A Probable Short Decimetric Type Ⅰ-like Noise Storm: Associated with Type Ⅲ Bursts?

A rare Type I-like noise storm was observed with the solar radio spectrometers (1.0-2.0 GHz and 2.60-3.8 GHz) at National Astronomical Observatories of China (NAOC) on September 23, 1998. We concentrate on checking the Type I-like noise storm occurred in the decay phase of a Type Ⅳ radio burst. This noise storm consists of many Type I bursts and isolated Type Ⅲ or Type Ⅲ pair bursts. It has a bandwidth of ≤0.5 GHz. The duration of each Type I burst is of the order of 100-300 ms. The total duration is greater than 11 minutes. The circular polarization degree of the components of Type Ⅰ and associated Type Ⅲ bursts are about 40%-100% and almost 100%, respectively, which is greater than that of the background continuum (nearly the precision of our instrument). This short decimetric Type Ⅰ-like storm may be another kind or the extension of the kind of metric Type Ⅰ storm, and may possess the duality of metric and decimetric radio emission. It may be in favor of an earlier emission mechanism of the fundamental plasma radiation due to the coalescence of Langmuir waves with low-frequency waves.

On the Zebra Structure in the Frequency Range near 3 GHz

We present 19 cases of zebra pattern structure (ZPS) and fiber bursts (FB) in radio bursts in frequency range around 3 GHz, and one such case in the range 5.2-7.6 GHz, using the new microwave spectrometer of NAOC between 2.6-3.8 and 5.2-7.6 GHz (China, Huairou station) with high resolution (10 MHz and 8 ms). The FB and ZPS have about the same spectral parameters: the frequency bandwidth of emission stripes △f-20MHz, the frequency separation between the emission and the neighboring low frequency absorption -△fea-30MHz and the frequency separation between emission stripes (when a periodic structure persists) △fs -60-70 MHz. Therefore we consider both these fine structures to be whistler manifestations, i.e., interactions of plasma electrostatic waves with whistler waves (generated by the same fast particles with loss-cone anisotropy) l + w → t. The duration of the fiber bursts of about 2 s corresponds to whistler waves propagating undamped at about 2s, which requires a whistler increment < 0.5 s-1. In the frequency range 3-7 GHz the relation between the ratios of plasma to cyclotron frequencies and whistler to cyclotron frequencies is almost independent of the decrement of whistler electron damping. This finding is used to obtain the magnetic field strength in the region of generation. For a reasonable value of electron temperature (2-20 MK), we find B = 125-190 G when the electron density is (8-18) × 1010 cm-3 and B = 520 - 610 G when the electron density is (35-60) × 1010 cm-3. In two remarkable events, 1998-04-15 and 2000-10-29, the right-hand polarization is strong for all the fine structures and corresponds to ordinary wave.