"""
    Advection function along v

```math
\\frac{df}{dt} = e \\frac{df}{dv}
```

"""
function (adv :: AmpereAdvection)( fᵗ  :: Array{ComplexF64,2}, 
                                   e   :: Vector{ComplexF64}, 
                                   dt  :: Float64 )
    fft!(fᵗ, 1)
    fᵗ .= fᵗ .* exp.(-1im * dt * adv.kx * transpose(e))
    ifft!(fᵗ, 1)

end