Implications of a direct photometric comparison of the EUV-corona and the K-Corona

Abstract

We report results of co-temporal and co-spatial measurements of the K-coronal brightness (pB) and the intensity of the coronal emission line λ625 Mg X in the inner corona. The pB measurements were made with the Mark III K-coronameter on Mauna Loa (HAO/NCAR), and the EUV measurements were obtained on rocket flights of the LASP EUV Coronal Spectrometer in 1980 and 1983. The data consist of radial limb profiles of pB and λ625 intensity as a function of height above the limb that overlap between 1.10 and 1.25 solar radii. One result of this comparison is that the quantity [Ac/Ap] $[(\overline {N^2 } )/(\bar N)^2 ]$ is much greater than unity in the coronal regions sampled (a polar coronal hole and an old active region). Here Ac and Ap are the abundances of magnesium in the corona and photosphere, respectively and $[(\overline {N^2 } )/(\bar N)^2 ]$ is the electron density “irregularity factor.” If as recent work suggest the photospheric and coronal abundances of magnesium are about equal (cf Meyer 1985), then the corona is more highly structured on small scale than is visible on existing coronal images of high spatial resolution. Some implications of this for coronal physics are discussed.