Origin of the chromospheric three-minute oscillations in sunspot umbrae

Abstract

Context. Sunspot umbrae show a change in the dominant period of their oscillations from five minutes (3.3 mHz) in the photosphere to three minutes (5.5 mHz) in the chromosphere. Aims. In this paper, we explore the two most popular models proposed to explain the three-minute oscillations: the chromospheric acoustic resonator and the propagation of waves with frequency above the cutoff value directly from lower layers. Methods. We employ numerical simulations of wave propagation from the solar interior to the corona. Waves are driven by a piston at the bottom boundary. We have performed a parametric study of the measured chromospheric power spectra in a large number of numerical simulations with differences in the driving method, the height of the transition region (or absence of transition region), the strength of the vertical magnetic field, and the value of the radiative cooling time.Results. We find that both mechanisms require the presence of waves with period in the three-minute band at the photosphere. These waves propagate upward and their amplitude increases due to the drop of the density. Their amplification is stronger than that of evanescent low-frequency waves. This effect is enough to explain the dominant period observed in chromospheric spectral lines. However, waves are partially trapped between the photosphere and the transition region, forming an acoustic resonator. This chromospheric resonant cavity strongly enhances the power in the three-minute band. Conclusions. The chromospheric acoustic resonator model and the propagation of waves in the three-minute band directly from the photosphere can explain the observed chromospheric three-minute oscillations. They are both important in different scenarios. Resonances are produced by waves trapped between the temperature minimum and the transition region. Strong magnetic fields and radiative losses remove energy from the waves inside the cavity, resulting in weaker amplitude resonances.

Linear Acoustic Waves in a Nonisothermal Atmosphere. II. Photospheric Resonator Model of Three-minute Umbral Oscillations

“…This led Khomenko & Collados (2015) to conclude that there is no strong observational support for an umbral resonance cavity model of any kind. By employing numerical simulations of wave propagation, Felipe (2019) investigated the propagating wave model and the chromospheric resonator model together, and concluded that the propagating wave model can explain the dominant period of the three-minute oscillations even without the chromospheric resonator, but the chromospheric resonator strongly enhances the power of the three-minute oscillations if it exists.…”

Oscillations observed in umbra, plage, quiet-Sun and the polarity inversion line of active region 11158 using Helioseismic Magnetic Imager/Solar Dynamics Observatory data

“…The umbral 3 min waves observed in the chromosphere were shown to be already present in the photosphere in a study by [ 28 ]. Numerical simulations [ 56 ] aided in the conclusion that propagation of waves in the 3 min band directly from the photosphere can explain the observed chromospheric 3 min oscillations. So it is not shocking to see the 3 min power, the surprising part is that it appears in the magnetogram data.…”

Signatures of sunspot oscillations and the case for chromospheric resonances

“…Their power is reduced with frequency as a result of the rapid decrease with frequency of the incident wave power from subphotospheric layers5 . Several numerical simulations of chromospheric resonances above sunspot umbrae also show no trace of a power increase at high frequencies6,8 . On the contrary, the paper by Jess et al identifies this power increase as a signature of the chromospheric…”

“…They show an enhanced power and a shorter dominant period, from waves with an amplitude of a few hundred meters per second in the five-minute band at the photosphere, to amplitudes of several kilometers per second in the three-minute band at the chromosphere. 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.…”