== Better Way ==

1. Define Scaler Mesh expression 

backFlow=min(0,dot({px,py,pz},normalize((coord(Mesh)-pos_p)))) 

2. Define radius from the center of Planet
rp=magnitude((coord(Mesh)-pos_p))

3. Add Operators -> Slicing -> Isosurface
   Select by Values
   Variable as "rp"

4. Do query -> Weighted Variable Sum

== Steps to get backflow flux ==

1. define expression
 backFlow=2*3.1415926*x*rho*vy

2. pseudocolor plot for variable backFlow

3. line plot of backFlow

4. run backFlow.py in commands. It will save the time and backflowflux to picked.txt