I have a point that is located right above a long straight wire with a current towards the right.
...............................x(P)

-------------------->-----------------------------
.......................I

Now, my question is the following: If the magnetic field at P decreases, but the direction remains the same, was this caused by a increase in the current, a reversing on the direction of the current , moving P farther away from the wire, moving P closer , moving P below the wire with same distance, or any other reason? I think that it would be due to the fact that P is moving further away, but I need an explanation and how can we know the strength of the magnetic field?