Topband: Back Scatter ?

[K1FZ-Bruce]

 Been working to optimize the F/B on some antennas, using ground wave stations in the upper BCB band. Noticed that some weaker ~20 mile distant 5 KW stations that the back (reverse) signal
has QSB on them.  Think we used to call it back scatter years ago. Some are down in the noise and QSB up to about 1 S unit. 
 
We used to think back scatter was reflective variations in the ionosphere.   Is there any more recent information ?  Maybe carbon in varying cloud layers ?
 
73
Bruce-k1fz
 
http://www.qsl.net/k1fz/beverage_antenna.html