278 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
Magnetohydrostatic equilibrium. I: Three-dimensional open magnetic flux tube in the stratified solar atmosphere
A single open magnetic flux tube spanning the solar photosphere (solar radius
R) and the lower corona (R + 10 Mm) is modelled in magnetohydrostatic
equilibrium within a realistic stratified atmosphere subject to solar gravity.
Such flux tubes are observed to remain relatively stable for up to a day or
more, and it is our aim to apply the model as the background condition for
numerical studies of energy transport mechanisms from the surface to the
corona. We solve analytically an axially symmetric 3D structure for the model,
with magnetic field strength, plasma density, pressure and temperature all
consistent with observational and theoretical estimates. The self similar
construction ensures the magnetic field is divergence free. The equation of
pressure balance for this particular set of flux tubes can be integrated
analytically to find the pressure and density corrections required to preserve
the magnetohydrostatic equilibrium. The model includes a number of free
parameters, which makes the solution applicable to a variety of other physical
problems and it may therefore be of more general interest.Comment: 9 pages, 8 figure
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
Державне регулювання житлового будівництва в Україні в контексті економічної циклічності
The article is devoted to the development of the concept of state regulation of housing construction in the context of economic cyclicality. The influence of economic cycles on the indicators of housing development in Ukraine is investigated. The expediency of state regulation of housing construction in the conditions of economic cyclicalness is substantiated and its targets are defined depending on the phase of the economic cycle. An indicative list of economic policy measures of the state that can be implemented in the process of state regulation of housing construction in terms of economic cyclicality is offered.Стаття присвячена розробці концепції державне регулювання житлового будівництва в контексті економічної циклічності. Доcліджено вплив економічних циклів на показники розвиток житлового будівництва в Україні. Обґрунтована доцільність державного регулювання житлового будівництва в умовах економічної циклічності та визначені його цільові орієнтири залежно від фази економічного циклу. Запропоновано орієнтовний перелік заходів економічної політики держави, які можуть бути здійснені в процесі державного регулювання житлового будівництва в умовах економічної циклічності
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
Evidence for the photospheric excitation of incompressible chromospheric waves
Observing the excitation mechanisms of incompressible transverse waves is vital for determining how energy propagates through the lower solar atmosphere. We aim to show the connection between convectively driven photospheric flows and incompressible chromospheric waves. The observations presented here show the propagation of incompressible motion through the quiet lower solar atmosphere, from the photosphere to the chromosphere. We determine photospheric flow vectors to search for signatures of vortex motion and compare results to photospheric flows present in convective simulations. Further, we search for the chromospheric response to vortex motions. Evidence is presented that suggests incompressible waves can be excited by the vortex motions of a strong magnetic flux concentration in the photosphere. A chromospheric counterpart to the photospheric vortex motion is also observed, presenting itself as a quasi-periodic torsional motion. Fine-scale, fibril structures that emanate from the chromospheric counterpart support transverse waves that are driven by the observed torsional motion. A new technique for obtaining details of transverse waves from time-distance diagrams is presented and the properties of transverse waves (e.g., amplitudes and periods) excited by the chromospheric torsional motion are measured
"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
The NonLinear Evolution of a Twist in a Magnetic Shocktube
The interaction between a small twist and a horizontal chromospheric shocktube is investigated. The magnetic flux tube is modeled using 1.5-D magnetohydrodynamics. The presence of a supersonic yet sub-Alfvénic flow along the flux tube allows the Alfvénic pulse driven at the photospheric boundary to become trapped and amplified between the stationary shock front and photosphere. The amplification of the twist leads to the formation of slow and fast shocks. The pre-existing stationary shock is destabilized and pushed forward as it merges with the slow shock. The propagating fast shock extracts the kinetic energy of the flow and launches rapid twists of 10–15 km s−1 upon each reflection. A cavity is formed between the slow and fast shocks where the flux tube becomes globally twisted within less than an hour. The resultant highly twisted magnetic flux tube is similar to those prone to kink instabilities, which may be responsible for solar eruptions. The generated torsional flux is calculated
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