A Quasi-Periodic Solar Radio Fluctuation at Microwave Band

A rare but interesting solar radio fine structure, quasi-periodic fluctuations, on 25 August 1999 was observed at microwave band for the first time. They fluctuated initially at a nearly stable frequency level then at a reverse drift component up to 5.49GHz. The individual fluctuation consists of a bi-directional drift component. The features are characterized by narrow bandwidth of Δf/f≤3%, quasi-periodicity of ～ms as well as a slowly reverse and a rapidly normal drift rates on the bi-directional drift component. The associated data of the Yohkoh soft and hard x-ray telescope and Nobeyama radio heliograph at 17GHz showed that there are several bright spots (i.e. inhomogeneities) along the soft x-ray loop, and the locations of both radio and soft x-ray sources are closely consistent. Therefore, the fluctuations are most likely caused by the inhomogeneities within a flare loop. Based on the two-component atmospheric model, we suggest a three-component atmospheric model with large scale length λ and small scale lengths λ₁ and λ₂ to describe equilibrium atmosphere and inhomogeneity. With the beam model, the characters of fluctuations may be interpreted reasonably by the quasi-equidistant inhomogeneity along a flare loop.