Two components of the solar coronal emission revealed by Extreme-ultraviolet spectroscopic observations

Recent spectroscopic observations have revealed the ubiquitous presence of blueward asymmetries of emission lines formed in the solar corona and transition region. These asymmetries are most prominent in loop footpoint regions, where a clear correlation of the asymmetry with the Doppler shift and line width determined from the single-Gaussian fit is found. Such asymmetries suggest at least two emission components: a primary component accounting for the background emission and a secondary component associated with high-speed upflows. The latter has been proposed to play a vital role in the coronal heating process and there is no agreement on its properties. Here we slightly modify the initially developed technique of red-blue (RB) asymmetry analysis and apply it to both artificial spectra and spectra observed by the Extreme-ultraviolet Imaging Spectrometer on board Hinode, and demonstrate that the secondary component usually contributes a few percent of the total emission, and has a velocity ranging from 50 to 150kms^-1 and a Gaussian width comparable to that of the primary one in loop footpoint regions. The results of the RB asymmetry analysis are then used to guide a double-Gaussian fit and we find that the obtained properties of the secondary component are generally consistent with those obtained from the RB asymmetry analysis. Through a comparison of the location, relative intensity, and velocity distribution of the blueward secondary component with the properties of the upward propagating disturbances revealed in simultaneous images from the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory, we find a clear association of the secondary component with the propagating disturbances.