Abstract
Abstract 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 = 0.04X 3 − 1.63X 2 + 16.30X − 41.24, with X = ln ( F MHz ) and with Y = ln ( S 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.
Highlights
Solar type III radio bursts are produced by electron beams that are propagating along open magnetic field lines in the corona and interplanetary medium (IPM)
We have extended for the first time the studies by Bonnin (2008b), Lecacheux (2000), and Krupar et al (2014) to metric–kilometric radio frequencies by combining the radio observations from space (Wind/Waves) with those from the ground-based Nançay Decameter Array (NDA; Boischot et al 1980; Lecacheux 2000) and Nançay Radioheliograph (NRH; Kerdraon & Delouis 1997; Mercier et al 2006) that are located at the Station de Radioastronomie de Nançay, France
For this event, which belongs to data set II, we have determined the peak flux density of the type III burst for every single frequency channel that was recorded during the time interval 13:10–13:25 UT
Summary
Solar type III radio bursts are produced by electron beams that are propagating along open magnetic field lines in the corona and interplanetary medium (IPM). It is widely accepted that type III bursts originate via a plasma emission mechanism (Ginzburg & Zhelezniakov 1958) in which the fast electrons form an unstable velocity distribution leading to a bump-on-tail instability (Bohm & Gross 1949) that excites Langmuir waves at the local plasma frequency ( fp = 8.98 ́ Ne , where fp is the plasma frequency in kHz and cm−3).
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