Solar U- and J-Bursts at the Decameter Waves

Abstract

The first solar U- and J-bursts observed with the UTR-2 radiotelescope at the decameter wavelengths in 2003–2004 are reported. During that period more than 50 J-bursts and only 7 U-bursts were recorded. It is the first case of ground based observations of J- and U-bursts with turning frequencies ranging between 10 and 25 MHz. The harmonic structure of J-bursts in the form of Jb–J pairs similar to well known Type IIIb–III pairs was first found. The mean harmonic ratio appeared to be 1.8. Also a group of J-bursts with unusual Turning Frequency Drift (TFD) of –2 kHz/s was detected. Such TFD corresponds to the coronal loop elevation velocity of about 60 km/s. Coronal loops with similar elevation velocities were also detected by SOHO/LASCO coronagraph in white light. The dynamic spectra of unusual U- and J-bursts are shown. Simplified model of the coronal loop in the form of semi-circle was created on the base of the U-burst dynamic spectrum and the Newkirk coronal density model. With this loop model the linear velocity of the source along the loop, the height of the turning frequency point and the geometrical size of the loop were calculated.