Sunspot seismic halos generated by fast MHD wave refraction

Astronomy and Astrophysics (Impact Factor: 4.38). 05/2009; 506(2). DOI: 10.1051/0004-6361/200913030
Source: arXiv


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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    • "Thus far, the mechanisms responsible for this phenomenon are not well understood. With regards to the morphology, Khomenko and Collados (2009) provide a concise summary, and the key points are: i) a disproportionate enhancement of acoustic power occurs in the 5.5 –7.5 mHz frequency range ii) the halos occur in low to intermediate fields (50 – 300 G) (Hindman and Brown, 1998), and diminish rapidly with greater field strengths iii) enhancement is aided by regions of near-horizontal field inclination, in particular between locations of opposite polarity (Schunker and Braun, 2011; Rajaguru et al., 2013) Numerous theories have suggested possible mechanisms responsible for the halo phenomenon. In simulations, Jacoutot et al. (2008) showed that highfrequency turbulent convective motions, in the presence of moderate magnetic fields, may enhance the local acoustic emission. "
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    ABSTRACT: The use of acoustic holography in the high-frequency $p$-mode spectrum can resolve the source distributions of enhanced acoustic emissions within halo structures surrounding active regions. In doing so, statistical methods can then be applied to ascertain relationships with the magnetic field. This is the focus of this study. The mechanism responsible for the detected enhancement of acoustic sources around solar active regions has not yet been explained. Furthermore the relationship between the magnetic field and enhanced acoustic emission has not yet been comprehensively examined. We have used vector magnetograms from the \Helioseismic and Magnetic Imager (HMI) on-board the Solar Dynamics Observatory (SDO) to image the magnetic-field properties in the halo. We have studied the acoustic morphology of an active region, with a complex halo and "glories," and we have linked some acoustic properties to the magnetic-field configuration. In particular, we find that acoustic sources are significantly enhanced in regions of intermediate field strength with inclinations no different from the distributions found in the quiet Sun. Additionally we have identified a transition region between the active region and the halo, in which the acoustic source power is hindered by inclined fields of intermediate field strength. Finally, we have compared the results of acoustic emission maps, calculated from holography, and the commonly used local acoustic maps, finding that the two types of maps have similar properties with respect to the magnetic field but lack spatial correlation when examining the highest-powered regions.
    Full-text · Article · Jul 2015 · Solar Physics
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    ABSTRACT: The cause of enhanced acoustic power surrounding active regions, the acoustic halo, is not as yet understood. We explore the properties of the enhanced acoustic power observed near disk center from 21 to 27 January 2002, including AR 9787. We find that (i) there exists a strong correlation of the enhanced high frequency power with magnetic-field inclination, with greater power in more horizontal fields, (ii) the frequency of the maximum enhancement increases along with magnetic field strength, and (iii) the oscillations contributing to the halos show modal ridges which are shifted to higher wavenumber at constant frequency in comparison to the ridges of modes in the quiet-Sun.
    Full-text · Article · Nov 2009 · Solar Physics
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