Comparison of active region upflow and core properties using simultaneous spectroscopic observations from IRIS and Hinode

Abstract

Context. The origin of the slow solar wind is still an open issue. It has been suggested that upflows at the edge of active regions are a possible source of the plasma outflow and therefore contribute to the slow solar wind. Aims. We investigate the origin and morphology of the upflow regions and compare the upflow region and the active region core properties. Methods. We studied how the plasma properties of flux, Doppler velocity, and non-thermal velocity change throughout the solar atmosphere, from the chromosphere via the transition region to the corona in the upflow region and the core of an active region. We studied limb-to-limb observations of the active region (NOAA 12687) obtained from 14 to 25 November 2017. We analysed spectroscopic data simultaneously obtained from IRIS and Hinode/EIS in the six emission lines Mg II 2796.4Å, C II 1335.71Å, Si IV 1393.76Å, Fe XII 195.12Å, Fe XIII 202.04Å, and Fe XIV 270.52Å and 274.20Å. We studied the mutual relationships between the plasma properties for each emission line, and we compared the plasma properties between the neighbouring formation temperature lines. To find the most characteristic spectra, we classified the spectra in each wavelength using the machine learning technique k-means. Results. We find that in the upflow region the Doppler velocities of the coronal lines are strongly correlated, but the transition region and coronal lines show no correlation. However, their fluxes are strongly correlated. The upflow region has a lower density and lower temperature than the active region core. In the upflow region, the Doppler velocity and non-thermal velocity show a strong correlation in the coronal lines, but the correlation is not seen in the active region core. At the boundary between the upflow region and the active region core, the upflow region shows an increase in the coronal non-thermal velocity, the emission obtained from the DEM, and the domination of the redshifted regions in the chromosphere. Conclusions. The obtained results suggest that at least three parallel mechanisms generate the plasma upflow: (1) The reconnection between closed loops and open magnetic field lines in the lower corona or upper chromosphere; (2) the reconnection between the chromospheric small-scale loops and open magnetic field; and (3) the expansion of the magnetic field lines that allows the chromospheric plasma to escape to the solar corona.