A chromospheric resonance cavity in a sunspot mapped with seismology

Abstract

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

An overall view of temperature oscillations in the solar chromosphere with ALMA

“…Moreover, longer-period internal-gravity waves, as well as various types of magnetohydrodynamic (MHD) waves of different periods and properties, have been observed throughout the chromosphere at all spatial scales, e.g. [39][40][41][42][43][44][45]. Such magneto-acoustic-gravity (MAG) waves are often considered as a prime means of transporting energy through the solar atmosphere, thus, contributing to the excess heating of the solar chromosphere and beyond, where they release their energy [46].…”

The dynamics of 3-min wavefronts and their relation to sunspot magnetic fields

“…In the central part of the umbra, the footpoints of thin magnetic tubes appear in the form of oscillating cells and dots [9]. One of the main physical mechanisms that determines the oscillatory power in the umbral core is related to the resonant cavity trapped between the photosphere and chromosphere [10,11]. Increasing angles of magnetic field inclination towards the sunspot periphery result in direct changes to the associated cutoff frequency [1].…”

Signatures of sunspot oscillations and the case for chromospheric resonances

“…Various models have been proposed to explain this behaviour [1][2][3] , including the presence of a chromospheric resonance cavity between the photosphere and the transition region [4][5][6][7][8] . Jess et al 9 claimed the detection of observational evidence supporting this model, obtained from the comparison of spectropolarimetric observations and numerical simulations. Here, it is shown that the observational insight reported by Jess et al is not a common property of sunspots.…”

“…In both panels, the red lines correspond to a simulation where the driver of the waves exhibits realistic amplitudes, whereas the simulation illustrated with the blue lines employed the same driver with a reduced amplitude in order to keep the simulations in the linear regime. Jess et al 9 presented an extraordinary data set of wave propagation in sunspot umbrae. They reported for the first time a striking chromospheric power enhancement at around 20 mHz.…”