Spectral Analysis of Solar Radio Type III Bursts from 20 kHz to 410 MHz

We present the statistical analysis of the spectral response of solar radio type III bursts over the wide frequency range between 20 kHz and 410 MHz. For this purpose, we have used observations that were carried out using both spaced-based (Wind/Waves) and ground-based (Nançay Decameter Array and Nançay Radioheliograph) facilities. In order to compare the flux densities observed by the different instruments, we have carefully calibrated the data and displayed them in solar flux units. The main result of our study is that type III bursts, in the metric to hectometric wavelength range, statistically exhibit a clear maximum of their median radio flux density around 2 MHz. Although this result was already reported by inspecting the spectral profiles of type III bursts in the frequency range 20 kHz–20 MHz, our study extends such analysis for the first time to metric radio frequencies (i.e., from 20 kHz to 410 MHz) and confirms the maximum spectral response around 2 MHz. In addition, using a simple empirical model we show that the median radio flux S of the studied data set obeys the polynomial form Y = 0.04X3 − 1.63X2 + 16.30X − 41.24, with $X=\mathrm{ln}({F}_{\mathrm{MHz}})$ and with $Y=\mathrm{ln}({S}_{\mathrm{SFU}})$. Using the Sittler and Guhathakurtha model for coronal streamers, we have found that the maximum of radio power therefore falls in the range 4 to 10 R⊙, depending on whether the type III emissions are assumed to be at the fundamental or the harmonic

Interplanetary Type III Bursts and Electron Density Fluctuations in the Solar Wind

Type III bursts are generated by fast electron beams originated from magnetic reconnection sites of solar flares. As propagation of radio waves in the interplanetary medium is strongly affected by random electron density fluctuations, type III bursts provide us with a unique diagnostic tool for solar wind remote plasma measurements. Here, we performed a statistical survey of 152 simple and isolated type III bursts observed by the twin-spacecraft Solar TErrestrial RElations Observatory mission. We investigated their time–frequency profiles in order to retrieve decay times as a function of frequency. Next, we performed Monte Carlo simulations to study the role of scattering due to random electron density fluctuations on time–frequency profiles of radio emissions generated in the interplanetary medium. For simplification, we assumed the presence of isotropic electron density fluctuations described by a power law with the Kolmogorov spectral index. Decay times obtained from observations and simulations were compared. We found that the characteristic exponential decay profile of type III bursts can be explained by the scattering of the fundamental component between the source and the observer despite restrictive assumptions included in the Monte Carlo simulation algorithm. Our results suggest that relative electron density fluctuations $\langle \delta {n}_{{\rm{e}}}\rangle /{n}_{{\rm{e}}}$ in the solar wind are 0.06–0.07 over wide range of heliospheric distances