The BY is just as green, and really high, I don't see the drop, at least not on the last 6 hour image. As to what the by means I don't know. I tried to figure that out once, and could find little on it. I have not been able to piece together any particular meaning on it from watching this stuff. so i ignore it. I have read what it measures on the xyz axis of data but I still don't know what it means.

a negative bz means the magnetosphere is open and the stuff is coming right on in and that fits with the other data, like what is always on the AE index.