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We present chromospheric activity index $S\rm_{HK}$ measurements for over13,000 F, G and K disk stars with high signal-to-noise ratio ($>$ 60) spectrain the Sloan Digital Sky Survey (SDSS) Data Release 7 (DR7) spectroscopicsample. A parameter $\delta$S is defined as the difference between $S\rm_{HK}$and a `zero' emission line fitted by several of the most inactive stars. The$S\rm_{HK}$ indices of subgiant stars tend to be much lower than dwarfs, whichprovide a way to distinguish dwarfs and giants with relatively low resolutionspectra. Cooler stars are generally more active and display a larger scatterthan hotter stars. Stars associated with the thick disk are in general lessactive than those of the thin disk. The fraction of K dwarfs that are activedrops with vertical distance from the Galactic plane. Metallicity affects$S\rm_{HK}$ measurements differently among F, G and K dwarfs in this sample.Using the open clusters NGC 2420, M67 and NGC6791 as calibrations, ages of mostfield stars in this SDSS sample range from 3-8 Gyr.

Tracers of chromospheric structure. I. CaII H$\&$K emission distribution of 13000 F, G and K stars in SDSS DR7 spectroscopic sample