Final answer:
Using the Right Hand Rule 2, a current-carrying wire with a current flowing southward, placed in the Earth's magnetic field with a horizontal component northward, experiences a magnetic force directed eastward.
Step-by-step explanation:
To determine the direction of a magnetic force experienced by a current-carrying wire that is immersed in the Earth's magnetic field, we can use the right-hand rule, commonly called the Right Hand Rule 2 (RHR-2). When a wire carries a current southward and is placed in a magnetic field directed northward, you would point your right thumb (representing the direction of the current) southward and your fingers (representing the magnetic field) northward.
The force, according to RHR-2, would then come out from the palm, which, in this case, would be directed eastward when considering the Earth's field is also pointing northward. If we also take into account Earth's magnetic field having a vertical component pointing downward, the net magnetic force acting on the wire is actually the result of both the horizontal and vertical components of Earth's field interacting with the current.
However, the question only asks about Earth's horizontal magnetic field, so the force felt by the wire would be perpendicular to both the direction of current (south) and the direction of the Earth's horizontal magnetic field (north), which using RHR-2, results in a force directed east.