Magnetic field reconstruction based on sunspot oscillations

Löhner-Böttcher, Johannes; González, N. Bello; Schmidt, Wolfgang

doi:10.1002/asna.201612430

Cited by 10 publications

(6 citation statements)

References 19 publications

(18 reference statements)

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“…3 off frequency to higher values 37 . A similar phenomenon has also been observed in IBIS spectral imaging observations of umbral oscillations 39 , and reiterates the appropriateness of defining the start of region II at 6 mHz.…”

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confidence: 81%

A chromospheric resonance cavity in a sunspot mapped with seismology

et al. 2019

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Sunspots are intense collections of magnetic fields that pierce through the Sun's photosphere, with their signatures extending upwards into the outermost extremities of the solar corona 1. Cutting-edge observations and simulations are providing insights into the underlying wave generation 2 , configuration 3, 4 , and damping 5 mechanisms found in sunspot atmospheres. However, the in-situ amplification of magnetohydrodynamic waves 6 , rising from a few hundreds of m/s in the photosphere to several km/s in the chromosphere 7 , has, until now, proved difficult to explain. Theory predicts that the enhanced umbral wave power found at chromospheric heights may come from the existence of an acoustic resonator 8-10 , which is created due to the substantial temperature gradients experienced at photospheric and transition region heights 11. Here we provide strong observational evidence of a resonance cavity existing above a highly magnetic sunspot. Through a combination of spectropolarimetric inversions and comparisons with high-resolution numerical simulations, we provide a new seismological approach to map the geometry of the inherent temperature stratifications across the diameter of the underlying sunspot, with the upper boundaries of the chromosphere ranging between 1300 ± 200 km and 2300 ± 250 km. Our findings will allow the three-dimensional structure of solar active regions to be conclusively determined from relatively commonplace two-dimensional Fourier power spectra. The techniques presented are also readily suitable for investigating temperature-dependent resonance effects in other areas of astrophysics, in

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confidence: 81%

A chromospheric resonance cavity in a sunspot mapped with seismology

et al. 2019

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“…However, their cutoff value is reduced by a factor cos θ, where θ is the field inclination from the solar vertical, due to the reduced gravity along the magnetic field (Bel & Leroy 1977;Jefferies et al 2006). This ramp effect allows the detection of low-frequency compressive waves in the chromosphere and corona of regions with strong magnetic fields (Orrall 1966;Giovanelli et al 1978;Lites et al 1993;De Moortel et al 2002;De Pontieu et al 2004;Vecchio et al 2007;Bloomfield et al 2007), and has been employed to reconstruct the magnetic field inclination in sunspots (Tziotziou et al 2006;Yuan et al 2014;Löhner-Böttcher et al 2016). An alternative process that leads to a reduction of the cutoff frequency is the radiative energy losses (Roberts 1983;Centeno et al 2006;Khomenko et al 2008), although the numerical simulations from Heggland et al (2011) indicate that field inclination is more relevant than radiation to explain long-period wave propagation.…”

Section: Introductionmentioning

confidence: 99%

Height variation of the cutoff frequency in a sunspot umbra

Felipe

Kuckein

Thaler

2018

A&A

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Context. In the solar atmosphere, the acoustic cutoff frequency is a local quantity which depends on the atmospheric height. It separates the low-frequency evanescent waves from the high-frequency propagating waves. Aims. We measure the cutoff frequency of slow magnetoacoustic waves at various heights of a sunspot umbra and compare the results with the estimations from several analytical formulae. Methods. We analyzed the oscillations in the umbra of a sunspot belonging to active region NOAA 12662 observed in the 10830 Å spectral region with the GREGOR Infrared Spectrograph and in the Fe i 5435 Å line with the GREGOR Fabry-Pérot Interferometer. Both instrumets are attached to the GREGOR telescope at the Observatorio del Teide, Tenerife, Spain. We have computed the phase and amplification spectra between the velocity measured from different pairs of lines that sample various heights of the solar atmosphere. The cutoff frequency and its height variation have been estimated from the inspection of the spectra. Results. At the deep umbral photosphere the cutoff frequency is around 5 mHz and it increases to 6 mHz at higher photospheric layers. At the chromosphere the cutoff is ∼ 3.1 mHz. The comparison of the observationally determined cutoff with the theoretically predicted values reveals an agreement in the general trend and a reasonable match at the chromosphere, but also significant quantitative differences at the photosphere. Conclusions. Our analyses show strong evidence of the variation of the cutoff frequency with height in a sunspot umbra, which is not fully accounted for by current analytical estimations. This result has implications for our understanding of wave propagation, the seismology of active regions, and the evaluation of heating mechanisms based on compressible waves.

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“…The amplitude of the high frequency waves (with frequency above the cutoff value, like waves in the 3 minute band) increases with height due to the drop of the density, and they dominate over the evanescent 5 minute oscillations. This model has been used to explain the chromospheric running penumbral waves (Bloomfield et al 2007;Madsen et al 2015), to determine the formation height of several spectral lines (Felipe et al 2010), to estimate the energy contribution of those waves to the chromospheric heating (Felipe et al 2011;Kanoh et al 2016), and to reconstruct the magnetic field inclination from measurements of the cutoff frequency (Yuan et al 2014;Löhner-Böttcher et al 2016), among other studies. The source of these waves can be located inside the sunspot, through magnetoconvection occurring in light bridges and umbral dots (Chae et al 2017), or they can be externally driven by p-modes (Krishna Prasad et al 2015).…”

Section: Introductionmentioning

confidence: 99%

Spiral-shaped wavefronts in a sunspot umbra

Felipe

Kuckein

Khomenko

et al. 2019

A&A

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Context. Solar active regions show a wide variety of oscillatory phenomena. The presence of the magnetic field leads to the appearance of several wave modes, whose behavior is determined by the sunspot thermal and magnetic structure. Aims. We aim to study the relation between the umbral and penumbral waves observed at the high photosphere and the magnetic field topology of the sunspot. Methods. Observations of the sunspot in active region NOAA 12662 obtained with the GREGOR telescope (Observatorio del Teide, Tenerife, Spain) were acquired on 2017 June 17. The data set includes a temporal series in the Fe i 5435 Å line obtained with the imaging spectrograph GREGOR Fabry-Pérot Interferometer (GFPI) and a spectropolarimetric raster map acquired with the GREGOR Infrared Spectrograph (GRIS) in the 10830 Å spectral region. The Doppler velocity deduced from the restored Fe i 5435 Å line has been determined, and the magnetic field vector of the sunspot has been inferred from spectropolarimetric inversions of the Ca i 10839 Å and the Si i 10827 Å lines. Results. A two-armed spiral wavefront has been identified in the evolution of the two-dimensional velocity maps from the Fe i 5435 Å line. The wavefronts initially move counterclockwise in the interior of the umbra, and develop into radially outward propagating running penumbral waves when they reach the umbra-penumbra boundary. The horizontal propagation of the wavefronts approximately follows the direction of the magnetic field, which shows changes in the magnetic twist with height and horizontal position. Conclusions. The spiral wavefronts are interpreted as the visual pattern of slow magnetoacoustic waves which propagate upward along magnetic field lines. Their apparent horizontal propagation is due to their sequential arrival to different horizontal positions at the formation height of the Fe i 5435 Å line, as given by the inclination and orientation of the magnetic field.

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Magnetic field reconstruction based on sunspot oscillations

Cited by 10 publications

References 19 publications

A chromospheric resonance cavity in a sunspot mapped with seismology

A chromospheric resonance cavity in a sunspot mapped with seismology

Height variation of the cutoff frequency in a sunspot umbra

Spiral-shaped wavefronts in a sunspot umbra

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