There is no simple answer to this question. Before I do try to answer it I
should at least briefly describe how the eye takes in colors. There area three
types of light-sensitive cells in the eye with various spectral sensitivity.
Some cells are sensitive to the red, others to the green and still others to the
blue part of the spectrum. The color is then determined by the ratio of how
each of the three types of cells are stimulated. To this purpose, the human eye
is equipped with a rather sophisticated calibration method. For each source, the
eye defines the white color by another ratio of the elements (red, green,
blue) so that the color of the objects we are viewing is preserved (at least
approximately) regardless of the source of light. Consequently, the color is
not determined only by the spectral composition of the light but by the
calibration of the eye, i.e. the white color definition, as well. And that is
the very problem when trying to answer the question, what color is the corona.
The spectral composition of the coronal light is very well-known. But how is
its color perceived by the human eye?
With the other two solar layers visible with the naked eye, i.e. the photosphere and
the chromosphere, the situation is very simple. The photosphere is white. There
is no whiter white than the color of the solar photosphere as the photosphere
is the source of the light that, at daytime, illuminates the surface of the
Earth and that is why our eye is, during daytime, calibrated so that it
considers the photosphere's light neutrally white. The chromosphere is red.
Its highly saturated red color is caused by the spectral line H-alpha radiation
in which the chromosphere shines intensively. The saturation of the red color is
so high that any color calibration is of no influence, i.e. the excitation of the
red-sensitive cells is so strong that the excitation of the green-sensitive and
blue-sensitive cells is almost immaterial. The solar corona, on the other hand,
has no distinctive color and its color hue greatly depends on what we define as white.
During the short period of totality the light of the solar photosphere is missing as it is hidden
by the Moon. The eye therefore misses the "classic" calibration source of white
light it is familiar with and the subjective opinions concerning the color of
the corona may differ. Most people evaluate the corona as bluish. The bluish
tinge of the corona in totality is given by the blue color of the Earth's sky.
The intensity of this blue color depends on the extent the human eye gets
accustomed, during the short period of totality, to the unusual light conditions
and manages to disregard the sky's color. Using objective methods to compare
the color of the corona with the color of the photosphere we would find that
the color of the corona would be only marginally different, i.e. the corona is
almost white (with a slight shift towards green). The color similarity does not
signify that the spectral composition of the photospherical and coronal light are
identical. It just means that the ratio of the red, green, and blue component
of the light are roughly similar.
Click on the picture to view it enlarged. The enlarged version has been scanned
so that it provides a better view of the more distant parts of the corona. The
immediate surroundings of the Sun are heavily overexposed but
it is possible to
"extract" even from the extremely overexposed
negative part the prominences by means of special digital processing. It makes
the fantastic quality of the Fujicolor Superia 800 film evident.
