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.

If magnitude I a child of spreadAngle and not one of its properties consider using FindFirstChild :)

...if that don't work idk just a passing though :P