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.
In solar facular regions (plages) three distinct classes of magnetic features are observed: small-scale flux tubes, knots, and pores. Small flux tubes have granular scales; they are in constant motion and can well be simulated numerically according to the concept of magneto-convection. On this dynamic background one observes quite stable, long-lived and bright objects called facular knots, with a diameter of 3-8 Mm and fine (less than 1 Mm) inner filamentary structure. Their magnetic field strength varies in the range from 250 to 1200 G. Our present article considers only these active formations. The stationary MHD problem is solved and analytical formulae are derived for calculation the pressure, density, temperature, and Alfven Mach number in the studied configuration from the corresponding magnetic field structure. The facular knot is modeled in a hydrostatic atmosphere defined by the Avrett & Loeser model (2008) and is surrounded by a weak (2 G) external field corresponding to the average global magnetic field strength on the solar surface. The constructed 3D analytical model presents the facular knot as a magnetic "fountain" with numerous slender fibrils and allows solving the following tasks: 1. Calculation of temperature profiles of the knot at any height of the atmosphere; 2. Description of ring brightening and fine azimuthal fibril structure observed in plages at high spatial resolution; 3. New interpretation of Center-to-Limb Variation problem that fits well with the observational data.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.