Long-Period Oscillations of Solar Facular Knots

Observations of facular areas of the Sun using the Helioseismic and Magnetic Imager device of the Solar Dynamics Observatory allow detecting long-period oscillations in the magnetic field of facular knots with periods ranging from 30 to 250 minutes. Statistically significant oscillation modes are selected by the method of time series decomposition in empirical modes. During the observation period, which is close to the knot's lifetime, its parameters (magnetic field, area on the magnetogram, etc.) may change noticeably, although the facular formation retains its structural identity all this time. These changes also affect the effective rigidity of the system (response to external disturbances), which leads to a change in the nature of the oscillations themselves. Based on this, we consider the observed oscillation in the magnetic field of facular knots with periods from 1 to 4 hours as the proper small oscillations of a system whose effective rigidity changes over time. A simple analytical model of the process is presented. It is shown that all three specific modes of low-frequency oscillations detected in facular knots are well described by the proposed model.