High-speed transition region and coronal upflows in the quiet sun

We study the line profiles of a range of transition region (TR) emission lines observed in typical quiet-Sun regions. In magnetic network regions, the Si IV 1402 Å, C IV 1548 Å, N V 1238 Å, O VI 1031 Å, and Ne VIII 770 Å spectral lines show significant asymmetry in the blue wing of the emission line profiles. We interpret these high-velocity upflows in the lower and upper TR as the quiet-Sun equivalent of the recently discovered upflows in the low corona above plage regions. The latter have been shown to be directly associated with high-velocity chromospheric spicules that are (partially) heated to coronal temperatures and play a significant role in supplying the active region corona with hot plasma. We show that a similar process likely dominates the quiet-Sun network. We provide a new interpretation of the observed quiet-Sun TR emission in terms of the relentless mass transport between the chromosphere and corona—a mixture of emission from dynamic episodic heating and mass injection into the corona as well as that from the previously filled, slowly cooling, coronal plasma. Analysis of the observed upflow component shows that it carries enough hot plasma to play a significant role in the energy and mass balance of the quiet corona. We determine the temperature dependence of the upflow velocities to constrain the acceleration and heating mechanism that drives these upflows. We also show that the temporal characteristics of these upflows suggest an episodic driver that sometimes leads to quasi-periodic signals. We suggest that at least some 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.