TYPE IV BURSTS AT FREQUENCIES 10-30 MHz

1. TYPE IV BURSTS AT FREQUENCIES 10-30 MHz V.N. Melnik (1), H.O. Rucker (2), A.A. Konovalenko (1), E.P. Abranin (1), V.V. Dorovskyy(1), A. A. Stanislavskyy (1), A. Lecacheux (3) Institute of Radio Astronomy, Ukrainian Academy of Sciences, Kharkov, Ukraine; Space Research Institute, Austrian Academy of Sciences, Graz, Austria; (3) Department de Radioastronomie,Observatoire de Paris, Paris, France I N T R O D U C T I O N Type IV bursts were identified by Boishot (1957) as continuum radio emission after flares and connected with CME. These bursts were observed from 20MHz up to meter and decimeter wavelengths and continued to microwaves (Chernov, 2007). There are stationary and moving Type IV bursts (Stewart, 1985). These bursts are rare phenomenon. Duration of Type IV bursts is about 30 min at 80 MHz and about 2 hours at lower frequencies. At any frequency the flux density and the source brightness temperature increase with time for several minutes and then slowly decay. At the same time the degree of circular polarization keeps increasing until the very end of the event and often reaches >90%. Brightness temperature of radio source reaches at 43 MHz and at 80 MHz (Stewart et al., 1978). Type IV source velocities are approximately constant and change from 200 km/s to 1600 km/s. Distribution of their velocities is similar to that for CME. It seems that Type IV bursts are connected with shocks, which responsible for Type II bursts. Observations at Nancay at 169 MHz (Trottet et al., 1981) show that Type IV bursts had pulsations with period about 2 s. Sometimes these bursts reveal fiber bursts and zebra patterns (Stewart, 1985). Radio emission of Type IV bursts is considered to come out from regions with closed magnetic structures (Smerd, and Dulk, 1971) and can be understood in the frame of plasma mechanism (Duncan, 1981). In this report the first results of Type IV bursts observations at UTR-2 radio telescope (Kharkov, Ukraine) on July, 2005 at frequencies 10-30 MHz are presented. Main observational properties of decameter Type IV bursts are discussed. O B S E R V A T I O N S Decameter Type IV bursts discussed in the paper were registered at radio telescope UTR-2 during summer 2005 campaign. Three sections of radio telescope, that is equivalent to effective area 30000 m2, were used. It provided the beam, 10x130. The 60-channel spectrometer with time resolution selectable up to 10 ms and a frequency resolution fixed at 300 kHz in the frequency band 10-30MHz was used. Fig.2. The time profile of Type IV burst at 25MHz. Fig.1. Type IV burst on July, 23, 2005. We observed some Type IV bursts at the end of July. The first one was registered on July, 23 (Fig.1). It consists of 2 parts - the first one from 10.00 UT to about 11.30 UT and the second one from 11.30 to 11.50 UT. The first one drifts with the rate about 10 kHz/s from high frequencies to low frequencies. We see also Type II burst against Type IV burst background, which drifts with the same frequency drift rate. The frequency profile of this part (Fig.3.) shows a fast decrease of flux from 10 s.f.u. at 25 MHz to about 1 s.f.u. at 10 MHz. Type IV burst (Fig.1) consists of a large number of sub-bursts with drift rates about 1-2 MHz/s and durations about 10 s. Good time resolution (Fig.4-5) shows that radio emission is continuum with small time variations. The second part of Type IV burst (at 11.30-11.50 UT) does not practically drift. Its flux is only about 2-3 s.f.u. Fig. 4. A fragment of Type IV dynamic spectrum Fig.3. Frequency profile of Type IV burst at 11.00.00 UT. Fig.5. A fragment of Type IV time profile