Wild (1950) was the first to register and describe Type III bursts properties. These bursts are tracks of radio emission, which drift from high frequencies to low frequencies. Profiles of Type III bursts have a quick rise and slower fall. Wild supposed that electrons with high velocities, up to 0.3c (c is the speed of light), were responsible for Type III bursts based on their high frequency drift rates. Ginzburg and Zhelezniakov (1958) suggested plasma mechanism of Type III burst radio emission, according to which at the first stage fast electrons generate Langmuir waves and at the second stage these waves are transformed into electromagnetic waves. Langmuir waves also influence the fast electron propagation and this propagation becomes a nonlinear one. The gas-dynamic theory of such phenomenon (Melnik, 1995) was proposed in 1995. According to this theory electrons move through plasma in the form of a beam-plasma structure, which consists of fast electrons and Langmuir waves. In the case of a single initial electron beam only one beam-plasma structure is formed, which is the source of radio emission in the form of Type III burst. If two electron beams are injected into the plasma then electrons propagate as a single beam-plasma structure at first and then at some distance it decays into two beam-plasma structures (Melnik and Kontar, 1998). In the radio emission it looks like a decay of Type III burst into two Type III bursts. Such a process was predicted in 1998. In this paper the first observation of such a decay is reported, parameters of Type III bursts and their sources are discussed in the gas-dynamic theory.

**Observation ****and interpretation**

A decaying Type III burst was observed on 2017 August 18 with radio telescopes GURT (frequency band of 8-70 MHz) (Figure 1a) and URAN-2 (frequency band of 8-33 MHz) (Figure 1b).

**Figure 1 –** Decaying Type III burst and newborn Type III bursts according to GURT (8-80 MHz) a) and URAN-2 (8-32 MHz) b). Profiles of decaying Type III burst and two newborn Type III bursts according to GURT (32-72 MHz) and URAN-2 (8-32 MHz) data. Each profile is normalized by a maximum flux at a given frequency (с).

Profiles of the decaying Type III burst and newborn Type III bursts are presented in the Figure 1с. We see that Type III burst drifted from high to low frequencies with a drift rate of about 4.4 MHz s^{-1} at 32 MHz and it decayed into new two Type III bursts in the frequency range of 30-35 MHz (Figure 2). These newborn Type III bursts drifted from high to low frequencies as well but with smaller drift rates, 2.7 and 1.7 MHz s^{-1} respectively, at frequency 32 MHz.

Their radio fluxes increased from 50 s.f.u. at 30 MHz to 1400 s.f.u. at 9 MHz. The full width half maximum (FWHM) duration of decaying Type III burst was 8.3 s at 35 MHz and durations of newborn Type III bursts at level of 90% were 3-3.6 s at 30 MHz. According to URAN-2 data polarizations of these bursts were not higher than 5%.

**Figure 2.** Tracks of flux maximums of Type III bursts (a) and their drift rates on frequency (b) according to GURT (32 -75 MHz) and URAN-2 (9-32 MHz) data.

In the gas-dynamic theory for the case of injection of two electron beams with velocities \(U_1\)>and \(U_2 > U_1\) in the initial point \(x=0\) (flare region)

\[ f(V, x = 0, t) = [n_{1} \delta (V – U_1) + n_{2} \delta (V – U_{2})] \exp (-t/\tau)\]

at the condition

\[\frac{n_1} {U_1} > \frac {n_2} {U_2 – U_1}\]

electrons at first propagate through plasma in the form of a single beam-plasma structure with constant velocity (Melnik and Kontar, 1998) \(V_{decay}= 1.03 \cdot 10^{10} \text{cm } \text{s}^{-1}\).

At the distance

\[x^* = \frac {(u_1 + u_2)(2 U_2 – U_1) \tau} {2 U_2} \ln \left( \frac {n_1(U_2 – U_1)} {n_2 U_1}\right) \tag{1} \]

this beam-plasma structure decays into two ones propagating with velocities \(V_1 = U_1 / 2\) and \(V_2 = U_2 / 2\).

As far as each beam-plasma structure is a source of radio emission in the form of Type III burst then decay of Type III burst into two Type III bursts with smaller drift rates must be observed at height \(x > x^*\).

In our case taking into account that newborn Type III bursts were generated at the second harmonic in the Newkirk corona (Newkirk, 1961), we derived velocities of beam-plasma structures \(V_1 = 0.4 \cdot 10^{10} \text{cm } \text{s}^{-1}\) and \(V_2 = 0.63 \cdot 10^{10} \text{cm } \text{s}^{-1}\). According to equation (1) the distance at which Type III burst decay happens equals \(R = 2.3R_s\) in the Newkirk model (from this distance radio emission of the second harmonic at 32 MHz is released) if the density ratio of slow and fast beam-plasma structures is \(n_1/n_2 \approx 3\). If radio emission of Type III bursts is fundamental then velocities of beam-plasma structures are \(V_1 = 0.23 \cdot 10^{10} \text{cm } \text{s}^{-1}\) and \(V_2 = 0.36 \cdot 10^{10} \text{cm } \text{s}^{-1}\) correspondingly, and decay distance is \(R = 1.74R_s\). The density ratio for this case is approximately the same \(n_1/n_2 \approx 3\).

**Conclusions**

The decay of the Type III burst into two Type III bursts has been observed for the first time. This was made possible due to wide frequency observations from 8 to 70 MHz, high sensitivity and high frequency-time resolutions of the radio telescopes GURT and URAN-2. Velocities of Type III sources, both decaying and newborn ones, turned out to be close to standard values for Type III bursts in the case of generation of bursts at the fundamental and harmonic frequencies. The phenomena of Type III burst decay is one more evidence in the favor of gas-dynamic theory of fast electron propagation through plasma.

Based on a recent paper V. N. Melnik *et al,* First Observation of the Solar Type III Burst Decay and Its Interpretation, *The Astrophysical Journal,* **885, **78 (2019) DOI:10.3847/1538-4357/ab46aa

**References**

Newkirk, G. Jr. 1961, ApJ 133, 983

Mel’nik, V. N.: 1995, Plasma Physics Reports, 21, 89-91.

Melnik, V. N., Kontar, E. P.: 1998, Physica Scripta, 58, 510-517.

**Full list of authors: **Valentin Melnik, Alexander Konovalenko, Sergey Yerin, Igor Bubnov, Anatoliy Brazhenko, Anatoliy Frantsuzenko, Vladimir Dorovskyy, Mykola Shevchuk, and Helmut Rucker