Solar Cycle Dependence of Umbral Magneto-Induced Line Broadening

Details Solar Cycle Dependence of Umbral Magneto-Induced Line Broadening Solar Cycle Dependence of Umbral Magneto-Induced Line Broadening

May 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agusmsp41b..06s&link_type=abstract

American Geophysical Union, Spring Meeting 2008, abstract #SP41B-06

Physics

7524 Magnetic Fields, 7536 Solar Activity Cycle (2162)

Scientific paper

Studies of the solar cycle dependence of peak umbral magnetic field strength have focused upon measurements of continuum intensity and the Zeeman splitting of infrared spectral lines. Here we extend the discussion into a measurement of effective line width using eleven years of spectromagnetograms from the Kitt Peak Vacuum Telescope (KPVT). The KPVT observed the 868.8 nm Fe I absorption line in opposing states of circular polarization between 1992 and 2003, deriving full-disk images of line-of-sight (LOS) velocity, LOS magnetic flux, continuum intensity, equivalent line width, and central line depth. We determine an effective spectral line width through a relation of the measured equivalent line widths and central line depths. Developing a basic model of the Stokes line profiles using the Seares formalism, we illustrate that a change in the effective line width within the umbra as determined using the KPVT data is consistent with the change in the Zeeman splitting. We discuss the effect of observed distance from disk center, stray light, and the unknown inclination angle of the magnetic field. Within individual sunspots observed near disk center, the determined effective line width decreases with distance from the umbral core consistent with the studied magnetic field gradient. Measurements of different sunspots show a clear dependence on umbral size consistent with previous studies of the umbral magnetic field. Using this effective line width as a diagnostic for magnetic field, we examine the dependence of maximum magnetic field strength on the phase of the solar cycle. We present a comprehensive statistical analysis using a sample size of over 3500 umbral measurements.

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