Velocity waves in the quiet solar chromosphere

Abstract

Propagation of velocity waves are investigated in the solar chromosphere, with a special view to high frequencies (periods ≥ 60 s). Four line profiles have been observed during 27 mn with the Sacramento Peak vacuum telescope (Hα, 3933, 8498 and 8542 Ca ii). Three Fourier analysis are performed according to the location in the cells of the chromospheric network. Phase-shifts and amplitude ratios between the line Doppler shifts are computed as functions of frequency. The “pollution” of high frequency results by energetic low frequency oscillations is investigated.

Hα Doppler shifts are probably affected by the large width of line formation layers (low “transfer function’). Using “formation altitudes for Doppler shifts” previously computed for the infra-red lines, we show that acoustic waves propagating upwards cannot account for the observations. In particular, the phase-shifts between oscillations in different chromospheric layers are much smaller than theoretical predictions. As a first attempt for a qualitative agreement, we suggest that most of the high frequency oscillations (10–15 mHz) are magnetoacoustic waves, travelling in layers where the gradient of the Alfvén-speed cannot be neglected, and reflected at the top of the chromosphere. The amplitudes of these waves are probably underestimated as derived from the observed Doppler shifts.