Evidence in support of the "nanoflare" picture of coronal heating from SUMER data

We study high signal-to-noise profiles of O IV emission lines obtained using the SUMER instrument on SOHO. Data for the quiet Sun obtained close to the disk center and at the solar limb were acquired. After careful data processing in which disk data were analyzed differentially against limb data, we find a systematic correlation between a density-sensitive emission-line ratio and Doppler shift across the same emission-line profiles. While unidentified blended lines cannot be completely discounted, the data suggest that the effects of such blends are small. Based on theoretical results in an earlier paper, we argue that if wave motions are responsible for the observed behavior, then the data reveal evidence for compressive waves propagating downward from the corona to the chromosphere. This analysis naturally lends support to the dominance of the "nanoflare" mechanism for coronal heating over other theories that invoke upward wave propagation, but other mechanisms capable of generating downward-propagating waves cannot be discounted. If, instead, steady flows are the cause of the observed behavior, such as return flows from spicules, then they must be such that the density is higher in the downflowing plasma. While these particular data do not allow us to discriminate between waves or steady flows, additional data from SOHO should be able to address this problem. This work required and achieved very accurate wavelength calibrations (better than 1/5 of a pixel on the detectors), taking SUMER close to its observational capabilities. We therefore present the elements of the analysis and calibration of SUMER data that may be of interest to other users.