Exploring the dynamical nature of the lower solar chromosphere

Abstract

We examine spectral time-series of two lower-chromospheric lines (N i 1319 Ä and C ii 1335 Ä) observed with the SUMER instrument on the SOHO spacecraft. We point out differences between (intensity and velocity) power spectra of network and internetwork regions and argue that the behaviour resembles that of Ca ii power spectra. No significant phase differences are found between the intensities of both lines. However, when phase spectra are averaged along the slit there is some evidence that the C ii intensity lags that of N i by 16 sec near 3 mHz. Intensity power spectra of C ii are affected at higher frequencies by streams of emitting structures. Using contrast-enhanced time slices we show that 1) there exists a grain-like pattern which is found in both network and internetwork regions; 2) streams of supersonically moving structures probably outline a wave interference pattern; 3) the sizes of structures observed in N i are smaller than when observed in C ii. At various points our findings disagree with earlier results from SUMER. A cookbook formalism is presented to derive confidence levels for power, phase, gain and coherency spectra.