Temporally resolved Type III solar radio bursts in the frequency range 3-13 MHz by A. Vecchio et al. – Community of European Solar Radio Astronomers

Type III radio bursts are the most common coherent radio emission produced by the Sun. They are characterized by a rapid drift in time towards lower frequencies and represent an indirect signature of energetic electrons produced at the Sun during a flare and propagating through the plasma of the corona and the interplanetary medium. Type III bursts are observed over a wide range of frequencies ranging from about ∼500 MHz down to tens of kHz close to 1 au, corresponding to a wide range of heliocentric distances.

Density fluctuations along the path of the solar radio waves can strongly affect the propagation and the properties of the detected type III bursts by means of frequency-dependent effects like scattering. Due to the scattering, the intensity-time profiles of a burst are characterized by a very fast rising phase followed by a long-lasting exponential decrease. Since decay times are directly related to the scattering, their analysis provides useful information about this process.

When the measured decay time τ is represented as a function of frequency, it follows a power law f ^−β, where β= -0.970±0.003 (Kontar et al., 2019). However, a data gap, marking the separation between measurements from space and on ground, is present in the range 3-13 MHz due to the lack of temporally resolved measurements. This range is only accessible from space as the Earth’s ionosphere partially reflects and absorbs the signals below ∼10 MHz. Accurate decay time measurements in this frequency range are therefore needed to confirm the expected trend and characterize, through observational data, the scattering in the radial distance belt between 2 and 5 R_☉ that currently remains unexplored. The few observations present in the literature (Hartz, 1964a, 1964b; Boischot, 1960,1967; Jebarai, 2023), performed with sampling time larger than 3.5 s, found quite different β values due to the various different low temporal resolutions of the data sets, not allowing for the measurements to be resolved. A sufficiently high temporal resolution (lower than 0.5s) is indeed needed to properly sample signals with expected decay times of the order of 1-10 s.

The SO/RPW/HFR Observations

The High Frequency Receiver (HFR) (Maksimovic et al., 2021; Vecchio et al, 2021) of the RPW instrument on board Solar Orbiter was configured to acquire five frequencies [3.225, 5.225, 6.875,10.125, 12.225] MHz for ten times followed by a sweep on 50 frequences between 0.425 and 16.325 MHz. This novel configuration, including performing an average on the lowest possible number of spectra and measuring at only one antenna, allows to reach, for each of the five frequencies, a time resolution of ∼0.07s and an average resolution of∼0.18 s, significantly better than previous measurements.

The dataset obtained over about 13 months of observation, is unique in the framework of space observations since the achieved sample time is up to 2 orders of magnitude higher than any other spacecraft measurements. By analyzing HFR power spectral densities more than 350 type III bursts have been identified. Decay times were obtained through an exponential fit on the data. The large number of detected events allowed to statistically characterize the decay time in the range 3-13 MHz.

Conclusions

1. For each of the considered frequency the decay time does not depend on the radial distance of the spacecraft but only on the frequency only (Figure 1 a).
2. The time resolution of the data set is the decisive factor for the proper measurement of decay times and the accurate determination of the τ spectral index in the considered frequency range. The time resolution of the measurements used in previous works was insufficient to accurately characterize the decay time at frequencies higher than 6 MHz since the sampling time is comparable to the expected decay time (Figure 1 b).
3. HFR measurements allowed to fully characterize the τ behavior as a function of frequency in the range 3-13 MHz and to fill the long-standing gap in the observations of type III burst decay times. Our observations show that the τ-f power law trend does not change in the radial distance range 2-5 R_⊙, and a spectral index β=-1 is representative in the full 1-100 R_⊙range (Figure 2).

Figure 1: a) Decay time as a function of the radial distance for the five considered frequencies. No radial distance dependence is observed. b) Decay times for the five considered frequencies obtained as the median value from the sample of measurements. Red: full time resolution original data set; black: data set with time resolution reduced to 3.5 s; green: PSP data set with 3.5 s resolution. Error bars represent the standard error. The black, red, and lime lines correspond to the power-law fit on the respective five data points. The blue dashed line shows the power-law function with β= -0.970±0 from Equation 1 of Kontar et al. (2019)]. The discrepancy with the blue line increases when the time resolution of the data set decreases, and a flatter trend is obtained. Figures from Vecchio et al. (2024).

Figure 2:  Median decay time values from measured type III bursts in the frequency range of 3–13 MHz (shaded region), superimposed on the data shown in Figure 10 of Kontar et al (2019) and the newly added data from Chrysaphi et al (2024). Error bars represent the standard error obtained from observations. The best-fit function is also printed. Figure from Vecchio et al. (2024).

Based on a recent paper by Antonio Vecchio, et al Temporally resolved Type III solar radio bursts in the frequency range 3-13 MHz, ApJL, 974, L18, 2024. DOI: https://doi.org/10.3847/2041-8213/ad7bbb

References

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Chrysaphi, N., Maksimovic, M., Kontar, E. P., et al. 2024, A&A, 687, L12

Hartz, T. R. 1964a, Annales d’Astrophysique, 27, 831; Hartz, T. R. 1964b, Annales d’Astrophysique, 27, 823

Jebaraj, I. C., Krasnoselskikh, V., Pulupa, et al., 2023, ApJL, 955, L20

Kontar, E. P., Chen, X., Chrysaphi, N., et al., 2019,  ApJ, 884, 122

Maksimovic, M., Souček, J., Chust, T., et al. 2021, A&A, 656, A41

Vecchio, A., Maksimovic, M., Krupar, V., et al. 2021, A&A, 656, A33