  We show that while the Fourier transform can figure only an "intensity" of a
periodic signal, there is an additional information embedded, which is a
"coherence" of the signal. Supposing that the periodicity is reflected only on
the "coherence," we introduce a time stability as a measure of "coherence"
excluding the "intensity," i.e. amplitude of signal. In stead of classical
strength-based significance, where strength implies the power or amplitude, we
adopt a stability-based significance as criterion to choose cycles in a random
signal. We inspect the time-stable solar periodicities and show that most
periodicities discovered in exterior solar activities such as the solar wind
inhere in interior solar activity such as the sunspot and that the time
stability can be an effective tool in spectral analysis of stochastic solar
activity.
