Thanks this [Nagel paper] might help.
As you see, the signal comes at regular intervals, once a day. If you can trigger the effect at irregular intervals, I think that would rule out cosmic rays as the source of your signal. By "irregular interval" I mean different times of day, separated by hours. Such as 9 a.m. one day, and 4 p.m. the next day. Or, if you can do it, twice the same day.