Fourier Power Spectra of Solar Noise Storms

We analyzed three noise storms recorded on 200?–?400 MHz Trieste Callisto radio spectra on 2 July 2012, 8 July 2012, and 16 July 2012 by the Fourier method. We divided intervals of the noise storms into five-minute intervals, and in these intervals we computed the mean Fourier spectra as a function of the wave numbers in the frequency and height-scale spaces. We found that these Fourier spectra, where the spectrum from the quiet-activity interval was subtracted, are power-law spectra. The mean power-law index of these spectra in the range (ln(k_{z}) = [1.8, 2.9]) (where (k_{z}) is the wave number in the height-scale space) is (-1.7pm0.14), (-1.6pm0.14), and (-1.5 pm0.12) for the 2 July 2012, the 8 July 2012, and the 16 July 2012 noise storms, respectively. It appears that as the number of Type-I bursts in the studied interval increases, the power-law index becomes closer to (-5/3); this is known as the Kolmogorov spectral index. The power-law index of the noise storms is very similar to that of the narrowband dm-spikes found in our previous studies. Furthermore, we found a break in the power spectra at (ln(k_{z}) approx2.9), and the mean power-law index values above this break are (-2.9pm0.46), (-3.1pm0.65), and (-3.4pm0.98), respectively.