DeForest and Gurman (1998), using EIT 171 Å data confirmed this discovery: Outwardly propagating perturbations of the intensity were observed at distances of , gathered in quasi-periodic groups of periods, with periods of about , The projected speeds are about and the relative amplitude (in density) was about .
A similar phenomenon was observed near the footpoints of coronal loops, with EIT (Berghmans and Clette, 1999) and TRACE (Nightingale et al., 1999; De Moortel et al., 2000, 2002a,b) imagers as near-isothermal EUV intensity disturbances, which start near the loop footpoints and propagate along the loops at the apparent speed lower than the sound speed. Multi-wavelength observations have been performed by Robbrecht et al. (2001) combining TRACE 171 Å and EIT 195 Å and by King et al. (2003) with TRACE 171 Å and 195 Å. Robbrecht et al. (2001) found the projected propagation speeds to vary roughly between and for both instruments, which is close to and below the expected sound speed in the coronal loops, respectively. King et al. (2003) pointed out the high correlation of the disturbances observed in the different bandpasses. Also, recently, a coordinated observation of this phenomenon with SOHO/CDS and TRACE instruments has been carried out Marsh et al. (2003). A propagating oscillation with a period of about , observed by TRACE in the 171 Å bandpass, was also observed in He I, O V and Mg IX emission lines with CDS, corresponding to the chromospheric, transition region, and coronal temperatures, respectively. This is consistent with about oscillations observed by O’Shea et al. (2001) with CDS in both velocity and intensity time series associated with the coronal lines Mg IX and Fe XVI, as well as in O V.
A comprehensive overview of observational properties of the longitudinal oscillations, based upon the analysis of examples, is given in De Moortel et al. (2002a), and a more recent one in Nakariakov (2003). The properties of propagating EUV disturbances may be summarised as follows: the projected propagation speed is ; the amplitudes are always less than in intensity (less than in density); the disturbances are quasi-periodic with the periods about . In most cases, only upwards propagating disturbances have been detected (from the footpoints to the apex of the loop). Sometimes, the waves can be present for several consecutive hours with, more or less, constant period. It is possible that the disturbances with shorter, about , periods are situated above sunspot regions, whereas disturbances propagating along the loops which are not associated with sunspots have longer periodicity, of about (De Moortel et al., 2002c). However, King et al. (2003) showed that both and perturbations can coexist in the same coronal structure, at least in the analysed example, so the question still remains open.
The propagation direction and speed, together with the fact that the observed waves are compressible suggest their interpretation as slow magnetoacoustic waves. Slow waves of the observed periodicities (shorter than ) can propagate without reflection in the corona, as the acoustic cut-off period is about . According to this interpretation, the waves propagate at about the sound speed in the loop. The observed speed of the waves is reduced by line-of-sight projection.
Waves were also recently detected in the Doppler velocity data in the and ranges by Sakurai et al. (2002). The line intensity and line width did not show clear oscillations, but their phase relationship with the Doppler velocity indicates propagating waves rather than standing waves. These waves were interpreted as superposition of propagating slow and Alfvén waves.
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