Trapped Eigenoscillations in the Lower Solar Atmosphere: Is there a Resonant Coupling?

Magnetic coupling through MHD waves and oscillations at the solar interior - lower corona interface is studied here. First, the effect of a magnetic solar atmosphere on solar global oscillations is investigated. Frequency shifts of acoustic eigenmodes are found due to the presence of the chromospheric and coronal magnetic fields. Potential application to local magneto-seismology is highlighted. Next, the propagation and leakage of global acoustic waves is studied in a multi-dimensional realistic model of the lower solar magnetized atmosphere with temperature, pressure and density profiles based on the VAL IIIc 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 harmonic 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 region by carrying out space or ground-based high-resolution and high-cadence observations.

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. For specific drivers of around 5 minutes, clear evidence of standing waves being set up in the lower atmospheric cavity is found, and the formation of surface waves travelling outwards along the transition region is demonstrated.

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 diagnostic 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.