Quasi-Periodic Oscillations of Microwave Emission in Solar Active Regions

In this paper we discuss QPOs registered at microwaves using the Nobeyama Radioheliograph (NoRH). Our main conclusions concerning the QPO, found from NoRH observations, include: In addition to widely known 3-min oscillations and comparatively less investigated shorter fluctuations, we have registered over sunspots at microwave range also a diversity of more long-term periodic processes having typical periods of tens and hundreds of minutes. In some sunspots these periods are dominating. In a single active region, different plasma structures can simultaneously oscillate within a long-term range with different periods. Most of the different plasma structures of the solar atmosphere registered at the Nobeyama radio maps at a wavelength of 1.76 cm at all heliographic latitudes show the presence of the long-term QPO. In observations of the QPO at microwaves, we have dealt mostly with nonstationary processes. We have thus used wavelet analysis to estimate the quality of proper plasma resonators responsible for QPOs. In our interpretation, three types of plasma oscillators were proposed to be of importance: resonators that coincide with the emitting region; resonators that are outside, but close to the radio emitter; and resonators of the global solar nature. We conclude that NoRH has opened a new era in the study of different quasi-periodic oscillations and waves in the plasma structures of the solar atmosphere.