136 research outputs found
Neutral /zero/ Points of Magnetic Fields
Geometry of force-free magnetic field in vicinity of neutral point
Calculation of Spectral Darkening and Visibility Functions for Solar Oscillations
Calculations of spectral darkening and visibility functions for the
brightness oscillations of the Sun resulting from global solar oscillations are
presented. This has been done for a broad range of the visible and infrared
continuum spectrum. The procedure for the calculations of these functions
includes the numerical computation of depth-dependent derivatives of the
opacity caused by p modes in the photosphere. A radiative-transport code was
used for this purpose to get the disturbances of the opacities from temperature
and density fluctuations. The visibility and darkening functions are obtained
for adiabatic oscillations under the assumption that the temperature
disturbances are proportional to the undisturbed temperature of the
photosphere. The latter assumption is the only way to explore any opacity
effects since the eigenfunctions of p-mode oscillations have not been obtained
so far. This investigation reveals that opacity effects have to be taken into
account because they dominate the violet and infrared part of the spectrum.
Because of this dominance, the visibility functions are negative for those
parts of the spectrum. Furthermore, the darkening functions show a
wavelength-dependent change of sign for some wavelengths owing to these opacity
effects. However, the visibility and darkening functions under the assumptions
used contradict the observations of global p-mode oscillations, but it is
beyond doubt that the opacity effects influence the brightness fluctuations of
the Sun resulting from global oscillations
A New Look at Mode Conversion in a Stratified Isothermal Atmosphere
Recent numerical investigations of wave propagation near coronal magnetic
null points (McLaughlin and Hood: Astron. Astrophys. 459, 641,2006) have
indicated how a fast MHD wave partially converts into a slow MHD wave as the
disturbance passes from a low-beta plasma to a high-beta plasma. This is a
complex process and a clear understanding of the conversion mechanism requires
the detailed investigation of a simpler model. An investigation of mode
conversion in a stratified, isothermal atmosphere, with a uniform, vertical
magnetic field is carried out, both numerically and analytically. In contrast
to previous investigations of upward-propagating waves (Zhugzhda and Dzhalilov:
Astron. Astrophys. 112, 16, 1982a; Cally: Astrophys. J. 548, 473, 2001), this
paper studies the downward propagation of waves from a low-beta to high-beta
environment. A simple expression for the amplitude of the transmitted wave is
compared with the numerical solution.Comment: 14 pages, 6 figure
Waves and granulation
Propagation of hydrodynamic waves is considered in frames of the one-dimensional model of granulation. An exact analytical solution of the problem is obtained. The theory is applied only to high-frequency p-modes, because the effect of gravity is neglected. It appears that acoustic waves in a structured atmosphere are not plane ones. Besides, there are hydrodynamic phonon and wave guide wave
modes. The results of calculations of the wave functions for high-frequency waves with periods 100–200 s are presented. Upgoing waves are captured in integranular lanes, while downgoing ones are trapped in granules. The effect of wave capture is of greater efficiency for phonon and wave guide modes. The phase velocities of waves differ from the mean sound speed in the photosphere
Three-minute wave enhancement in the solar photosphere
It is a well-known result that the power of five-minute oscillations is
progressively reduced by magnetic fields in the solar photosphere. Many authors
have pointed out that this fact could be due to a complex interaction of many
processes: opacity effects, MHD mode conversion and intrinsic reduced acoustic
emissivity in strong magnetic fields. While five-minute oscillations are the
dominant component in the photosphere, it has been shown that chromospheric
heights are in turn dominated by three-minute oscillations. Two main theories
have been proposed to explain their presence based upon resonance filtering in
the atmospheric cavity and non linear interactions. In this work we show,
through the analysis of IBIS observations of a solar pore in the photospheric
Fe I 617.3 nm line, that three-minute waves are already present at the height
of formation of this line and that their amplitude depends on the magnetic
field strength and is strictly confined in the umbral region.Comment: A&A accepte
MHD waves in sunspots
The review addresses the spatial frequency morphology of sources of sunspot
oscillations and waves, including their localization, size, oscillation
periods, height localization with the mechanism of cut-off frequency that forms
the observed emission variability. Dynamic of sunspot wave processes, provides
the information about the structure of wave fronts and their time variations,
investigates the oscillation frequency transformation depending on the wave
energy is shown. The initializing solar flares caused by trigger agents like
magnetoacoustic waves, accelerated particle beams, and shocks are discussed.
Special attention is paid to the relation between the flare reconnection
periodic initialization and the dynamics of sunspot slow magnetoacoustic waves.
A short review of theoretical models of sunspot oscillations is provided.Comment: 20 pages, 6 figures, Chapter in AGU Monograph (in press), Review
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Magneto-acoustic waves in sunspots: first results from a new 3D nonlinear magnetohydrodynamic code
Waves observed in the photosphere and chromosphere of sunspots show complex
dynamics and spatial patterns. The interpretation of high-resolution sunspot
wave observations requires modeling of three-dimensional non-linear wave
propagation and mode transformation in the sunspot upper layers in realistic
spot model atmospheres. Here we present the first results of such modeling. We
have developed a 3D non-linear numerical code specially designed to calculate
the response of magnetic structures in equilibrium to an arbitrary
perturbation. The code solves the 3D nonlinear MHD equations for perturbations;
it is stabilized by hyper-diffusivity terms and is fully parallelized. The
robustness of the code is demonstrated by a number of standard tests. We
analyze several simulations of a sunspot perturbed by pulses of different
periods at subphotospheric level, from short periods, introduced for academic
purposes, to longer and realistic periods of three and five minutes. We present
a detailed description of the three-dimensional mode transformation in a
non-trivial sunspot-like magnetic field configuration, including the conversion
between fast and slow magneto-acoustic waves and the Alfv\'en wave, by
calculation of the wave energy fluxes. Our main findings are the following: (1)
the conversion from acoustic to the Alfv\'en mode is only observed if the the
driving pulse is located out of the sunspot axis, but this conversion is
energetically inefficient; (2) as a consequence of the cut-off effects and
refraction of the fast magneto-acoustic mode, the energy of the evanescent
waves with periods around 5 minutes remains almost completely below the level
beta=1; (3) waves with frequencies above the cut-off propagate field-aligned to
the chromosphere and their power becomes dominating over that of evanescent
5-minute oscillations, in agreement with observations
Dynamics of the Solar Magnetic Network. II. Heating the Magnetized Chromosphere
We consider recent observations of the chromospheric network, and argue that
the bright network grains observed in the Ca II H & K lines are heated by an as
yet unidentified quasi-steady process. We propose that the heating is caused by
dissipation of short-period magnetoacoustic waves in magnetic flux tubes
(periods less than 100 s). Magnetohydrodynamic (MHD) models of such waves are
presented. We consider wave generation in the network due to two separate
processes: (a) by transverse motions at the base of the flux tube; and (b) by
the absorption of acoustic waves generated in the ambient medium. We find that
the former mechanism leads to an efficient heating of the chromosphere by slow
magnetoacoustic waves propagating along magnetic field lines. This heating is
produced by shock waves with a horizontal size of a few hundred kilometers. In
contrast, acoustic waves excited in the ambient medium are converted into
transverse fast modes that travel rapidly through the flux tube and do not form
shocks, unless the acoustic sources are located within 100 km from the tube
axis. We conclude that the magnetic network may be heated by magnetoacoustic
waves that are generated in or near the flux tubes.Comment: 30 pages, 8 figures, Accepted in Astrophysical Journa
Buoyancy-driven Magnetohydrodynamic Waves
Turbulent motions close to the visible solar surface may generate low-frequency internal gravity waves (IGWs) that propagate through the lower solar atmosphere. Magnetic activity is ubiquitous throughout the solar atmosphere, so it is expected that the behavior of IGWs is to be affected. In this article we investigate the role of an equilibrium magnetic field on propagating and standing buoyancy oscillations in a gravitationally stratified medium. We assume that this background magnetic field is parallel to the direction of gravitational stratification. It is known that when the equilibrium magnetic field is weak and the background is isothermal, the frequencies of standing IGWs are sensitive to the presence of magnetism. Here, we generalize this result to the case of a slowly varying temperature. To do this, we make use of the Boussinesq approximation. A comparison between the hydrodynamic and magnetohydrodynamic cases allows us to deduce the effects due to a magnetic field. It is shown that the frequency of IGWs may depart significantly from the Brunt–Väisälä frequency, even for a weak magnetic field. The mathematical techniques applied here give a clearer picture of the wave mode identification, which has previously been misinterpreted. An observational test is urged to validate the theoretical findings
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