14 research outputs found
Buildup of Magnetic Shear and Free Energy During Flux Emergence and Cancellation
We examine a simulation of flux emergence and cancellation, which shows a
complex sequence of processes that accumulate free magnetic energy in the solar
corona essential for the eruptive events such as coronal mass ejections (CMEs),
filament eruptions and flares. The flow velocity at the surface and in the
corona shows a consistent shearing pattern along the polarity inversion line
(PIL), which together with the rotation of the magnetic polarities, builds up
the magnetic shear. Tether-cutting reconnection above the PIL then produces
longer sheared magnetic field lines that extend higher into the corona, where a
sigmoidal structure forms. Most significantly, reconnection and upward
energy-flux transfer are found to occur even as magnetic flux is submerging and
appears to cancel at the photosphere. A comparison of the simulated coronal
field with the corresponding coronal potential field graphically shows the
development of nonpotential fields during the emergence of the magnetic flux
and formation of sunspots
Simulation of Flux Emergence from the Convection Zone to the Corona
Here, we present numerical simulations of magnetic flux buoyantly rising from
a granular convection zone into the low corona. We study the complex
interaction of the magnetic field with the turbulent plasma. The model includes
the radiative loss terms, non-ideal equations of state, and empirical corona
heating. We find that the convection plays a crucial role in shaping the
morphology and evolution of the emerging structure. The emergence of magnetic
fields can disrupt the convection pattern as the field strength increases, and
form an ephemeral region-like structure, while weak magnetic flux emerges and
quickly becomes concentrated in the intergranular lanes, i.e. downflow regions.
As the flux rises, a coherent shear pattern in the low corona is observed in
the simulation. In the photosphere, both magnetic shearing and velocity
shearing occur at a very sharp polarity inversion line (PIL). In a case of
U-loop magnetic field structure, the field above the surface is highly sheared
while below it is relaxed
Dynamic Coupling of Convective Flows and Magnetic Field during Flux Emergence
We simulate the buoyant rise of a magnetic flux rope from the solar
convection zone into the corona to better understand the energetic coupling of
the solar interior to the corona. The magnetohydrodynamic model addresses the
physics of radiative cooling, coronal heating and ionization, which allow us to
produce a more realistic model of the solar atmosphere. The simulation
illustrates the process by which magnetic flux emerges at the photosphere and
coalesces to form two large concentrations of opposite polarities. We find that
the large-scale convective motion in the convection zone is critical to form
and maintain sunspots, while the horizontal converging flows in the near
surface layer prevent the concentrated polarities from separating. The foot
points of the sunspots in the convection zone exhibit a coherent rotation
motion, resulting in the increasing helicity of the coronal field. Here, the
local configuration of the convection causes the convergence of opposite
polarities of magnetic flux with a shearing flow along the polarity inversion
line. During the rising of the flux rope, the magnetic energy is first injected
through the photosphere by the emergence, followed by energy transport by
horizontal flows, after which the energy is subducted back to the convection
zone by the submerging flows
Radiative Hydrodynamic Models of the Optical and Ultraviolet Emission from Solar Flares
We report on radiative hydrodynamic simulations of moderate and strong solar
flares. The flares were simulated by calculating the atmospheric response to a
beam of non-thermal electrons injected at the apex of a one-dimensional closed
coronal loop, and include heating from thermal soft X-ray, extreme ultraviolet
and ultraviolet (XEUV) emission. The equations of radiative transfer and
statistical equilibrium were treated in non-LTE and solved for numerous
transitions of hydrogen, helium, and Ca II allowing the calculation of detailed
line profiles and continuum emission. This work improves upon previous
simulations by incorporating more realistic non-thermal electron beam models
and includes a more rigorous model of thermal XEUV heating. We find XEUV
backwarming contributes less than 10% of the heating, even in strong flares.
The simulations show elevated coronal and transition region densities resulting
in dramatic increases in line and continuum emission in both the UV and optical
regions. The optical continuum reaches a peak increase of several percent which
is consistent with enhancements observed in solar white light flares. For a
moderate flare (~M-class), the dynamics are characterized by a long gentle
phase of near balance between flare heating and radiative cooling, followed by
an explosive phase with beam heating dominating over cooling and characterized
by strong hydrodynamic waves. For a strong flare (~X-class), the gentle phase
is much shorter, and we speculate that for even stronger flares the gentle
phase may be essentially non-existent. During the explosive phase, synthetic
profiles for lines formed in the upper chromosphere and transition region show
blue shifts corresponding to a plasma velocity of ~120 km/s, and lines formed
in the lower chromosphere show red shifts of ~40 km/s.Comment: 21 pages, 15 figures. Will appear in 2005 September 1 issue of the
Ap
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The photospheric boundary of Sun-to-Earth coupled models
The least understood component of the Sun-to-Earth coupled system is the solar atmosphere-the visible layers of the Sun that encompass the photosphere, chromosphere, transition region and low corona. Coronal mass ejections (CMEs), principal drivers of space weather, are magnetically driven phenomena that are thought to originate in the low solar corona. Their initiation mechanism, however, is still a topic of great debate. If we are to develop physics-based models with true predictive capability, we must progress beyond simulations of highly idealized magnetic configurations, and develop the techniques necessary to incorporate observations of the vector magnetic field at the solar photosphere into numerical models of the solar corona. As a first step toward this goal, we drive the SAIC coronal model with the complex magnetic fields and flows that result from a sub-photospheric MHD simulation of an emerging active region. In particular, we successfully emerge a twisted Omega-loop into a pre-existing coronal arcade. To date, it is not possible to directly measure the magnetic field in the solar corona. Instead, we must rely on nonpotential extrapolations to generate the twisted, pre-eruptive coronal topologies necessary to initiate data-driven MHD simulations of CMEs. We therefore investigate whether a non-constant-a force-free extrapolation can successfully reproduce the magnetic features of a self-consistent MHD simulation of flux emergence through a stratified model atmosphere. We generate force-free equilibria from simulated photospheric and chromospheric vector magnetograms, and compare these results to the MHD calculation. We then apply these techniques to an IVM (Mees Solar Observatory) vector magnetogram of NOAA active-region 8210, a source of a number of eruptive events on the Sun. (C) 2004 Elsevier Ltd. All rights reserved