Exploration of solar chromosphere using high resolution spectroscopic observations

Abstract

Brazilian Solar Spectroscope (BSS), operating in the frequency range of (1000 i 2500) MHz with high time and frequency resolutions, has observed various fine structures showing intensity variations as functions of time and frequency. Here we are reporting radio bursts observed by BSS, from August to October/2001, exhibiting fine structures with total duration of the order of 300 ms, covering a frequency range of ~ 200 MHz. These characteristics are similar to those of the narrow band type III bursts observed above 1000 MHz. Hence these fine structures are variants of decimetric narrow band type III-like bursts. The fine structures observed exhibit variation of intensity as a function of frequency and curvatures in the frequency-time plane. These fine structures are explained as a propagation effect. We suggest that these structures are the signatures of the chromospheric density inhomogeneities lying in the line-of-sight path between the observer and the radio source. If the emitted frequencies are lower than the plasma frequency of the inhomogeneity, there will be absorption over a certain band, corresponding to the dimension of the inhomogeneity. The curved like structures are due to lower group velocities of the lower frequencies in the density inhomogeneities. The estimated scale sizes of the irregularities obtained are of the order of 101 - 104 km. Thus an alternative method to total solar eclipse observation is suggested for investigations of chromospheric density irregularities.