Path-lengths in the quiet Sun transition region using O iv lines from IRIS

Abstract

ABSTRACT We use spectroscopic diagnostics to determine different plasma parameters in the transition region of the quiet Sun. The electron number density from O iv lines using high-resolution spectral observations from the Interface Region Imaging Spectrograph (IRIS) are estimated to be around 1010 cm−3, in agreement with literature values and our re-analysis of earlier observations from HRTS and SUMER. We also find a large scatter in the densities obtained from IRIS and SUMER. We use these densities to estimate the size of the emitting region (the path-length) using an isothermal assumption. We find evidence for some centre-to-limb variations, with median values of path-lengths for disc centre and near the limb from IRIS O iv being 10 and 31 km, respectively. These values are close to those obtained from HRTS. We also use new atomic models for the quiet Sun, which include electron density effects, photoionization, and charge transfer, to find path-lengths from Si iv and C iv in close agreement with those from O iv. Finally, we estimate the emission measure from HRTS data to show that the isothermal assumption overestimates the path-lengths by factors of 5–8. Such small path-lengths should be taken into account when modelling the transition region for the interpretation of observations from e.g. IRIS and the Solar Orbiter SPICE spectrometer.