MAPPING THE STRUCTURE OF THE CORONA USING FOURIER BACKPROJECTION TOMOGRAPHY

Estimating the structure, or density distribution, of the solar corona from a set of two-dimensional white-light images made by coronagraphs is a critical challenge in coronal physics. This work describes new data-analysis procedures which are used to create global maps of the coronal structure at heights where the corona becomes approximately radial (⩾ 3 R☉). The technique, which is named Qualitative Solar Rotational Tomography (QSRT), uses total brightness white light observations, processed with a suitable background subtraction and a Normalizing Radial Graded Filter (NRGF). These observations are made with high frequency by the Large Angle and Spectrometric Coronagraph Experiment (LASCO) C2 coronagraph, which allows a standard Fourier-transform-based tomographical reconstruction. In this paper, we first test the technique using a model corona. QSRT is then applied to a set of observations made during Carrington Rotation (CR) 2000–2001 (2003 March 16 to 2003 March 31). Since the maps are constructed from data which are normalized using the NRGF process, QSRT cannot give electron density directly. Nevertheless, the tests using the model corona demonstrate the technique\u27s ability to give a good qualitative reconstruction of the coronal structure at high latitude, with decreasing but acceptable accuracy at the equator. These tests also demonstrate QSRT\u27s insensitivity to noise. For the LASCO C2 observations, good agreement is found between synthetic images calculated from the reconstructed corona and the original observations, and good agreement is found between the distribution of density in a QSRT reconstruction and that found using a global MHD model. Despite their lack of quantitative information on absolute electron density, the resulting maps (which are constructed directly from high-resolution coronal data observed at the appropriate height), contain useful information on the distribution of density in the corona