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). Type III bursts drift from high frequency (~1 GHz) to 20 kHz at 1 AU and sometimes even beyond. It has been known for a long time that type IIIs are extremely variable, both in radio flux density and in observing frequency range. A statistical study of type III bursts (Bonnin 2008b) observed by the Wind/Waves instrument have also found a maximum response of type III bursts at around 1 MHz previously reported by Weber (1978). More recently, using the S/Waves instruments onboard STEREO twin spacecraft, Krupar et al. (2014) have confirmed the statistical maximum of flux density around 1 MHz. In this article, we have studied the spectral response of the type III bursts over a much wider bandwidth (i.e., 20 kHz–410 MHz) and found that the maximum spectral response lies between 1 and 2 MHz.

Observations

In this article, we have performed the first-ever statistical analysis of the spectral response of solar-type III bursts over the wide frequency range between 20 kHz and 410 MHz. For this purpose, we have used the observations that were carried out using both spaced-based (Wind/Waves) and ground-based (NDA, Nançay Decameter Array and NRH, 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 (SFU). We have studied 1434 type III bursts that were observed by Wind/Waves solely and 115 type III bursts that were observed simultaneously by both Wind/Waves and NDA. Note that in the latter case, both instruments observed type III bursts that were roughly in the same line of sight. In addition, we have included the data derived using calibrated observations of the NRH at some specific frequencies.

For instance, the calibrated dynamic spectrogram observed on 2014 July 31 is shown in Figure-1.  The top panel shows the observation of type III bursts carried out using the NDA in the frequency range 10–80 MHz. The middle and lower panels show observations carried out using the Wind/Waves instrument from space. The frequency range 20 kHz–1040 kHz (i.e., lower panel) is observed using the RAD1 receiver, and the frequency range 1.075–13.825 MHz (i.e., middle panel) is carried out using RAD2 receiver. We note that a type III burst at 13:30 UT is not seen in RAD2 because of missing information.

Results

The main results of our study is that type III bursts in the metric to hectometric wavelengths range statistically exhibit a maximum of their radio power at around 1 to 2 Mhz as shown in Figure – 2. Using the Sittler and Guhathakurtha model for coronal streamers, we have found that this frequency range corresponds to the heliocentric distance range ≈3–8 solar radii if one assume a radio emission at the fundamental plasma frequency. On the other hand if the type III bursts are a combination of both fundamental and harmonic emissions, the maximum of the radio flux would originate in the heliocentric distance range of ~3 to ~20 solar radii. Note that this radial range is crucial for the solar wind since this is where it becomes both supersonic, after the Parker sonic point, and super-Alfvénic.

Finally, there is no doubt that our findings will soon be compared to in situ observations. Indeed the Parker Solar Probe is expected to reach 10 solar radii in 2024. The in-situ measurements of the various plasma parameters including density fluctuations, Langmuir waves, and energetic electrons should provide crucial information for fully explaining the maximum radio flux of solar type III bursts.

Figure 2 – Spectral response of type III bursts observed from 20 kHz to 410 MHz is shown. The yellow circle, magenta cross, and blue plus markers indicate the maximum flux density (in SFU) of the 115 type III bursts observed using RAD1 and RAD2 receivers of the Wind/Waves instrument and Nançay Decameter Array, respectively. The orange asterisk markers indicate the observations of Nançay Radioheliograph at selected frequencies (i.e., 164, 236, 327, and 410 MHz). The black solid curve represents the median (i.e., 50th percentile) of the flux densities of the data shown in every frequency channel. The red filled region shows the first and third quartiles of the data shown. Note that the galactic contribution is subtracted.

Conclusions

In this article, 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 (NDA and NRH) 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. Using the Sittler and Guhathakurtha model for coronal streamers, we have found that the maximum of radio power therefore falls in the range of ~3 to ~20 solar radii, depending on whether the type III emissions are assumed to be at the fundamental or the harmonic.

Based on the recently published article: K. Sasikumar Raja, et al,  “Spectral Analysis of Solar Radio Type III Bursts from 20 kHz to 410 Mhz”, The Astrophysical Journal, 924(2):58, January 2022. doi:10.3847/1538-4357/ac34ed.

References

1. Bonnin, X. 2008b, PhD thesis, Université Paris-Diderot—Paris VII, https:// hal.archives-ouvertes.fr/tel-00461521

2. Krupar, V., Maksimovic, M., Santolik, O., et al. 2014, SoPh, 289, 3121

3. Weber, R. R. 1978, SoPh, 59, 377

*Full list of authors: K. Sasikumar Raja, Milan Maksimovic, Eduard P. Kontar, Xavier Bonnin, Philippe Zarka, Laurent Lamy, Hamish Reid, Nicole Vilmer, Alain Lecacheux, Vratislav Krupar, Baptiste Cecconi, Lahmiti Nora, and Laurent Denis