http://arxiv.org/abs/1412.4107
We processed magnetograms that were obtained with the Michaelson Doppler Imager onboard the Solar and Heliospheric Observatory (SOHO/MDI). The results confirm the basic properties of long-period oscillations of sunspots that have previously been established and also reveal new properties. We show that the limiting (lowest) eigenmode of low-frequency oscillations of a sunspot as a whole is the mode with a period of 10 – 12 up to 32 – 35 hours (depending on the sunspot’s magnetic- field strength). This mode is observed consistently throughout the observation period 5 – 7 days, but its amplitude is subject to quasi-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 power spectrum of sunspot oscillations. But this lowest mode is apparently not an eigenmode of a sunspot because its period does not depend on the magnetic field of the sunspot. Perhaps, the mode reflects the quasi-periodic sunspot perturbations caused by supergranulation cells that surround it. We also analyze SOHO/MDI artifacts, which may affect the low-frequency power spectra of sunspots.
V. Efremov, L. Parfinenko, A. Solovev, et. al.
Mon, 15 Dec 14
50/53
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