Diagnostics of a coronal hole and the adjacent quiet sun by the hinode/EUV Imaging Spectrometer (EIS)

Abstract

A comparison between a coronal hole (CH) and the adjacent quiet Sun (QS) has been performed using the spectroscopic diagnostics of the Hinode/EUV Imaging Spectrometer (EIS). Based on spectral signatures, we identify the coronal funnels that play an important role in the formation and propagation of the nascent fast solar wind. From the observed line profiles, we estimate the intensity, Doppler velocity, line width (FWHM), and electron density over CH and the adjacent QS region of a north polar coronal hole (NPCH). Excesswidth regions (excess FWHM above a threshold level) have been identified in QS and CH. The plasma-flow inversion (average red-shifts changing to blue-shifts at a specific height) in CH and excess-width regions of QS take place at ≈5.01 × 105 K. Furthermore, the high electron-density concentration in excess-width regions of QS provides an indication that these regions are the footprints of coronal funnels. We also find that non-thermal velocities of CH are higher in comparison to QS indicating that the CHs are the source regions of the fast solar wind. Doppler and non-thermal velocities, as recorded by different-temperature lines, have also been compared with previously published results. As we go from lower to the upper solar atmosphere, down flows are dominant in the lower atmosphere, while coronal lines are dominated by up flows with a maximum value of ≈ 10 – 12 km s−1 in QS. The non-thermal velocity increases first but after log Te = 5.47 it decreases further in QS.This trend can be interpreted as a signature of the dissipation of Alfven waves, while an increasing trend reported earlier may be attributed to the signature of the growth of Alfven waves at lower heights. The predominance of the occurrence of nano-flares around the O VI formation temperature could also explain the non-thermal-velocity trend.