Radio Signature of a Distant behind-the-limb CME on 2017 September 6

Abstract

Abstract We discuss properties of a Type IV burst, which was observed on 2017 September 6, as a result of the powerful flare X 9.3. At decameter wavelengths this burst was observed by the radio telescopes STEREO A, URAN-2, and the Nancay Decameter Array at frequencies 5–35 MHz. This moving Type IV burst was associated with a coronal mass ejection (CME) propagating in the southwest direction with a speed of 1570 km s−1. The maximum radio flux of this burst was about 300 s.f.u. and the polarization was more than 40%. In the frequency range of 8–33 MHz it continued for more than 2 hr. For STEREO A the associated CME was behind the limb, and its longitudinal angle was about 160°. This moving Type IV burst was observed by STEREO A at frequencies of 5–15 MHz in spite of the low sensitivity of STEREO A. This means that the radio emission directivity of a Type IV burst is rather wide. Assuming the plasma mechanism of Type IV radio emission we derived the plasma density distribution in the CME core at distances of 5.6 Rs and 9.8 Rs (Rs is the solar radius), and its mass to be about 1016 g. It is planned that the minimum perihelion of the Parker Solar Probe (PSP) spacecraft will be at about 9 Rs . So we discuss in what conditions PSP will be in if it crosses a similar CME core.