Mapping the Structure of the Corona Using Fourier Backprojection Tomography

Abstract

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-transformbased 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 March 16 to 2003. 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's 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's 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.

Automatically Detecting and Tracking Coronal Mass Ejections. I. Separation of Dynamic and Quiescent Components in Coronagraph Images

“…The choice of date is arbitrary, but a clean reconstruction with streamers distributed up to high mid-latitudes was preferred. This longitude-latitude map is a shell of the corona at 4 R , and the density is in normalized units (not absolute electron density; see Morgan et al 2009). The depicted structure shows accurately the distribution of streamers during the observational period, and the structure is used as a basis to create a set of synthetic coronal observations.…”

“…When viewed in white light coronagraph images, their emission is integrated over an extended line of sight (LOS) which includes other non-CME structures as well as the CME itself. Despite the increasing sophistication of solar tomography techniques (see Morgan et al 2009;Morgan & Habbal 2010b, and references within), in general, detailed information is lacking on these non-CME structures (i.e., the quiescent coronal structure of streamers and coronal holes). Therefore CMEs are observed, not in isolation, but in the presence of the fine structural detail of the quiescent corona.…”

From the Sun to the Earth: The 13 May 2005 Coronal Mass Ejection

“…This is different to building a standard coronagraph Carrington or synoptic map, since tomographical techniques are used which help to resolve each line of sight (LOS). The method used is described by Morgan, Habbal, and Lugaz (2009).…”

“…Reconstructed/modelled synoptic-map overviews of CR2029 can be seen in Figures 2 and 3 using three different methods. The first ( Figure 2) shows a reconstructed white-light Carrington map of SOHO|LASCO data at 4 R using the technique briefly described in Section 2.2 and in detail by Morgan, Habbal, and Lugaz (2009). The density is in normalised (arbitrary) units (see Morgan, Habbal, and Lugaz, 2009).…”

“…The first ( Figure 2) shows a reconstructed white-light Carrington map of SOHO|LASCO data at 4 R using the technique briefly described in Section 2.2 and in detail by Morgan, Habbal, and Lugaz (2009). The density is in normalised (arbitrary) units (see Morgan, Habbal, and Lugaz, 2009). The second ( Figure 3) is a comparison of 3-D reconstructed IPS results using the UCSD kinematic, time-dependent solar-wind model with STELab IPS data (top) along with a 3-D, time-dependent coronal MHD model (bottom).…”