Lifetime of Surface Features and Stellar Rotation: A Wavelet   Time-Frequency Approach
  We explore subtle variations in disk-integrated measurements spanning   $\lsim$ 18 years of stellar surface magnetism by using a newly developed time-frequency gapped wavelet algorithm.   We present results based on analysis of the Mount Wilson   Ca II H and K emission fluxes in four, magnetically-active stars (HD 1835 [G2V], 82885 [G8IV-V], 149661 [K0V] and 190007 [K4V]) and sensitivity tests using artificial data.   When the wavelet basis is appropriately modified (i.e., when the time-frequency resolution is optimized), the results are consistent with the existence of spatially localized and long-lived   Ca II features (assumed here as activity regions that tend to recur in narrowly-confined latitude bands), especially in HD 1835 and 82885.   This interpretation is based on the observed persistence of relatively localized Ca II wavelet power at a narrow range of rotational time scales, enduring as long as $\gsim$ 10 years. 
Willie Soon, Peter Frick, Sallie Baliunas