Radio continuum emission in the halo of NGC 4217. Left: JVLA 3 GHz radio continuum emission at 7″ angular resolution, shown as the grey-scale background map. Contour lines show the 2, 3, and 5σ levels at 144 MHz (red, green, and blue) observed with LOFAR at 20″ angular resolution (indicated by the filled circle in the bottom-left corner). Confusing background sources have been subtracted from the 20″ map. Right: LOFAR radio continuum emission at 144 MHz imaged with enhanced diffuse extended emission at 20″ angular resolution (for details see Sect. 2.2). The contour line is at 320 μJy beam⁻¹ with a resolution of 40″, equating to 0.8σ. Confusing background sources have been subtracted. The coloured emission displays polarised intensity at ≈9″ resolution at 3 GHz observed with JVLA, and the corresponding vectors show the orientation of the magnetic field (the vector length is proportional to the polarised intensity). Thick grey lines show the assumed shape of the CRE transport in the superbubble (Sect. 3.4).