Tomography and triangulation

In *Aso et al.* [1998b] initial results and model comparisons using a
modified version of a Simultaneous Iterative Reconstruction
Technique (SIRT) [*Gilbert*, 1972] were applied to data from
the March 1995 ALIS-Japan 1 campaign. The method was also verified by
numerical simulations. An auroral fold occurring at 23:40:30 UTC on 26
March 1995 was selected for testing the tomographic analysis with the
modified SIRT method assuming magnetic field-aligned auroral
structures. Projecting back the reconstructed volume onto the
original images reveals a disparity of about % or less. The
modified SIRT method was found to be a promising CT application in the
field of auroral studies. In total, four joint ALIS-Japan campaigns
have been undertaken so far, and some of these data are still in the
analysis phase.

*Aso et al.* [2000] reports on tomographic analysis of auroral images
by the modified SIRT method. Auroral images were obtained on 9 February
1997 at 19:46:00 UTC in the 5577 Å emission line as well as at
18:31:30 UTC in the 1Neg. 4278 Å emission line. The latter case showed
a bright folded arc south of Kiruna and a faint thin arc north of
Kiruna. The peak height of the intense aurora appears lower than the
faint arc. Given favourable conditions for auroral tomography, it is
possible to study the basic auroral formation process. To demonstrate
this *Aso et al.* [2000] also performed a numerical simulation for the
reconstruction of a slightly folded auroral structure using data from
seven stations. Furthermore, the authors report on initial
triangulation results applied to the studies of nacreous clouds. Some
more reports of ALIS observations of nacreous clouds are summarised in
Section 6.6.1.

Triangulation and tomography-like methods were applied to the studies
of HF pump-enhanced airglow (Section 6.4). A solution to the inverse
problem for a particularly ill-posed problem, with data from only
three stations south of the emission region, is discussed in
*Gustavsson et al.* [2001a].

In *Gustavsson et al.* [2001b] a constrained tomographic inversion was
used to estimate the 1Neg. 4278 Å altitude distribution, which allowed
an estimate of the energy distribution of the electron flux
(Section 6.5.1).

This section has provided a rather brief summary of a very large
subject. For further, more detailed studies, the reader is encouraged
to read the cited papers, and, in particular, the main reference for
three-dimensional imaging with ALIS by *Gustavsson* [2000].

Future developments to be expected in this field include the
investigation of smooth basis functions and approximated projection
algorithms for faster tomography [*Rydesäter and Gustavsson*, 2001].