Abstract
Abstract Eruptive activity in the solar corona can often lead to the propagation of shock waves. In the radio domain the primary signature of such shocks are type II radio bursts, observed in dynamic spectra as bands of emission slowly drifting toward lower frequencies over time. These radio bursts can sometimes have an inhomogeneous and fragmented fine structure, but the cause of this fine structure is currently unclear. Here we observe a type II radio burst on 2019 March 20th using the New Extension in Nançay Upgrading LOFAR, a radio interferometer observing between 10–85 MHz. We show that the distribution of size scales of density perturbations associated with the type II fine structure follows a power law with a spectral index in the range of α = −1.7 to −2.0, which closely matches the value of −5/3 expected of fully developed turbulence. We determine this turbulence to be upstream of the shock, in background coronal plasma at a heliocentric distance of ∼2 R ⊙. The observed inertial size scales of the turbulent density inhomogeneities range from ∼62 Mm to ∼209 km. This shows that type II fine structure and fragmentation can be due to shock propagation through an inhomogeneous and turbulent coronal plasma, and we discuss the implications of this on electron acceleration in the coronal shock.
Highlights
Coronal mass ejections (CMEs) are eruptions of magnetized plasma from the solar corona into the heliosphere
Our goal is to attempt to identify any evidence of turbulence being responsible for the substructure that we see in the type II burst fine structure
For this we search for a power-law distributions of the size scales associated with the inhomogeneity in the radio burst, similar to the analyses performed by Chen et al (2018) and Reid & Kontar (2021) for type III bursts
Summary
Coronal mass ejections (CMEs) are eruptions of magnetized plasma from the solar corona into the heliosphere These eruptions can drive shocks through the solar atmosphere, and the primary radio signature of such shocks are known as type II radio bursts (Nelson & Melrose 1985; Mann et al 1996). Type II bursts usually last tens of minutes and are characterized by bands of emission slowly drifting to lower frequencies over time. They can often show a fine structure, which sometimes has the appearance of fragmented, short duration, and narrow-band bursts of emission (Armatas et al 2019). Type II fragmentation may be important in the study of coronal turbulence, as well as the implications of turbulence on particle acceleration in the coronal shock (Guo & Giacalone 2010)
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