Sunspot seismic halos generated by fast MHD wave refraction

Astronomy and Astrophysics (Impact Factor: 4.48). 05/2009; 506(2). DOI: 10.1051/0004-6361/200913030
Source: arXiv

ABSTRACT We suggest an explanation for the high-frequency power excess surrounding
active regions known as seismic halos. The idea is based on numerical
simulations of magneto-acoustic waves propagation in sunspots. We propose that
such an excess can be caused by the additional energy injected by fast mode
waves refracted in the higher atmosphere due to the rapid increase of the
Alfven speed. Our model qualitatively explains the magnitude of the halo and
allows to make some predictions of its behavior that can be checked in future

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    ABSTRACT: Context. We present evidence for the conversion and transmission of wave modes on the magnetic flux tubes that constitute mottles and form the magnetic canopy in a quiet Sun region. Aims. Our aim is to highlight the details and the key parameters of the mechanism that produces power halos and magnetic shadows around the magnetic network observed in H� . Methods. We use our previous calculations of the magnetic field vector and the height of the magnetic canopy, and based on simple assumptions, we determine the turning height, i.e., the height at which the fast magneto-acoustic waves reflect at the chromosphere. We compare the variation of 3, 5, and 7 min power in the magnetic shadow and the power halo with the results of a two-dimensional model on mode conversion and transmission. The key parameter of the model is the attack angle, which is related to the inclination of the magnetic field vector at the canopy height. Our analysis takes also into account that 1) there are projection effects on the propagation of waves, 2) the magnetic canopy and the turning height are curved layers, 3) waves with periods longer than 3 min only reach the chromosphere in the presence of inclined magnetic fields (ramp effect), 4) mottles in H� are canopy structures, and 5) the wings of H� contain mixed signal from low- and high-� plasma. Results. The dependence of the measured power on the attack angle follows the anticipated by the two-dimensional model very well. Long-period slow waves are channeled to the upper chromospheric layers following the magnetic field lines of mottles, while short period fast waves penetrate the magnetic canopy and are reflected back higher, at the turning height. Conclusions. Although both magnetoacoustic modes contribute to velocity signals, making the interpretation of observations a challenging task, we conclude that conversion and transmission of the acoustic waves into fast and slow magnetoacoustic waves are responsible for forming power halos and magnetic shadows in the quiet Sun region.
    Astronomy and Astrophysics 06/2014; DOI:10.1051/0004-6361/201423986 · 4.48 Impact Factor
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    ABSTRACT: The Helioseismic and Magnetic Imager (HMI) and the Atmospheric Imaging Assembly (AIA) instruments onboard the Solar Dynamics Observatory satellite produce Doppler velocity and continuum intensity at 6173 Å as well as intensity maps at 1600 Å and 1700 Å, which can be used for helioseismic studies at different heights in the solar photosphere. We perform a Hankel–Fourier analysis in an annulus centered around sunspots or quiet-Sun regions, to estimate the change in power of waves crossing these regions of interest. We find that there is a dependence of power-reduction coefficients α on measurement height in the photosphere: Sunspots reduce the power of outgoing waves with frequencies ν lower than ν≈4.5 mHz at all heights, but enhance the power of acoustic waves in the range ν≈4.5 – 5.5 mHz toward chromospheric heights, which is likely the signature of acoustic glories (halos). Maximum power reduction seems to occur near the continuum level and to decrease with altitude. Sunspots also impact the frequencies of outgoing waves in an altitude-dependent fashion. The quiet Sun is shown to behave like a strong power reducer for outgoing f- and p-modes at the continuum level, with a power reduction α≈15 – 20 %, and like a weak power enhancer for p-modes higher in the atmosphere. It is speculated that the surprising power reduction at the continuum level is related to granulation. In Doppler-velocity data, and unlike in intensity data, the quiet Sun behaves like a strong power reducer for granular flows.
    Solar Physics 11/2012; 282(1). DOI:10.1007/s11207-012-0128-0 · 3.81 Impact Factor
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    ABSTRACT: The wave characteristics derived from ring-diagram analysis of HMI Doppler data in magnetically quiet regions near active regions are compared with those with no nearby active regions using 5° patches during 2.5 years of cycle 24 ascending phase. We search for perturbations that may be associated with propagation of the acoustic oscillations through the nearby sunspot. We observe significant variations in the mode parameters and flows. We analyse their dependence on the direction of the wave propagation. The observed mode dependence of the variations in mode amplitude, line width and frequency does not have the same functional form as that observed for the differences between quiet and active regions.
    Journal of Physics Conference Series 06/2013; 440(1):2008-. DOI:10.1088/1742-6596/440/1/012008


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