QUASI-PERIODIC PROPAGATING SIGNALS IN THE SOLAR CORONA: THE SIGNATURE OF MAGNETOACOUSTIC WAVES OR HIGH-VELOCITY UPFLOWS?

Since the discovery of quasi-periodic propagating oscillations with periods of order 3-10 minutes in coronal loops with TRACE and SOHO/EIT (and later with STEREO/EUVI and Hinode/EIS), they have been almost universally interpreted as evidence for propagating slow-mode magnetoacoustic waves in the low plasma beta coronal environment. Here we show that this interpretation is not unique, and that for coronal loops associated with plage regions (as opposed to sunspots), the presence of magnetoacoustic waves may not be the only cause for the observed quasi-periodicities. We focus instead on the ubiquitous, faint upflows at 50-150 km s(-1) that were recently discovered as blueward asymmetries of spectral line profiles in footpoint regions of coronal loops, and as faint disturbances propagating along coronal loops in EUV/X-ray imaging time series. These faint upflows are most likely driven from below and have been associated with chromospheric jets that are (partially) rapidly heated to coronal temperatures at low heights. These two scenarios (waves versus flows) are difficult to differentiate using only imaging data, but careful analysis of spectral line profiles indicates that faint upflows are likely responsible for some of the observed quasi-periodic oscillatory signals in the corona. We show that recent EIS measurements of intensity and velocity oscillations of coronal lines (which had previously been interpreted as direct evidence for propagating waves) are actually accompanied by significant oscillations in the line width that are driven by a quasi-periodically varying component of emission in the blue wing of the line. This faint additional component of blue-shifted emission quasi-periodically modulates the peak intensity and line centroid of a single Gaussian fit to the spectral profile with the same small amplitudes (respectively a few percent of background intensity and a few km s(-1)) that were previously used to infer the presence of slow-mode magnetoacoustic waves. Our results indicate that it is possible that a significant fraction of the quasi-periodicities observed with coronal imagers and spectrographs that have previously been interpreted as propagating magnetoacoustic waves are instead caused by these upflows. The different physical cause for coronal oscillations would significantly impact the prospects of successful coronal seismology using propagating disturbances in coronal loops.