Soon after starting the MEG electronics, as they warm up, there is sometimes large noise from the system itself. We regularly see this during the empty-room noise recordings, and have to wait a few minutes and redo the noise recording. As such, please always plan to start the electronics 5-10 minutes before you collect the empty-room recording and if you do see the artefact, wait 2-3 minutes for it to disappear. As always, keep me informed of anything new or unusual, like if this startup noise lasts more than a few minutes.
The artefact is described further in this post.