Comparison of SOHO UVCS and MLSO MK4 coronameter densities

Abstract

We have compared the density distributions of solar corona obtained by SOHO Ultraviolet Coronagraph Spectrometer (UVCS) and Mauna Loa Solar Observatory (MLSO) MK4 coronameter. This is the first attempt to compare the coronal densities estimated by the two instruments. In the spectral data of UVCS, we have selected two emission lines (O VI 1032 A and 1037.6 A), which have both radiative and collisional components. The coronal number density is determined from the ratio of these two components. The MK4 coronameter has a field of view ranging from 1.08 to 2.85 solar radii. The coronal density can be determined by inverting MLSO MK4 polarization maps. We find that the mean electron number density in a helmet streamer observed by MK4 on 2003 April 28 is fairly consistent with that observed by UVCS. For a coronal hole and an active region observed on 1999 October 19 and 24, the MK4 coronal densities are close to those from the UVCS within a factor of two; the former values are twice the latter at 1.7 solar radii and closer to the latter at higher altitudes. Our results demonstrate that MK4 polarization data can provide us with a coronal density distribution in a large field of view with a time cadence of about three minutes. We suggest that the MK4 data can be used to derive 2-D density distributions of coronal structures and further to estimate the heights of CME-associated type II shocks.