A geologist measures the inclination (dip angle) of Earth's magnetic field at a surface location and finds it is nearly horizontal — approximately 5°. Where is she most likely located?
ANear a magnetic pole, where field lines are nearly vertical
BNear the magnetic equator, where field lines are parallel to the surface
CIn the northern hemisphere at mid-latitudes, where inclination is typically 45–70°
DAt an anomalous location where non-dipole components dominate
The dipole model predicts that field line inclination varies systematically with magnetic latitude: at the magnetic poles, inclination is ±90° (field lines point straight down or up); at the magnetic equator, inclination is 0° (field lines are horizontal). A nearly horizontal field (inclination ≈ 5°) indicates a location near the magnetic equator. This relationship is captured by the equation tan(I) = 2 tan(λ), where I is inclination and λ is magnetic latitude — the same relationship used in paleomagnetism to reconstruct ancient plate positions.
Question 2 Multiple Choice
Earth's magnetic dipole model accounts for roughly 90% of the observed surface field. What does the remaining ~10% represent, and what does it tell us?
AThe angular offset between the geographic and magnetic poles, which accounts for the declination at every surface location
BHigher-order terms — quadrupole, octupole, and further — that describe regional departures from the simple dipole and change over time through secular variation
CThe contribution of crustal rocks to the total field, which is fixed and does not vary temporally
DInterference from the solar wind, which distorts the perfect dipole pattern near the surface
The real field is more complex than a pure dipole. Spherical harmonic analysis decomposes the field into its components: the dipole (degree 1) dominates, but quadrupole, octupole, and higher-degree terms account for regional anomalies. These non-dipole features drift and change over decades to centuries — a phenomenon called secular variation — reflecting shifting convection patterns in the outer core. Navigators have tracked declination changes for centuries because these regional non-dipole features cause magnetic north to wander noticeably over human timescales.
Question 3 True / False
Because Earth's magnetic dipole axis is tilted about 11° from the geographic rotation axis, magnetic declination (the angle between true north and magnetic north) varies depending on where you are on Earth's surface.
TTrue
FFalse
Answer: True
The tilt of the magnetic dipole means that magnetic north and geographic north only coincide along a line (the agonic line) where both poles happen to be on the same meridian. At all other locations, a compass needle points toward magnetic north, which is offset from true north by the declination angle. Declination ranges from near zero in some regions to 20° or more in others, and it changes slowly over time due to secular variation. Accurate navigation requires knowing and correcting for the local declination.
Question 4 True / False
Earth's magnetic field polarity has been stable throughout geologic history, with the north magnetic pole generally located near the geographic north pole.
TTrue
FFalse
Answer: False
Paleomagnetic evidence shows that Earth's field has reversed polarity hundreds of times throughout geologic history — the north and south magnetic poles swap. These reversals happen at irregular intervals averaging roughly every 200,000–300,000 years, though some stable polarity intervals (chrons) have lasted tens of millions of years. The record of reversals is preserved in magnetized rocks and is the basis for magnetostratigraphy. We are currently in the Brunhes Normal Chron (polarity like today), which began about 780,000 years ago.
Question 5 Short Answer
Earth's interior is too hot for a permanent bar magnet to maintain Earth's magnetic field. Explain what actually generates the field and why a solid permanent magnet deep in the Earth is impossible.
Think about your answer, then reveal below.
Model answer: Earth's field is generated by the geodynamo: convecting electrically conducting liquid iron in the outer core produces electric currents, which generate a magnetic field that in turn organizes the fluid flow — a self-sustaining feedback loop. A permanent bar magnet is impossible because Earth's deep interior temperature far exceeds the Curie temperature of iron (~770°C for iron, but the outer core is ~3,000–5,000°C). Above the Curie temperature, thermal agitation destroys the magnetic ordering of atomic magnetic moments, making permanent magnetism impossible regardless of the material.
This is a fundamental misconception worth correcting. The geodynamo requires the outer core to be liquid (for convection) and electrically conducting (to carry currents). The inner core is solid iron but does not maintain permanent magnetism for the same reason — it is also above the Curie temperature. The field is dynamic and self-generated, which is why it can undergo secular variation and polarity reversals that a permanent magnet could never produce.