Abstract
Although theoretically predicted, the simultaneous excitation of several resonant modes in sunspots has not been observed. Like any harmonic oscillator, a solar magnetic flux tube can support a variety of resonances, which constitute the natural response of the system to external forcing. Apart from a few single low order eigenmodes in small scale magnetic structures, several simultaneous resonant modes were not found in extremely large sunspots. Here we report the detection of the largest-scale coherent oscillations observed in a sunspot, with a spectrum significantly different from the Sun’s global acoustic oscillations, incorporating a superposition of many resonant wave modes. Magnetohydrodynamic numerical modeling agrees with the observations. Our findings not only demonstrate the possible excitation of coherent oscillations over spatial scales as large as 30–40 Mm in extreme magnetic flux regions in the solar atmosphere, but also paves the way for their diagnostic applications in other astrophysical contexts.
Highlights
Theoretically predicted, the simultaneous excitation of several resonant modes in sunspots has not been observed
As of apart from a few single low order eigenmodes, which were identified in small scale magnetic structures[14,15,16,17], several simultaneous resonant modes were not found at the same time in extremely large sunspots with diameters of tens of Mm
In this work we analyze a 40 Mm sized sunspot observed on May 20, 2016 and report the largest coherent oscillations observed in a sunspot, which agree with numerical modeling
Summary
The simultaneous excitation of several resonant modes in sunspots has not been observed. As of apart from a few single low order eigenmodes (i.e. sausage and kink), which were identified in small scale magnetic structures (diameters up to a few Mm)[14,15,16,17], several simultaneous resonant modes were not found at the same time in extremely large sunspots with diameters of tens of Mm. From an observational point of view, sunspots are found to be dominated by 5-m oscillation periods in the photosphere[18], which arise as a result of p-mode conversion by the magnetic field[19,20]. The results can be interpreted as a superposition of several resonant modes of the magnetic flux tube
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