Abstract
Line-depth ratios, i.e., the ratios of the central depths of suitably-chosen spectral lines, have proved to be an effective thermometer for stars on the cool half of the HR diagram. Several individual stars show line-depth-ratios changing in concert with Ca II H&K emission over decade time scales; some have amplitudes of many degrees. The disk-integrated solar spectrum shows variations in line-depth ratios that are quite weak, amounting to a degree or so. Although we have observations for only one solar cycle, there can be little doubt that the variation is part of the cycle process. The physical cause of the variation is likely the changing fraction of the surface infused with magnetic field (plage), where the run of temperature with optical depth differs from the non-magnetic regions. Line-depth ratios may prove useful in understanding the variation in photospheric structure with changing magnetic activity, and for monitoring secular changes over many solar cycles.
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