How to do a quadratic Sphere Gradiant ?

Hello every one,
I am looking to make an Electric Dipole Propagation Pattern as such :



I have seen the ramps exemple but is shows only a linear gradiant.

Could you help me to achieve that effect on the torus ?

Thank you very much :)

I have seen the ramps exemple but is shows only a linear gradiant.
Could you help me to achieve that effect on the torus ?
Thank you very much :)

Here's an example for you.
torus_example.blend

Wouaw once again thank you for your quick answer Mikhail !
It's very clever how it's done ;)

Seb