SOHO/CDS observations of waves above the network

Abstract

We analyze temporal variations in the intensities and the Doppler shifts of He i 584.33 A (chromosphere), O v 629.73 A (transition region), and Mg ix 368.07 A (corona) measured in and above chromospheric network near disk center with the Coronal Diagnostic Spectrometer (CDS) onboard the Solar and Heliospheric Observatory (SOHO). There is significant correlation between the He i and O v modulations, with O v intensity leading He i intensity by 27.3 s ± 4.6 s but no significant time shift in the Doppler shift. Cross-correlation between the O v and Mg ix intensities reveals multiple maxima without correlation between their Doppler shifts. Wavelet power analysis gives evidence of intermittent chromospheric and transition-region oscillations with periodicities in the 250−450 s range and of coronal oscillations in the 110−300 s range. Wavelet phase difference analysis shows that the determined time shift between variations of the He i and O v intensities is dominated by waves with about 300 s periodicity. We interpret these results as giving evidence of compressive waves that propagate downward from the transition region to the chromosphere in the particular chromospheric network. We discuss different scenarios regarding origin and source localization of waves, and we speculate on their role in coronal heating above chromospheric network.