Fourier Power Spectra of Solar Noise Storms

Abstract

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 $-1.7\pm0.14$, $-1.6\pm0.14$, 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 $-2.9\pm0.46$, $-3.1\pm0.65$, and $-3.4\pm0.98$, respectively.