119 research outputs found
Magnetohydrodynamic code for gravitationally-stratified media
Aims. We describe a newly-developed magnetohydrodynamic (MHD) code with the capacity to simulate the interaction of any arbitrary perturbation (i.e., not necessarily limited to the linearised limit) with a magnetohydrostatic equilibrium background.
Methods. By rearranging the terms in the system of MHD equations and explicitly taking into account the magnetohydrostatic equilibrium condition, we define the equations governing the perturbations that describe the deviations from the background state of plasma for the density, internal energy and magnetic field. We found it was advantageous to use this modified form of the MHD equations for numerical simulations of physical processes taking place in a stable gravitationally-stratified plasma. The governing equations are implemented in a novel way in the code. Sub-grid diffusion and resistivity are applied to ensure numerical stability of the computed solution of the MHD equations. We apply a fourth-order central difference scheme to calculate the spatial derivatives, and implement an arbitrary Runge-Kutta scheme to advance the solution in time.
Results. We have built the proposed method, suitable for strongly-stratified magnetised plasma, on the base of the well-documented Versatile Advection Code (VAC) and performed a number of one- and multi-dimensional hydrodynamic and MHD tests to demonstrate the feasibility and robustness of the code for applications to astrophysical plasmas
Photospheric high-frequency acoustic power excess in sunspot umbra: signature of magneto-acoustic modes
We present observational evidence for the presence of MHD waves in the solar
photosphere deduced from SOHO/MDI Dopplergram velocity observations. The
magneto-acoustic perturbations are observed as acoustic power enhancement in
the sunspot umbra at high-frequency bands in the velocity component
perpendicular to the magnetic field. We use numerical modelling of wave
propagation through localised non-uniform magnetic field concentration along
with the same filtering procedure as applied to the observations to identify
the observed waves. Guided by the results of the numerical simulations we
classify the observed oscillations as magneto-acoustic waves excited by the
trapped sub-photospheric acoustic waves. We consider the potential application
of the presented method as a diagnostic tool for magnetohelioseismology.Comment: 6 pages, 5 figures, to appear in Annales Geophysica
"Explosively growing" vortices of unstably stratified atmosphere
©2016. American Geophysical Union.A new type of "explosively growing" vortex structure is investigated theoretically in the framework of ideal fluid hydrodynamics. It is shown that vortex structures may arise in convectively unstable atmospheric layers containing background vorticity. From an exact analytical vortex solution the vertical vorticity structure and toroidal speed are derived and analyzed. The assumption that vorticity is constant with height leads to a solution that grows explosively when the flow is inviscid. The results shown are in agreement with observations and laboratory experiment
Observational Evidence of Sausage-Pinch Instability in Solar Corona by SDO/AIA
We present the first observational evidence of the evolution of sausage-pinch
instability in Active Region 11295 during a prominence eruption using data
recorded on 12 September 2011 by the Atmospheric Imaging Assembly (AIA) onboard
the Solar Dynamics Observatory (SDO). We have identified a magnetic flux tube
visible in AIA 304 \AA\ that shows curvatures on its surface with variable
cross-sections as well as enhanced brightness. These curvatures evolved and
thereafter smoothed out within a time-scale of a minute. The curved locations
on the flux tube exhibit a radial outward enhancement of the surface of about
1-2 Mm (factor of 2 larger than the original thickness of the flux tube) from
the equilibrium position. AIA 193 \AA\ snapshots also show the formation of
bright knots and narrow regions inbetween at the four locations as that of 304
\AA\ along the flux tube where plasma emission is larger compared to the
background. The formation of bright knots over an entire flux tube as well as
the narrow regions in < 60 s may be the morphological signature of the sausage
instability. We also find the flows of the confined plasma in these bright
knots along the field lines, which indicates the dynamicity of the flux tube
that probably causes the dominance of the longitudinal field component over
short temporal scales. The observed longitudinal motion of the plasma frozen in
the magnetic field lines further vanishes the formed curvatures and plasma
confinements as well as growth of instability to stablize the flux tube.Comment: 12 pages, 5 figure
Modelling 3D magnetic networks in a realistic solar atmosphere
This is the final version. Available from OUP via the DOI in this recordThe magnetic network extending from the photosphere (solar radius ≃ R☉ ) to lower corona
(R☉ + 10Mm) plays an important role in the heating mechanisms of the solar atmosphere.
Here we further develop the models with realistic open magnetic flux tubes of the authors in
order to model more complicated configurations. Closed magnetic loops, and combinations
of closed and open magnetic flux tubes are modelled. These are embedded within a stratified
atmosphere, derived from observationally motivated semi-empirical and data-driven models
subject to solar gravity and capable of spanning from the photosphere up into the chromosphere
and lower corona. Constructing a magnetic field comprising self-similar magnetic flux
tubes, an analytic solution for the kinetic pressure and plasma density is derived. Combining
flux tubes of opposite polarity it is possible to create a steady background magnetic field configuration
modelling a solar atmosphere exhibiting realistic stratification. The result can be
applied to SOHO/MDI and SDO/HMI and other magnetograms from the solar surface, upon
which photospheric motions can be simulated to explore the mechanism of energy transport.
We demonstrate this powerful and versatile method with an application to Helioseismic and
Magnetic Imager data.Academy of FinlandScience and Technology Facilities Council (STFC
Response to “Comment on ‘Large-scale Alfvén vortices’” [Phys. Plasmas 23, 034703 (2016)]
[Phys. Plasmas 23, 034703 (2016)
3D Simulations of Magnetohydrodynamic Waves in the Magnetized Solar Atmosphere
We present results of three-dimensional numerical simulations of
magnetohydrodynamic (MHD) wave propagation in a solar magnetic flux tube. Our
study aims at understanding the properties of a range of MHD wave modes
generated by different photospheric motions. We consider two scenarios observed
in the lower solar photosphere, namely, granular buffeting and vortex-like
motion, among the simplest mechanism for the generation of waves within a
strong, localized magnetic flux concentration. We show that granular buffeting
is likely to generate stronger slow and fast magnetoacoustic waves as compared
to swirly motions. Correspondingly, the energy flux transported differs as a
result of the driving motions. We also demonstrate that the waves generated by
granular buffeting are likely to manifest in stronger emission in the
chromospheric network. We argue that different mechanisms of wave generation
are active during the evolution of a magnetic element in the intergranular
lane, resulting in temporally varying emission at chromospheric heights.Comment: Appeared in ApJ, 11 pages, 12 figure
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