The
photosphere is the visible surface of the Sun, the source of the continuous
radiation that constitutes
over 99% of the energy measured by spacecraft radiometers such as ACRIM. The temperature
decreases with height in the photosphere out to a minimum region and then
begins to increase
in the chromosphere. The cores of the strong Fraunhofer absorption lines are
formed just above
the temperature minimum region. They do not have emission cores because of
scattering effects.
The weaker Fraunhofer lines are formed at and below the temperature minimum
region. Since 1980 we
have used the 1-meter Fourier Transform Spectrometer (FTS) on Kitt Peak to
monitor the solar
irradiance spectrum, and in particular, the line-center fluxes of the strong
Sodium D and Magnesium
b lines and 22 other weaker lines, mostly due to iron. A unique feature of this
FTS instrument
and its feed optics is that, over this 23 year interval, nothing, to our
knowledge, has been changed
or modified in any way. The central fluxes of all of the above lines decreased
between the 1980
activity maximum and the 1986 minimum, as expected, and then increased as
active regions again
appeared on the disk. In 1990, however, as solar activity began to diminish,
the Fraunhofer line fluxes
continued to increase. They did not return to their 1986 minimum values during
the 1996 minimum.
Instead, the central fluxes of the strong lines increased by 10-15%. We have
not found any instrumental
effects that might account for this result. Preliminary quiet-Sun model
calculations indicate that a
temperature rise of a few hundred K in the temperature minimum region could
account for such increases.
However, the Ca K-index (residual flux in a 1A passband at the Ca II line
center) was the same in
1996 as in 1986, and this places severe constraints on possible increases of
quiet-Sun temperatures.
Both the Fraunhofer and Ca II line observations represent line-to-continuum
ratios, with the
various lines and continua formed at different depths. We are investigating
whether any combination
of temperature variations at different depths might account for these
observations. We note
that the Willson and Mordvinov composite Total Solar Irradiance (TSI) signal
indicated increased output
from the Sun in 1995-98 compared to 1984-87. We hope to find an explanation for
the Fraunhofer
line increases between these two epochs, and, if so, to calculate the
corresponding TSI change
that would be expected.
Abstract