Earth's liquid outer core is highly electrically conductive. If convection in the outer core suddenly stopped — but the core remained liquid and conductive — what would happen to Earth's magnetic field over the next ~20,000 years?
AIt would remain roughly constant because the static liquid iron is still highly conductive
BIt would gradually decay to near zero through Ohmic dissipation
CIt would reverse polarity as the field is no longer being actively maintained
DIt would strengthen briefly because fluid motions had previously been opposing the field
Even a highly conductive static conductor cannot sustain a magnetic field indefinitely — electrical resistance dissipates the currents driving the field into heat. The induction equation shows that when fluid velocity v = 0, only the decay term (η/μ₀)∇²B remains. The estimated decay time for Earth's outer core is 10,000–20,000 years. Earth's field persisting for 3.5 billion years is direct evidence that convection continuously regenerates it. Option A confuses high conductivity with perfect conductivity — finite resistivity always causes decay, just more slowly.
Question 2 Multiple Choice
Which of the following best explains why Earth's rotation is essential to the geomagnetic dynamo?
ARotation generates the electrical charges needed to produce a magnetic field
BRotation keeps the inner core from melting, maintaining the temperature gradient that drives convection
CThe Coriolis force organizes convective flow into helical columns that provide the systematic twist needed for the α-effect
DRotation creates differential pressure that drives liquid iron outward from the inner core boundary
The Coriolis force imparts a systematic helical structure to convective motions — the α-effect — which twists toroidal field back into poloidal field, completing the self-sustaining regeneration cycle. Without rotation, convection would be turbulent but lack the organized helicity needed for this. All planetary dynamos require both a conducting fluid and significant rotation. Options A and D misidentify the mechanism entirely; option B confuses Earth's rotational dynamics with its thermal budget.
Question 3 True / False
Paleomagnetic reversals occur at regular, predictable intervals because they reflect a periodic instability in the dynamo.
TTrue
FFalse
Answer: False
Paleomagnetic reversals are NOT periodic — they emerge from the inherently chaotic and nonlinear dynamics of the dynamo. Intervals between reversals range from tens of thousands to tens of millions of years with no regularity. This is analogous to a chaotic pendulum that occasionally flips over its pivot: the event is possible, even inevitable over long times, but not scheduled. Treating reversals as periodic confuses a statistically recurrent but aperiodic phenomenon with a cyclic one.
Question 4 True / False
The geomagnetic dynamo is self-sustaining because fluid motions continuously regenerate the magnetic field from existing field, counteracting Ohmic decay.
TTrue
FFalse
Answer: True
This is exactly what the induction equation captures. The term ∇ × (v × B) represents stretching and amplification of existing field lines by fluid motion — the existing field B contributes to the currents that maintain it. When this induction term wins over the Ohmic decay term, the dynamo is self-sustaining. This is why the dynamo is described as a feedback system: it requires an initial seed field, but once established, it regenerates itself from its own output via the coupling between fluid velocity and magnetic field.
Question 5 Short Answer
Why must Earth's magnetic field be continuously regenerated, and what process accomplishes this?
Think about your answer, then reveal below.
Model answer: Any magnetic field in a conductor with finite resistivity will decay through Ohmic dissipation — electrical resistance converts current energy into heat. For Earth's outer core, the estimated decay time is ~10,000–20,000 years. Since Earth's field has persisted for at least 3.5 billion years, something must regenerate it continuously. That process is the MHD dynamo: convection-driven fluid motion in the liquid iron outer core stretches and amplifies magnetic field lines via the induction equation's ∇ × (v × B) term, regenerating the field faster than resistivity destroys it.
This is the fundamental motivation for dynamo theory. If Earth were a static ball of iron — no matter how conductive — its field would have decayed in geologically short timescales. The persistence of the geomagnetic field is the observational demand that the dynamo must meet. Understanding this also clarifies why the α-ω mechanism matters: it describes the specific organized fluid motions that make the regeneration efficient and self-sustaining.