Observational Evidence for Variations of the Acoustic Cutoff Frequency With Height in the Solar Atmosphere

Abstract

Direct evidence for the existence of an acoustic cutoff frequency in the solar atmosphere is given by observations performed by using the HELioseismological Large Regions Interferometric DEvice operating on the Vacuum Tower Telescope located on Tenerife. The observational results demonstrate variations of the cutoff with atmospheric heights. The observed variations of the cutoff are compared to theoretical predictions made by using five acoustic cutoff frequencies that have been commonly used in helioseismology and asteroseismology. The comparison shows that none of the theoretical predictions is fully consistent with the observational data. The implication of this finding is far reaching as it urgently requires either major revisions of the existing methods of finding acoustic cutoff frequencies or developing new methods that would much better account for the physical picture underlying the concept of cutoff frequencies in inhomogeneous media.

The Frequency-dependent Damping of Slow Magnetoacoustic Waves in a Sunspot Umbral Atmosphere

“…This is perhaps a consequence of the variation of acoustic cutoff frequency with height (Murawski et al 2016;Wiśniewska et al 2016). Wiśniewska et al (2016) performed observations of the quiet solar atmosphere using multiple spectral lines and demonstrated that the acoustic cutoff frequency initially increases with height, before levelling off at greater atmospheric heights.…”

“…This is perhaps a consequence of the variation of acoustic cutoff frequency with height (Murawski et al 2016;Wiśniewska et al 2016). Wiśniewska et al (2016) performed observations of the quiet solar atmosphere using multiple spectral lines and demonstrated that the acoustic cutoff frequency initially increases with height, before levelling off at greater atmospheric heights. In addition, the presence of harmonic peaks at 6.57 and 13.1 mHz (152 and 76 s; see Figure 6) in the Ca II K and IRIS2796 Å channels may support the existence of a resonant cavity, possibly below the photosphere (Thomas & Scheuer 1982), rather than in the chromosphere (Zhugzhda et al 1983), since the oscillations are found to be predominantly upwardly propagating at chromospheric heights.…”

Observed Local Dispersion Relations for Magnetoacoustic-gravity Waves in the Sun’s Atmosphere: Mapping the Acoustic Cutoff Frequency

“…Although the cut-off frequency is expected to be a locally defined quantity (Chae & Litvinenko 2018), only its variation with height has received any attention (Felipe et al 2018;Murawski et al 2016;Wiśniewska et al 2016). To the best of our knowledge, no one has examined how the cut-off frequency varies spatially in latitude and longitude.…”

Nonlinear Effects in Three-minute Oscillations of the Solar Chromosphere. I. An Analytical Nonlinear Solution and Detection of the Second Harmonic

“…It is generally accepted that these three-minute (3 minute) oscillations indicate the presence of vertically propagating acoustic waves with frequencies slightly above the acoustic cutoff 0 w in a gravitationally stratified medium (e.g., Felipe et al 2010;Chae & Goode 2015;Wiśniewska et al 2016). The key physical idea, originally formulated by Fleck & Schmitz (1991) and Kalkofen et al (1994), is that the waves constitute a response of the solar atmosphere to a general velocity disturbance at its base.…”