I would suggest you create this in 2 or even 3 layers. One as a shape with low opacity and Screen as blend mode (as mentioned above). Then, on a separate layer above that create a selection from the shape you initially drew, with the a generous feather radius. Fill the selection with white and set the blending mode to Linear Dodge (Add), then adjust the opacity until it resembles spreading light. Create a selection of the second layer and modify said selection (either expand or contract) and adjust the opacity again so it doesn't "wash out" too much.
I prefer this approach because it gives me more control over the amount of spread.