  The CoRoT satellite has made available high precision photometric
observations of a large number of stars of different spectral types. Continuous
photometric time series allow the characterization of stellar microvariability
in a systematic way. We determine an index indicating the level of activity,
derived from photometric data, for a large sample of stars with different color
temperatures. We also assess to what extent this index can be related to an
estimated Rossby number for stars whose rotation period can be estimated. We
also estimate a characteristic lifetime of the surface heterogeneities. Our
work is based on the Fourier analysis of stellar light curves. We analyzed the
Fourier power spectra of 430 selected light curves obtained by CoRoT during
three observation runs. The low-frequency contribution of the stellar
variability is modelled by a "generalized semi-lorentzian" profile. An activity
index is derived from the fitted amplitude and width of the semi-lorentzian
model. Some of the Fourier spectra exhibit a rotational modulation which
enables the determination of the rotation period. In addition, a convective
turnover time is derived from a grid of stellar models, so that a Rossby number
can be estimated. A characteristic lifetime of the phenomena causing the
observed power at low frequency is assessed from the fitted model of the power
spectrum and is compared to the rotation period. Higher values of the
microvariability index are observed among the coolest stars from our sample. 28
light curves show a clear rotational modulation. The estimated Rossby number of
most of the observed stars with a rotational modulation is less than 1. The
activity index decreases with increasing Rossby number. The quality of the
CoRoT data enables the determination of the characteristic lifetime of active
structures. It is shown to increase with the rotation period.
