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%%%% ID=P5-6

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\title{Shadow Bands Observed During the Total Solar Eclipse of 4 December 2002, by High-Resolution Imaging.}
\titlemark{Shadow Bands During the Total Solar Eclipse of 4 Dec 2002}

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\author{Szymon Gladysz, Michael Redfern}
\affil{Experimental Physics Department, National University of Ireland, Galway, Ireland}

\author{Barrie W. Jones}
\affil{Physics and Astronomy Department, The Open University, Milton Keynes, MK7 6AA, UK}



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\contact{Szymon Gladysz }
\email{Szymon.Gladysz@NUIGalway.ie }

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\paindex{Gladysz, S.}
\aindex{Redfern, M.}
\aindex{Jones, B. W.}

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\authormark{Gladysz, Redfern, Jones}

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\keywords{image: processing, scintillation, atmospheric effects}

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%			       Abstract
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\begin{abstract}          % Leave intact
% Place the text of your abstract here - NO BLANK LINES
We present the results of comparison between characteristics
of highly atypical shadow bands recorded during total solar
eclipse of 4 December 2002 in Botswana and theory of Codona.
For the first time the analysis was based on images of the
phenomenon and not photometric data. Thanks to this, use was
made of high spatial resolution and detailed plots of power
spectra were obtained. The plots' shapes are in excellent
agreement with the one predicted by theory. Due to the novel
nature of the recording process, some noteworthy image
processing techniques were used.
\end{abstract}

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\section{Introduction}

Shadow bands are natural phenomena that appear just before and just after
totality during solar eclipses. They arise from the superposition of atmospheric
speckle patterns from elements of an incoherent, extended line source (the
remaining visible crescent of the Sun). They are linear patterns moving across
the ground with typical speeds of a few m/s in the direction perpendicular to
their elongation. They align parallel to the tangent to the centre of the solar
crescent (Marschall et al., 1984). It has been observed that shadow band spacing
decreases and their contrast increases as totality approaches (Codona, 1986;
Jones, 1996, 1999).

\section{Obtaining The Data}

Shadow bands were recorded during total eclipse of 4 December 2002 in Botswana.
The imaging system consisted of a white, diffusely - reflective screen (surface
was perpendicular to the line of sight to the eclipsed Sun) and a digital video
recorder on a tripod in front of it. For the purpose of observation the aperture
was locked fully open, exposure time was 1 ms (so that the phenomenon was
"frozen"). The recorder was capturing 25 non-interlaced frames per second.

The bands were also observed visually. They were weak and disorganized and
resembled the surface of boiling water rather than linear patterns.

\begin{figure}
\epsscale{0.3}
\plotone{P5-6_1.eps}
\caption{A typical image of the screen. Shadow bands are of extremely low contrast.} \label{P5-6:screen}
\end{figure}

\section{Image Processing}

\begin{itemize}
\item Because we could not place the camera between the source and the screen
there was a certain amount of perspective elongation that had to be removed from
the images (Fig. 1). Perspective transformation arises if a planar object is
viewed from a fixed point in space (Glasbey, Mardia, 1998):

\begin{eqnarray}
u & = &  \frac{a_{10}x + a_{01}y + a_{00}}{c_{10}x + c_{01}y + 1}\;,
\nonumber \\ & & \\
v & = & \frac{b_{10}x + b_{01}y + b_{00}}{c_{10}x + c_{01}y + 1} \nonumber
\end{eqnarray}

where x, y are the coordinates of a point in the object plane, and u, v are the
coordinates of the same point in the image plane. The inverse transformation,
(u, v) $\rightarrow$ (x, y), was applied to every image using the reciprocal
formulae. The eight parameters $(a_{10}, a_{01}, a_{00}, b_{10}, b_{01}, b_{00},
c_{10}\ and\ c_{01})$ were found using known positions of the four corners of
the screen before and after the transformation. This meant solving a system of
eight linear equations with eight unknowns. Bi-cubic interpolation was used for
image scaling.


\item Subsequently images were flat-fielded using the ensemble-average as a
flat-field. This is a simple variant of a technique proposed by Lindler et al.
(1993) for Hubble Space Telescope's Faint Object Spectrograph and usually
referred to as 'superflats'. The general decreasing (increasing) trend in
overall brightness before (after) totality was removed by normalization. That
produced images with the same intensity level as the first (last) image in the
sequence. Background around the screen was cropped.


\item The intensity values on the screen were in a very small range (30, 40
values on a 0-255 scale). We decided to enhance the contrast by applying
Gaussian curve fixing algorithm to the histogram of intensity and this way
determining the range that had to be widened (Fig. 2). Afterwards we transformed
each value in the histogram to a corresponding new 'bin' between 0 and 255.
Thanks to this we achieved values that subtended whole scale.

\end{itemize}

\begin{figure}
\epsscale{0.4}
\plottwo{P5-6_2.eps}{P5-6_3.eps}
\caption{Typical intensity histogram with Gaussian curve fit (left). Histogram of an image after contrast enhancement (right).} \label{P5-6:histograms}
\end{figure}

\section{Results}

Two batches of 1480 images (59.2 s) before and after totality were used for
further analysis. Operations that could be used to detect shadow bands are:

\begin{itemize}

\item computation of the cross-correlation between two successive images to
demonstrate the movement of coherent pattern between frames \item application of
a FFT algorithm to a single image in order to find the shape of power spectral
density function, psd(k), defined by Kay and Marple (1981), as predicted by
Codona, and search for shadow bands' characteristic spatial frequencies.

\end{itemize}

Cross-correlation for different pairs of raw (without contrast enhancement)
images revealed nothing but a peak at (0, 0) coordinates. This means that
structure of the screen (hardly visible with an un-aided eye) is more pronounced
than the shadow bands (meaning zero-shift between the images). Besides that, 25
fps means 40 ms between frames while coherence time (t0) for the atmosphere is
usually not more than 10 ms (Monnier, 2003). Shadow bands change their shape too
much from one image to another for the cross-correlation to detect them; the
temporal-spatial approach was not useful in practice. The theory describing
shadow bands is mostly based in the spatial domain, so the use of spatial
approach in the analysis is more relevant.

Power spectral density functions were more likely to reveal shadow bands as they
are calculated in the spatial domain so that low fps limit of the digital video
recorder played no role here. We recorded ambient noise at a low light level
with the recorder (same settings as on the observation day), and calculated
averaged and normalized psds for the resulting ensemble of images. The psd of an
image is a two-dimensional function but for the purpose of this study a
one-dimensional psd was calculated in order to make comparisons with Codona's
plots. Power spectra for the pixel vectors at a right angle to the tangent to
the crescent were calculated separately, added and averaged to produce a psd(k)
plot. The psd of the noise was then subtracted from each psd of shadow bands. 60
such functions were computed before and after totality, setting the interval
between measurements to 1 s. Smoothing with a rectangular window, 4 pixels wide,
was used. The cut-off frequency was imposed by the Nyquist theorem. In some
cases noise cancelled the signal in the very low frequency range, but that part
of the plots was not significant for further analysis.

\begin{figure}
\epsscale{0.4}
\plottwo{P5-6_4.eps}{P5-6_5.eps}
\caption{Shadow bands' smoothed psd function (left), 41 s before second contact and Codona's (1986) prediction of a psd(k) shape (right).} \label{P5-6:psds}
\end{figure}

The qualitative resemblance between our power spectral density functions and the
plot resulting from theoretical assumptions in Codona's paper is striking.
According to Codona, shadow bands' psds display three characteristic scales.
First characteristic scale is controlled by the geometry of the crescent and
corresponds to the low-frequency peak in the spectrum. The next scale relates to
the first minimum in the plots and is due to the first null of the source's
spectrum. Third scale corresponds to the fast oscillating term in the analytic
expression, called the "Fresnel filter" (starting at the second minimum in the
plots). Which of them will be the dominant shadow band scale visible to the
human eye depends on the distance of the atmospheric scattering layer from the
observer and the time to totality. Codona derived formulae relating the height
of the scattering layer, z, to the dominant spatial frequency of a psd plot. The
derivation of the values of z is the goal of the next stage of the shadow bands'
data analysis.
%-----------------------------------------------------------------------
%			      References
%-----------------------------------------------------------------------
% List your references below within the reference environment
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%                Nucl.Phys., 274, 1
%           \reference H\'enon, M.\  1961, Ann.d'Ap., 24, 369
%           \reference King, I.\ R.\  1966, \aj, 71, 276
%           \reference King, I.\ R.\  1975, in Dynamics of Stellar 
%                Systems, ed.\ A.\ Hayli (Dordrecht: Reidel), 99
%           \reference Tody, D.\  1998, \adassvii, 146
%           \reference Zacharias, N.\ \& Zacharias, M.\ 2003,
%                \adassxii, \paperref{P7.6}
% 
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%
\begin{references}

\reference Codona, C., 1986. Astronomy and Astrophysics, 164, 415
\reference Glasbey, C. A., Mardia, K. V., 1998. Journal of Applied Statistics, 25, 155
\reference Jones, B. W., 1996. Journal of Atmospheric and Terrestrial Physics, 58, 1309
\reference Jones, B. W., 1999. Journal of Atmospheric and Solar-Terrestrial Physics, 61, 965
\reference Kay, S. M., Marple, S. L., 1981. Proceedings of the IEEE, 69, 1380
\reference Lindler, D., Bohlin, R., Hartig, G., Keyes, C., 1993. FOS Instrument Science Report CAL/FOS-088
\reference Marschall, L. A., Mahon, R., Henry, R. C., 1984. Applied Optics, 23, 4390
\reference Monnier, J. D., 2003. Reports on Progress in Physics, 66, 789


\end{references}

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\end{document}  % Leave intact
