Curious ripple in the data

The strange ripple in the dynamic spectrum of the solar radio event from a few weeks back has caught some interest, and we don't have an explanation for it... yet. At first, it was thought it might be an RFI effect, although it is very broad. It also drifts in frequency, which is unusual.

These data are from a single dipole. So, it might be coupling between that and other dipoles, or perhaps Cas A rippling through a sidelobe. For the UK station (which doesn't see the effect -- REF), these sources would be at different position angles. There are several things we have now checked:

We see it on different dipoles. These might have different couplings (due to antenna spacings) or be at different position angles (with respect to, say, a local RFI source).

We also see it on both polarisations. The following two plots are from different polarisations.

LBA aerial #L04, X-polarisation (Image: D. McKay-Bukowski)

LBA aerial #L04, Y-polarisation (Image: D. McKay-Bukowski)

We may still be seeing some sort of ground effect or perhaps a reflection from the reindeer fence. Alternatively, it might be a "snow interferometer" (like the old sea-interferometer, but with a reflection off the snow). The fact is that the sun is at quite a low elevation and the wavelengths are quite long at these frequency bands.

In any case, we remain puzzled. Any suggestions, anyone?