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The propagation and leakage of acoustic waves is studied in a 2-dimensional realisticmodel of the lower solar atmosphere, with temperature, pressure and density profiles based on the VALc model. The higher atmosphere, on the other hand, is the McWhirter atmospheric model. Acoustic waves, mainly identified by solar global oscillations manifest at photospheric heights. Their leakage into the lower atmosphere is approximated by a sinusoidal point velocity driver at a range of realistic driver periods measured at photospheric heights, positioned just above the temperature minimum in the photosphere. Convective instability may thus be ignored.
The excited high-frequency waves are seen to propagate through the lower atmosphere to the transition region, and dependant on the wave period are transmitted into the lower corona. It was found that for periods close to the lower atmospheric resonant cavity period reflection from the transition region and trapping in the cavity formed right below the transition region is manifested in the form of chromospheric standing waves. We urge observers to justify these standing waves in the region between the photosphere and transition regions in space or ground-based data.
Further, it is observed in the simulations that waves driven below the cut-off period propagate through into the higher atmosphere with only a slight reflected component. Waves driven at a higher period, in contrast, are largely trapped in the lower atmosphere, with some leakage through the transition region.
Time distance and power spectrum analysis are applied to the model data, and for specific drivers of around 5 minutes we see clear evidence of standing waves being setup in the lower atmospheric cavity, and the formation of surface waves travelling outwards along the transition region.
When the lower atmospheric magnetic canopy is also considered, global oscillations can resonantly interact at a much wider range of frequencies as opposed to quiet Sun regions. The properties of this interaction allow us to carry out local magneto-seismology, i.e. to derive diagnistic information about the chromospheric magnetic field. This technique can be further used to improve the missing details of wave leakage, spicule and chromospheric jet formation. |
