SPACE WEATHER — Direction of storm-time magnetic field perturbation. During a geomagnetic (magnetic) storm, the ring current produces a perturbation field that is, relative to Earth’s main dipole field at the surface, in which direction?

Introduction / Context:
Geomagnetic storms inject and intensify a westward ring current of charged particles around Earth. This current system creates its own magnetic field that modifies the observed field at Earth’s surface, tracked by indices such as Dst.

Given Data / Assumptions:

• We are comparing the storm-time perturbation field with the main geomagnetic field near the surface.
• Ring current is westward and encircles Earth near the equatorial plane.
• We consider the net effect on surface measurements.

Concept / Approach:
A westward ring current generates a magnetic field that opposes Earth’s internal dipole field at low latitudes. As a result, the total surface field magnitude decreases during storm main phases, which is why the Dst index turns more negative.

Step-by-Step Solution:
Identify driver: enhanced ring current during storm.Apply superposition: ring-current field counteracts Earth’s dipole at the surface.Observed outcome: depressed field strength (negative Dst).Therefore, the perturbation is in the opposite sense to the main field.

Verification / Alternative check:
Storm-time magnetometer records show a decrease in H-component at mid–low latitudes, confirming opposition to the main dipole field.

Why Other Options Are Wrong:
Same — would increase field strength, which contradicts observations.Southward/Northward — describe interplanetary magnetic field orientation, not the surface perturbation direction relative to the main field.Random — the effect is systematic, not random.

Common Pitfalls:
Confusing IMF southward orientation (a coupling condition) with the direction of the storm-time perturbation at Earth’s surface.

Final Answer:
opposite