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Eruptive activity in the solar corona can often lead to the propagation ofshock waves. In the radio domain the primary signature of such shocks are typeII radio bursts, observed in dynamic spectra as bands of emission slowlydrifting towards lower frequencies over time. These radio bursts can sometimeshave inhomogeneous and fragmented fine structure, but the cause of this finestructure is currently unclear. Here we observe a type II radio burst on2019-March-20th using the New Extension in Nan\c{c}ay Upgrading LOFAR(NenuFAR), a radio interferometer observing between 10-85 MHz. We show that thedistribution of size-scales of density perturbations associated with the typeII fine structure follows a power law with a spectral index in the range of$\alpha=-1.7$ to -2.0, which closely matches the value of $-5/3$ expected offully developed turbulence. We determine this turbulence to be upstream of theshock, in background coronal plasma at a heliocentric distance of $\sim$2R$_{\odot}$. The observed inertial size-scales of the turbulent densityinhomogeneities range from $\sim$62 Mm to $\sim$209 km. This shows that type IIfine structure and fragmentation can be due to shock propagation through aninhomogeneous and turbulent coronal plasma, and we discuss the implications ofthis on electron acceleration in the coronal shock.

Observations of shock propagation through turbulent plasma in the solar corona