Variability and Redundancy across the Solar Spectrum

Peter Fox [pfox@ucar.edu], High Altitude Observatory/ESSL/National Center for Atmospheric Research, Boulder, Colorado.

    The shape of the solar spectrum and its variation in magnitude fordifferent levels and location of solar activity is similiar overcertain wavelength ranges. There are three reasons for this:

1)  The solar photosphere is close to local thermodynamic equilibrium(LTE) and not very far from radiative equilibrium; therefore, thespectral distribution is close to a Planck function. Under conditionsof radiative equilibrium, the entire spectrum will be determined bya single variable, the effective temperature.

2)  Radiation emergent from a stellar atmosphere does not originatefrom a single level in the atmosphere, rather it is the accumulation ofradiation from inside the atmosphere along the line of sight to the observer.The basic function in this accumulation process is the contribution(or source) function which has a maximum near unit optical depthand a width (FWHM) spanning about two density scale heights.

3) The wavelength dependence of the opacity allows this contributionfunction to shift up and down in the atmosphere according towhether the opacity increases or decreases, respectively. Sincethe photospheric temperature decreases outward, such shifts in theregion of radiation formation cause changes in the emergent intensity.

    The object of this study is to determine how a particular (small)wavelength region may represent a wider band or other bands in thesolar spectrum.To exploit the redundancy in the spectrum requires an understanding ofhow the relative variability in well defined wavelength regions respondsto changing solar activity and whether the correspondence depends on thetime scale of the solar activity, e.g. that of active region emergencecompared to the solar cycle itself. We present details of the investigationof these factors.

NCAR is sponsored by the National Science Foundation. Thiswork is partly supported by the NSF RISE program.