Fourier analysis of active-region plage

Abstract

We study the dynamical interaction of the solar chromosphere with the transition region in mossy and non-mossy active-region plage. We carefully align image sequences taken with the Transition Region And Coronal Explorer (TRACE) in the ultraviolet passbands around 1550, 1600, and 1700 A and the extreme ultraviolet passbands at 171 and 195 A. We compute Fourier phase-difference spectra that are spatially averaged separately over mossy and non-mossy plage to study temporal modulations as a function of temporal frequency. The 1550 versus 171 A comparison shows zero phase difference in non-mossy plage. In mossy plage, the phase differences between all UV and EUV passbands show pronounced upward trends with increasing frequency, which abruptly changes into zero phase difference beyond 4-6 mHz. The phase difference between the 171 and 195 A sequences exhibits a shallow dip below 3 mHz and then also turns to zero phase difference beyond this value. We attribute the various similarities between the UV and EUV diagnostics that are evident in the phase-difference diagrams to the contribution of the C IV resonance lines in the 1550 and 1600 A passbands. The strong upward trend at the lower frequencies indicates the presence of upward-traveling disturbances. It points to correspondence between the lower chromosphere and the upper transition region, perhaps by slow-mode magnetosonic disturbances, or by a connection between chromospheric and coronal heating mechanisms. The transition from this upward trend to zero phase difference at higher frequencies is due to the intermittent obscuration by fibrils that occult the foot points of hot loops, which are bright in the EUV and C IV lines, in oscillatory manner.