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Helioseismic Detection of Emerging Magnetic Flux  

Stathis Ilonidis   Submitted: 2012-03-12 22:11

Investigating the properties of magnetic flux emergence is one of the most important problems of solar physics. In this study we present a newly developed deep-focus time-distance measurement scheme which is able to detect strong emerging flux events in the deep solar interior, before the flux becomes visible on the surface. We discuss in detail the differences between our method and previous methods, and demonstrate step-by-step how the signal-to-noise (S/N) ratio is increased. The method is based on detection of perturbations in acoustic phase travel times determined from cross-covariances of solar oscillations observed on the surface. We detect strong acoustic travel-time reductions of an order of 12 - 16 seconds at a depth of 42 - 75 Mm. These acoustic anomalies are detected 1 - 2 days before high peaks in the photospheric magnetic flux rate implying that the average emerging speed is 0.3 - 0.6 km s-1. The results of this work contribute to our understanding of solar magnetism and benefit space weather forecasting.

Authors: Stathis Ilonidis, Junwei Zhao, Alexander G. Kosovichev
Projects: SoHO-MDI

Publication Status: Accepted in Proc. of the 61st Fujihara Seminar: Progress in solar/stellar physics with helio- and asteroseismology
Last Modified: 2012-03-13 13:19
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Determining Absorption, Emissivity Reduction, and Local Suppression Coefficients inside Sunspots  

Stathis Ilonidis   Submitted: 2010-10-02 20:41

The power of solar acoustic waves is reduced inside sunspots mainly due to absorption, emissivity reduction, and local suppression. The coefficients of these power-reduction mechanisms can be determined by comparing time-distance cross-covariances obtained from sunspots and from the quiet Sun. By analyzing 47 active regions observed by SOHO/MDI without using signal filters, we have determined the coefficients of surface absorption, deep absorption, emissivity reduction, and local suppression. The dissipation in the quiet Sun is derived as well. All of the cross-covariances are width corrected to offset the effect of dispersion. We find that absorption is the dominant mechanism of the power deficit in sunspots for short travel distances, but gradually drops to zero at travel distances longer than about 6 degrees. The absorption in sunspot interiors is also significant. The emissivity-reduction coefficient ranges from about 0.44 to 1.00 within the umbra and 0.29 to 0.72 in the sunspot, and accounts for only about 21.5% of the umbra's and 16.5% of the sunspot's total power reduction. Local suppression is nearly constant as a function of travel distance with values of 0.80 and 0.665 for umbrae and whole sunspots respectively, and is the major cause of the power deficit at large travel distances.

Authors: Stathis Ilonidis and Junwei Zhao
Projects: SoHO-MDI

Publication Status: Published in Solar Physics
Last Modified: 2010-10-04 18:07
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Time-Distance Solar Far-Side Imaging Using Three-Skip Acoustic Signals  

Stathis Ilonidis   Submitted: 2009-08-15 21:28

The purpose of this work is to image solar far-side active regions using acoustic signals with three skips and improve the quality of existing images. The mapping of far-side active regions was first made possible using the helioseismic holography technique by use of four-skip acoustic signals. The quality of far-side images was later improved with the combination of four- and five-skip signals using the time-distance helioseismology technique. In this work, we explore the possibility of making three-skip far-side images of active regions, and improving the image quality by combining the three-skip images with the images obtained from existing techniques. A new method of combining images is proposed that increases the signal-to-noise ratio and reduces the appearance of spurious features.

Authors: Stathis Ilonidis, Junwei Zhao and Thomas Hartlep
Projects: None

Publication Status: published in Solar Physics
Last Modified: 2009-08-16 11:01
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Abstracts by Author
Helioseismic Detection of Emerging Magnetic Flux
Determining Absorption, Emissivity Reduction, and Local Suppression Coefficients inside Sunspots
Time-Distance Solar Far-Side Imaging Using Three-Skip Acoustic Signals

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