**3D HELIOSPHERIC RECONSTRUCTIONS FROM THE SECCHI WHITE LIGHT
CORONAGRAPHS ONBOARD STEREO: RESULTS FROM THE EARLY RECONSTRUCTIONS**

**J.W. Cook, J.S. Newmark (Naval Research Laboratory), P.A. Reiser, P.C.
Crane (Interferometrics), A. Yahil, T. Gosnell, and R. Puetter (Pixon
LLC)**

The twin STEREO spacecraft will carry onboard the SECCHI (Sun‑Earth
Connection Coronal and Heliospheric Investigation) experiment,
consisting of an EUVI disk imager and three white light coronagraphs on
each spacecraft. At NRL we are investigating the tomographic electron
density

reconstructions, and their limitations, which are achievable from just
two viewpoints using the coronagraph observations. We employ the PIXON
technique for reconstruction. In this presentation we discuss results
from early reconstructions.

We have started with simple geometrical volumes of constant 1×10^8^
cm^-3^ electron number density: cylinders (toy coronal polar plumes),
and hemispherical shells with a main axis along a line to Sun center
(toy coronal mass ejections). We have results for all model problems
with 64×64×64 resolution volumes, and selected problems with 128×128×128
resolution. Our model coronagraph observes to 15 R~~, with an occulter
extending to 1.3 R~~. We render from the starting known electron
density distribution within a 3D volume (the input IMAGE) a synthetic
white light input DATA 2D picture, in radial and tangential
polarizations. We use two viewpoints in the ecliptic plane, at 0°
(behind B spacecraft) and 37°(ahead A spacecraft), appropriate for early
in the STEREO mission, or three viewpoints, at -37° (B), 0° (Earth), and
37° (A). From these input synthetic observations, our PIXON technique
produces an electron density reconstruction (3D output IMAGE), using the
fewest PIXON elements, that renders 2D output DATA pictures which best
agree with the 2D input DATA pictures. We discuss the following test
problems, illustrating the degree of agreement of the PIXON output IMAGE
reconstructions with the known starting input IMAGE model electron
density distributions, and of the rendered input and output DATA
pictures. We are particularly interested in the limitations present when
only two viewpoints are available. The test problems are:

1\. Plumes, finite geometry, 2 viewpoints, polarization

2\. Plumes, *infinite* geometry, 2 viewpoints, polarization

3\. Plumes, finite geometry, *3 viewpoints*, polarization

4\. Plumes, finite geometry, 2 viewpoints, *brightness*

5\. CME, finite geometry, 2 viewpoints, polarization

6\. CME, *infinite* geometry, 2 viewpoints, polarization

7\. CME, finite geometry, *3 viewpoints*, polarization

8\. CME, finite geometry, 2 viewpoints, *brightness*

9-13. CME, finite geometry, 2 viewpoints, polarization, *CME at angles
to the plane of the sky of 60°, 45°, 0°, -45°, and -60°*. We illustrate
that reconstructions are possible for structures far from the plane of
the sky. The fundamental consideration is the signal-to-noise level of
the input DATA observations used. (In interpreting our results, to
express DATA intensities in terms of B/B~O~, for our choice of problem
numerical values, B~O~ = 5.6×10^20^ ph cm^-2^ s^-1^ sr^-1^. Our test
objects are unrealistically bright, both from the assumed electron
density and from the choice of solar spectral parameters employed. We
are still learning!)

We have for each test problem a group of diagnostic figures to evaluate
the reconstruction. We illustrate below selected diagnostic figures for
test problems 1 and 5, the standard plumes and CME problems, at
128×128×128 resolution. For problems 1 and 5 we show:

DATA rendered from the input and PIXON output IMAGE 3D electron density
distributions. Look for differences;

The above rendered synthetic DATA placed in the model CCD field-of-view;

Two pictorial quantitative representations of the difference between the
input and PIXON output DATA;

A visualization of the input and PIXON output IMAGE 3D electron density
distributions along four representative viewpoints. Notice the
differences viewed from the +z axis, where the PIXON reconstruction is
least constrained by the input DATA;

Two histogram representations of the quantitative difference between the
input and PIXON output DATA;

Two histogram representations of the quantitative difference between the
input and PIXON output IMAGE 3D electron density distributions.

We have with us the diagnostic figures for all of the problems, and will
use them in individual discussions about the results from our series of
test problems. We compare reconstructions using realistic, finite
geometry lines-of-sight vs. infinite geometry with all lines-of-sight
parallel (Sun at infinity); reconstructions using two viewpoints vs. an
extra third viewpoint; and reconstructions using tangential and radial
polarizations vs. brightness alone. With only two viewpoints it is
possible to produce \"shadow\" reconstructed artifacts of the features
which were present in the starting model volume, via the symmetry of the
two viewpoint angles. These artifacts are partly removed with the
addition of the third viewpoint to the reconstruction.

We will make selected reconstructions available to the STEREO community
as test problems. We solicit for the SECCHI effort the contributions of
outside researchers to this very difficult problem. The test problems,
with information describing the test data and model parameters used, can
be found at http://stereo.nrl.navy.mil by following the "3D R & V" link.

This work is supported by NASA under S‑13631‑Y, and by the Office of
Naval Research. The SECCHI experiment is an international collaboration
led by the Naval Research Laboratory.