection site changes dramatically depending on the phase of the
evolution of the passage of the two flux sources. In the initial phase, where
the sources become connected, the heating is confined to a compact region. For
the disconnecting phase the energy gets distributed over a larger area due to
the reconnected field line connectivity.Comment: 6 pages, 8 figures. Procedings of SOHO 15 Coronal Heating, ESA
publicatio
MHD consistent cellular automata (CA) models II. Applications to solar flares
In Isliker et al. (2000b), an extended cellular automaton (X-CA) model for
solar flares was introduced. In this model, the interpretation of the model's
grid-variable is specified, and the magnetic field, the current, and an
approximation to the electric field are yielded, all in a way that is
consistent with Maxwell's and the MHD equations. Here, we reveal which relevant
plasma physical processes are implemented by the X-CA model and in what form,
and what global physical set-up is assumed by this model when it is in its
natural state (SOC). The basic results are: (1) On large-scales, all variables
show characteristic quasi-symmetries. (2) The global magnetic topology forms
either (i) closed magnetic field lines, or (ii) an arcade of field lines above
the bottom plane line, if the model is slightly modified. (3) In case of the
magnetic topology (ii), loading can be interpreted as if there were a plasma
which flows predominantly upwards, whereas in case of the magnetic topology
(i), as if there were a plasma flow expanding from the neutral line. (4) The
small-scale physics in the bursting phase represent localized diffusive
processes. (5) The local diffusivity usually has a value which is effectively
zero, and it turns locally to an anomalous value if a threshold is exceeded,
whereby diffusion dominates the quiet evolution (loading). (6) Flares
(avalanches) are accompanied by the appearance of localized, intense electric
fields. (7) In a variant on the X-CA model, the magnitude of the current is
used directly in the instability criterion. First results indicate that the SOC
state persists. (8) The current-dissipation during flares is spatially
fragmented into a large number of dissipative current-surfaces of varying
sizes, which show a highly dynamic temporal evolution.Comment: 13 pages, 12 figures; in press at Astronomy and Astrophysics (2001
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