Ráfagas de Radio Solar III temporarily resolved in the frequency range of 3-13 MHz by A. Vecchio et al. – European astronomer radio community

Radio type III bursts are the most common coherent radio emission produced by the Sun. They are characterized by a rapid drift over time towards lower frequencies and represent an indirect firm of energy electrons produced in the sun during a flare and spread through the crown plasma and the interplanetary medium. Type III bursts are observed in a wide range of frequencies ranging from approximately ∼500 MHz to tens of KHz about 1 au, corresponding to a wide range of heliocentric distances.

Density fluctuations along the path of solar radio waves can strongly affect the propagation and properties of type III explosions detected by means of frequency dependent effects such as dispersion. Due to the dispersion, the intensity time profiles of a burst are characterized by a very rapid increase phase followed by a long -term exponential decrease. Since decomposition times are directly related to dispersion, their analysis provides useful information about this process.

When the measured decomposition time τ is represented according to the frequency, a law of power follows F ^−βwhere β = -0.970±0.003 (Kontart et al., 2019). However, a data gap, which marks the separation between space and soil measurements, is present in the range of 3-13 MHz due to the lack of temporarily resolved measurements. This range can only be accessed from space, since the land ionosphere reflects and partially absorbs the signals below ∼10 MHz. Therefore, the precise decomposition time measurements in this frequency range are necessary to confirm The expected trend and characterize, through observation data, the dispersion in the radial distance strap between 2 and 5 r_☉ That currently remains unexplored. The few observations present in the literature (Hartz, 1964a1964b; Boischot, 1960,1967; Jebarai, 2023), made with a sampling time greater than 3.5 s, found quite different β Values ​​due to the different low temporary resolutions of the data sets, not allowing the measurements to be resolved. In fact, a highly enough temporary resolution is needed (less than 0.5s) to correctly test the signals with the expected decomposition times of the order of 1-10 s.

Observations SO/RPW/HFR

The high frequency receiver (HFR) (Maksimovic et al., 2021; Vecchio et al, 2021) of the RPW instrument aboard the 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 50 frequency sweep between 0.425 and 16,325 MHz. This new configuration, which includes making an average in the lowest possible number of spectra and measuring only in one antenna, allows to reach, for each of the five frequencies, a time resolution of ∼0.07 and an average resolution of 0.18 s, significantly better than the previous measures.

The data set obtained for approximately 13 months of observation is unique within the framework of space observations since the sample time is reached is up to 2 more high orders than any other measurement of the spaceship. When analyzing the spectral densities of HFF power, more than 350 bursts type III have been identified. The decomposition times were obtained through an exponential adjustment in the data. The large number of events detected are statistically characterized by the decomposition time in the range of 3-13 MHz.

Conclusions

1. For each of the frequency considered, the decomposition time does not depend on the radial distance of the spacecraft but only on frequency (Figure 1 a).
2. The time resolution of the data set is the decisive factor for the proper measurement of the decomposition times and the precise determination of the spectral index τ in the frequency range considered. The time resolution of the measurements used in previous works was insufficient to accurately characterize the decomposition time at frequencies greater than 6 MHz since the sampling time is comparable to the expected decomposition time (Figure 1 B).
3. HFR measurements are allowed to completely characterize the behavior τ based on the frequency in the range of 3-13 MHz and fill the long-standing vacuum in the observations of the decomposition times of Ráfaga Type III. Our observations show that the tendency of the power law τ-F does not change in the radial distance range 2-5 r_⊙and a spectral index β = -1 is representative in the complete 1-100 r_⊙range (Figure 2).

Figure 1: a) Decomposition time depending on the radio distance for the five frequencies considered. No dependence on radial distance is observed. b) Decomposition times for the five frequencies considered obtained as the average value of the measurement sample. Red: original full -time resolution set; Black: data set with time resolution reduced to 3.5 s; Green: PSP data set with a resolution of 3.5 s. Error bars represent the standard error. The black, red and lime lines correspond to the adjustment of the power law in the respective five data points. The blue discontinuous line shows the function of the power law with β = -0.970 ± 0 of equation 1 of Kontar et al. (2019)]Discrepancy with the blue line increases when the time resolution of the data set decreases and a flatter trend is obtained. Figures of Vecchio et al. (2024).

Figure 2: Average discompression time values ​​of type III explosions measured in the frequency range of 3 to 13 MHz (shaded region), superimposed on the data shown in Figure 10 of Kontar et al (2019) and the freshly added data of Chrysaphi et al (2024). The error bars represent the standard error obtained from the observations. The best adjustment function is also printed. Figure of Vecchio et al. (2024).

Based on a recent article by Antonio Vecchio, et al SOLAR RADIO RADIO RADY TYPE III temporarily resolved in the frequency range of 3-13 MHz, APJL, 974, L182024. DOI: https://doi.org/10.3847/2041-8213/ad7bbb

References

Boischot, A. 1967, Annals of Astrophysique, 30, 85; Boischot, A., Lee, Rh and Warwick, JW 1960, APJ, 131, 61

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

Hartz, Tr 1964a, Annals of Astrophysique, 27, 831; Hartz, Tr 1964b, Annals of Astrophysique, 27, 823

Jebaraj, IC, Krasnososkikh, V., Pulupa, et al., 2023, APJL, 955, L20

Kontar, EP, 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