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