A moving Type IV radio burst and its relation to the coronal magnetic field

A moving Type IV burst, observed with the Culgoora radioheliograph on 1970 April 29, moved out to about 3 R and attained high circular polarization before fading. The appearance of the moving Type IV source suggests an isolated, self-contained, synchrotron emitting plasmoid. Magnetic field maps of the corona derived from photospheric observations indicate that the plasmoid moved almost radially outward from the flare region along open field lines. To explain the observed source structure and high unipolar polarization, we suggest that a ring of electric current was ejected from the low corona and guided by coronal magnetic field lines; the radio emission was synchrotron radiation generated by mildly-relativistic electrons trapped in the poloidal magnetic field of the ring current.Part of the research reported here was carried out while the author was at the Division of Radiophysics, C.S.I.R.O., Sydney, Australia.The National Center for Atmospheric Research is sponsored by the National Science Foundation.