Long-term quasi-periodic oscillations of EUV-flux on the Sun

Abstract

Two sequences of OSO-4 spectroheliograms in Mg x and Si xii obtained during October–November 1967 and covering the intervals of 83 and 22 hr, respectively, have been analyzed to reveal quasi-periodic oscillations of EUV flux from solar sources with a periodicity of 5–14 hr. The oscillation periods of the emission flux from local sources over sunspots and magnetic field enhancements in plages without spots have been investigated in correlation with characteristics of the respective AR and plages. The greatest periods (> 8 hr) are shown to be peculiar of small sunspots or sunspot groups at the initial or final stage of their development, whereas the smallest periods (∼ 5–6 hr) are observed in the case of large well-developed groups at the maximum stage of development. In quiet regions on the Sun and plages without spots, the oscillation periods are ∼ 6–8 hr. The surface areas in which the oscillations are synchronous and coincide in phase have typical dimensions of ≲1′ in quiet and 1 to 5′ in active regions. These areas form a spatial structure similar to the chromospheric network and supergranules. The characteristic lifetime of the structure elements is ∼ 1.5–2 days.