264 research outputs found
Clusters of small eruptive flares produced by magnetic reconnection in the sun
We report on the formation of small solar flares produced by patchy magnetic
reconnection between interacting magnetic loops. A three-dimensional (3D)
magnetohydrodynamic (MHD) numerical experiment was performed, where a uniform
magnetic flux sheet was injected into a fully developed convective layer. The
gradual emergence of the field into the solar atmosphere results in a network
of magnetic loops, which interact dynamically forming current layers at their
interfaces. The formation and ejection of plasmoids out of the current layers
leads to patchy reconnection and the spontaneous formation of several small
(size ? 1-2Mm) flares. We find that these flares are short-lived (30 s - 3 min)
bursts of energy in the range O(10^25 - 10^27) ergs, which is basically the
nanoflare-microflare range. Their persistent formation and co-operative action
and evolution leads to recurrent emission of fast EUV/X-ray jets and
considerable plasma heating in the active corona.Comment: 5 pages, 5 figure
Eruption of magnetic flux ropes during flux emergence
Aims: We investigate the formation of flux ropes in a flux emergence region
and their rise into the outer atmosphere of the Sun.
Methods: We perform 3D numerical experiments solving the time-dependent and
resistive MHD equations.
Results: A sub-photospheric twisted flux tube rises from the solar interior
and expands into the corona. A flux rope is formed within the expanding field,
due to shearing and reconnection of field lines at low atmospheric heights. If
the tube emerges into a non-magnetized atmosphere, the flux rope rises, but
remains confined inside the expanding magnetized volume. On the contrary, if
the expanding tube is allowed to reconnect with a preexisting coronal field,
the flux rope experiences a full eruption with a rise profile which is in
qualitative agreement with erupting filaments and Coronal Mass Ejections
Flux emergence and coronal eruption
Our aim is to study the photospheric flux distribution of a twisted flux tube
that emerges from the solar interior. We also report on the eruption of a new
flux rope when the emerging tube rises into a pre-existing magnetic field in
the corona. To study the evolution, we use 3D numerical simulations by solving
the time-dependent and resistive MHD equations. We qualitatively compare our
numerical results with MDI magnetograms of emerging flux at the solar surface.
We find that the photospheric magnetic flux distribution consists of two
regions of opposite polarities and elongated magnetic tails on the two sides of
the polarity inversion line (PIL), depending on the azimuthal nature of the
emerging field lines and the initial field strength of the rising tube. Their
shape is progressively deformed due to plasma motions towards the PIL. Our
results are in qualitative agreement with observational studies of magnetic
flux emergence in active regions (ARs). Moreover, if the initial twist of the
emerging tube is small, the photospheric magnetic field develops an undulating
shape and does not possess tails. In all cases, we find that a new flux rope is
formed above the original axis of the emerging tube that may erupt into the
corona, depending on the strength of the ambient field.Comment: 5 pages, 3 figures, accepted for publication in A&
Modelling magnetic flux emergence in the solar convection zone
[Abridged] Bipolar magnetic regions are formed when loops of magnetic flux
emerge at the solar photosphere. Our aim is to investigate the flux emergence
process in a simulation of granular convection. In particular we aim to
determine the circumstances under which magnetic buoyancy enhances the flux
emergence rate (which is otherwise driven solely by the convective upflows). We
use three-dimensional numerical simulations, solving the equations of
compressible magnetohydrodynamics in a horizontally-periodic Cartesian domain.
A horizontal magnetic flux tube is inserted into fully developed hydrodynamic
convection. We systematically vary the initial field strength, the tube
thickness, the initial entropy distribution along the tube axis and the
magnetic Reynolds number. Focusing upon the low magnetic Prandtl number regime
(Pm<1) at moderate magnetic Reynolds number, we find that the flux tube is
always susceptible to convective disruption to some extent. However, stronger
flux tubes tend to maintain their structure more effectively than weaker ones.
Magnetic buoyancy does enhance the flux emergence rates in the strongest
initial field cases, and this enhancement becomes more pronounced when we
increase the width of the flux tube. This is also the case at higher magnetic
Reynolds numbers, although the flux emergence rates are generally lower in
these less dissipative simulations because the convective disruption of the
flux tube is much more effective in these cases. These simulations seem to be
relatively insensitive to the precise choice of initial conditions: for a given
flow, the evolution of the flux tube is determined primarily by the initial
magnetic field distribution and the magnetic Reynolds number.Comment: 12 pages, 15 figures, 2 tables. Accepted for publication in Astronomy
and Astrophysic
Validation and Benchmarking of a Practical Free Magnetic Energy and Relative Magnetic Helicity Budget Calculation in Solar Magnetic Structures
In earlier works we introduced and tested a nonlinear force-free (NLFF)
method designed to self-consistently calculate the free magnetic energy and the
relative magnetic helicity budgets of the corona of observed solar magnetic
structures. The method requires, in principle, only a single, photospheric or
low-chromospheric, vector magnetogram of a quiet-Sun patch or an active region
and performs calculations in the absence of three-dimensional magnetic and
velocity-field information. In this work we strictly validate this method using
three-dimensional coronal magnetic fields. Benchmarking employs both synthetic,
three-dimensional magnetohydrodynamic simulations and nonlinear force-free
field extrapolations of the active-region solar corona. We find that our
time-efficient NLFF method provides budgets that differ from those of more
demanding semi-analytical methods by a factor of ~3, at most. This difference
is expected from the physical concept and the construction of the method.
Temporal correlations show more discrepancies that, however, are soundly
improved for more complex, massive active regions, reaching correlation
coefficients of the order of, or exceeding, 0.9. In conclusion, we argue that
our NLFF method can be reliably used for a routine and fast calculation of free
magnetic energy and relative magnetic helicity budgets in targeted parts of the
solar magnetized corona. As explained here and in previous works, this is an
asset that can lead to valuable insight into the physics and the triggering of
solar eruptions.Comment: 32 pages, 14 figures, accepted by Solar Physic
Recurrent solar jets in active regions
We study the emergence of a toroidal flux tube into the solar atmosphere and
its interaction with a pre-existing field of an active region. We investigate
the emission of jets as a result of repeated reconnection events between
colliding magnetic fields. We perform 3D simulations by solving the
time-dependent, resistive MHD equations in a highly stratified atmosphere. A
small active region field is constructed by the emergence of a toroidal
magnetic flux tube. A current structure is build up and reconnection sets in
when new emerging flux comes into contact with the ambient field of the active
region. The topology of the magnetic field around the current structure is
drastically modified during reconnection. The modification results in a
formation of new magnetic systems that eventually collide and reconnect. We
find that reconnection jets are taking place in successive recurrent phases in
directions perpendicular to each other, while in each phase they release
magnetic energy and hot plasma into the solar atmosphere. After a series of
recurrent appearance of jets, the system approaches an equilibrium where the
efficiency of the reconnection is substantially reduced. We deduce that the
emergence of new magnetic flux introduces a perturbation to the active region
field, which in turn causes reconnection between neighboring magnetic fields
and the release of the trapped energy in the form of jet-like emissions. This
is the first time that self-consistent recurrency of jets in active regions is
shown in a three-dimensional experiment of magnetic flux emergence.Comment: 4 pages, 3 figures, accepted for publication (A&A
3D MHD Flux Emergence Experiments: Idealized models and coronal interactions
This paper reviews some of the many 3D numerical experiments of the emergence
of magnetic fields from the solar interior and the subsequent interaction with
the pre-existing coronal magnetic field. The models described here are
idealized, in the sense that the internal energy equation only involves the
adiabatic, Ohmic and viscous shock heating terms. However, provided the main
aim is to investigate the dynamical evolution, this is adequate. Many
interesting observational phenomena are explained by these models in a
self-consistent manner.Comment: Review article, accepted for publication in Solar Physic
Validation of the magnetic energy vs. helicity scaling in solar magnetic structures
We assess the validity of the free magnetic energy - relative magnetic
helicity diagram for solar magnetic structures. We used two different methods
of calculating the free magnetic energy and the relative magnetic helicity
budgets: a classical, volume-calculation nonlinear force-free (NLFF) method
applied to finite coronal magnetic structures and a surface-calculation NLFF
derivation that relies on a single photospheric or chromospheric vector
magnetogram. Both methods were applied to two different data sets, namely
synthetic active-region cases obtained by three-dimensional
magneto-hydrodynamic (MHD) simulations and observed active-region cases, which
include both eruptive and noneruptive magnetic structures. The derived
energy--helicity diagram shows a consistent monotonic scaling between relative
helicity and free energy with a scaling index 0.840.05 for both data sets
and calculation methods. It also confirms the segregation between noneruptive
and eruptive active regions and the existence of thresholds in both free energy
and relative helicity for active regions to enter eruptive territory. We
consider the previously reported energy-helicity diagram of solar magnetic
structures as adequately validated and envision a significant role of the
uncovered scaling in future studies of solar magnetism
Sunspot rotation. I. A consequence of flux emergence
Context. Solar eruptions and high flare activity often accompany the rapid
rotation of sunspots. The study of sunspot rotation and the mechanisms driving
this motion are therefore key to our understanding of how the solar atmosphere
attains the conditions necessary for large energy release.
Aims. We aim to demonstrate and investigate the rotation of sunspots in a 3D
numerical experiment of the emergence of a magnetic flux tube as it rises
through the solar interior and emerges into the atmosphere. Furthermore, we
seek to show that the sub-photospheric twist stored in the interior is injected
into the solar atmosphere by means of a definitive rotation of the sunspots.
Methods. A numerical experiment is performed to solve the 3D resistive
magnetohydrodynamic (MHD) equations using a Lagrangian-Remap code. We track the
emergence of a toroidal flux tube as it rises through the solar interior and
emerges into the atmosphere investigating various quantities related to both
the magnetic field and plasma.
Results. Through detailed analysis of the numerical experiment, we find clear
evidence that the photospheric footprints or sunspots of the flux tube undergo
a rotation. Significant vertical vortical motions are found to develop within
the two polarity sources after the field emerges. These rotational motions are
found to leave the interior portion of the field untwisted and twist up the
atmospheric portion of the field. This is shown by our analysis of the relative
magnetic helicity as a significant portion of the interior helicity is
transported to the atmosphere. In addition, there is a substantial transport of
magnetic energy to the atmosphere. Rotation angles are also calculated by
tracing selected fieldlines; the fieldlines threading through the sunspot are
found to rotate through angles of up to 353 degrees over the course of the
experiment
On the Saturation of Astrophysical Dynamos: Numerical Experiments with the No-cosines flow
In the context of astrophysical dynamos we illustrate that the no-cosines
flow, with zero mean helicity, can drive fast dynamo action and study the
dynamo's mode of operation during both the linear and non-linear saturation
regime: It turns out that in addition to a high growth rate in the linear
regime, the dynamo saturates at a level significantly higher than normal
turbulent dynamos, namely at exact equipartition when the magnetic Prandtl
number is on the order of unity. Visualization of the magnetic and velocity
fields at saturation will help us to understand some of the aspects of the
non-linear dynamo problem.Comment: 8 pages, 5 figures, submitted to the proceedings of "Space Climate 1"
to be peer-reviewed to Solar Physic
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