Effect of Magnetic Fields on Wave Propagation in the Solar Atmosphere
"We studied 1 - 24 mHz intensity fluctuations in 3-hour sequences of high-cadence, high-resolution, broad-band filtergram images taken by the SOT-FG on board the Hinode spacecraft. The observations consist of near simultaneous, co-registered G-band (GB), Ca II H-line (HL) intensity images, and line-of-sight magnetic images calibrated to Gauss with MDI magnetograms. GB is typically used as a proxy for magnetic fields while HL is a diagnostic of chromospheric heating. We estimate the height z1 of the ""magnetic canopy,” where magnetic and gas pressures balance, using potential field extrapolation and the Fontenla 2006 model atmosphere. When z1 is above the height of formation of both signals, the coherence of the GB and HL oscillations is strong for frequencies between 3 mHz and 6 mHz and maximal near 5 mHz, around the acoustic cutoff frequency. Near 3 mHz there is no time delay between the HL and GB signals indicating a pure standing wave. Above 7 mHz the time delay settles near 20 sec indicating an upward propagating acoustic wave. When z1 is below the GB and HL heights of formation the coherence between the signals drops and has a maximum near 4 mHz. The time delay remains zero at 3 mHz, but above 7 mHz it is less than 20 sec and decreases with frequency, suggesting that the original acoustic fluctuations have undergone mode conversion. A similar analysis of ground-based data acquired with the Rapid Oscillations in the Solar Atmosphere (ROSA) instrument at the Dunn Solar Telescope finds longer time delays of 34 sec when z1 is high and 21 sec when z1 is low. This is consistent with the greater effective formation height of the ROSA narrowband (1 Å) Ca II K-line core compared to the more broadband (3 Å) H-line in the Hinode data."