Radio signature of a distant behind-the-limb CME on 2017 September 6

TL;DR

This study analyzes a Type IV radio burst from a powerful solar flare and CME on 2017 September 6, revealing its properties, directivity, and plasma density distribution, with implications for Parker Solar Probe observations.

Contribution

It provides the first detailed analysis of a distant behind-the-limb Type IV burst and estimates plasma density and mass of the CME core using radio observations.

Findings

Type IV burst observed at 5-35 MHz for over 2 hours.

Radio emission directivity of Type IV bursts is wide.

Estimated plasma density in CME core at 5.6 and 9.8 Rs.

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 NDA 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. 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, its longitudinal angle was about 160 degrees. 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 and its mass to be about 10^16 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.