Long-Period Oscillations of Sunspots Observed by SOHO/MDI

摘要

We processed magnetograms that were obtained with the Michaelson DopplerImager onboard the Solar and Heliospheric Observatory (SOHO/MDI). The resultsconfirm the basic properties of long-period oscillations of sunspots that havepreviously been established and also reveal new properties. We show that thelimiting (lowest) eigenmode of low-frequency oscillations of a sunspot as awhole is the mode with a period of 10 - 12 up to 32 - 35 hours (depending onthe sunspot's magnetic- field strength). This mode is observed consistentlythroughout the observation period 5 - 7 days, but its amplitude is subject toquasi-cyclic changes, which are separated by about 1.5 - 2 days. As a result,the lower mode with periods of about 35 - 48 hours appears in the powerspectrum of sunspot oscillations. But this lowest mode is apparently not aneigenmode of a sunspot because its period does not depend on the magnetic fieldof the sunspot. Perhaps, the mode reflects the quasi-periodic sunspotperturbations caused by supergranulation cells that surround it. We alsoanalyze SOHO/MDI artifacts, which may affect the low-frequency power spectra ofsunspots.