Long-Period Oscillations of Sunspots Observed by SOHO/MDI

TL;DR

This study analyzes long-period oscillations in sunspots using SOHO/MDI data, confirming known properties and revealing new insights into their eigenmodes and possible external influences like supergranulation.

Contribution

It identifies the fundamental low-frequency eigenmode of sunspots and explores its properties, including amplitude variations and external perturbation effects.

Findings

The lowest eigenmode has a period of 10-12 to 32-35 hours.

Amplitude of the eigenmode varies quasi-cyclically every 1.5-2 days.

A lower mode of 35-48 hours may be caused by external supergranulation effects.

Abstract

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.