Abstract

Since the discovery of quasi-periodic oscillations with periods of order 3-10 minutes in coronal loops with TRACE and EIT (and later with EUVI and EIS), these oscillations have mostly been interpreted as evidence for propagating slow-mode magnetoacoustic waves in a low plasma beta environment ori... Show moreSince the discovery of quasi-periodic oscillations with periods of order 3-10 minutes in coronal loops with TRACE and EIT (and later with EUVI and EIS), these oscillations have mostly been interpreted as evidence for propagating slow-mode magnetoacoustic waves in a low plasma beta environment originating, most-likely, in the chromosphere. We show that this interpretation is not unique, and that at least for plage-related coronal loops, it may not be the most likely cause for the observed quasi-periodicities. We use Monte Carlo simulations to show that current oscillation detection methods based on wavelet analysis, and wave tracking cannot distinguish the quasi-periodic signals of such waves in coronal imaging timeseries with those caused by the faint signal from upflows at 50-150 km/s that have lifetimes of order 1-2 minutes and that occur randomly in time and occur on granular timescales. Such upflows were recently discovered as blueward line asymmetries with EIS and have been linked to chromospheric, spicular upflows that are rapidly heated to coronal temperatures. We use EIS and SUMER spectra to show that these faint upflows at the footpoints of coronal loops sometimes occur quasi-periodically on timescales of order 5-15 minutes. Finally, we show that recent EIS measurements of intensity and velocity oscillations, that have been interpreted as direct evidence for propagating waves, are fully compatible with a scenario in which faint upflows at high speed occur quasi-periodically. We show evidence from spectral line asymmetry analysis that support this scenario. We suggest that a significant fraction of the quasi-periodicities observed with coronal imagers and spectrographs that have previously been interpreted as propagating magnetoacoustic waves, may instead be caused by these upflows. The uncertainty in the identification of the physical cause for coronal oscillations significantly impacts the prospects of successful coronal seismology using propagating, slow-mode magneto-acoustic waves. Show less