Magnitude cannot be assigned to(??)

Ok, basically, I'm trying to make the rocket plume extend as you go up. This is the code:

                if script.Parent.PrimaryPart.Position.Y >300 then
                    while script.Parent.Plume.ParticleEmitter.SpreadAngle.Unit ~= NumberRange.new(100,100) do
                        script.Parent.Plume.ParticleEmitter.SpreadAngle.Magnitude = Vector2.new(0 +.1,0 +.1)
                    end
                end

(the "go up" part is in another section)

Please help me with this. I'm back in scripting after almost 4-6 months of playing some games, so state reasons i got it wrong even if it is the most obvious thing in the world. I should also say i'm making use of some commands i never did before, so this is all new for me.