The interfringe intensity in coronal interferograms

The paper discusses the low contrast of the Fabry-Perot (FP) fringes observed in the interferograms of the solar corona. The various factors which affect the interfringe intensity of Fabry-Pérot line profiles are considered. The effect of continuum radiation on the interfringe intensity of Fabry-Pérot line profiles is calculated by assuming a coronal model. For the given FP, it was found that unless the ratio of line to continuum intensity (integrated line intensity to continuum per angström) is less than 10, continuum contribution has no significant effect on the fringe contrast. The analysis may be pertinent in the design aspects of a FP interferometer for astronomical observations. The excess interfringe intensity found in the coronal line profiles is interpreted as due to the presence of fast moving plasma components in corona along the line of sight. A detailed analysis qualitatively shows that the line excitation conditions existing in ambient corona and coronal loops are different in active regions while they are more similar in polar regions.