Earth’s magnetism: During secular variation of the magnetic meridian, how do the oscillation characteristics (range and period) behave at different places on Earth?

Introduction / Context:
Magnetic declination at a given location changes slowly over years and decades, a phenomenon known as secular variation. Surveyors must understand these long-term trends to interpret historical bearings, update maps, and convert between magnetic and true directions reliably.

Given Data / Assumptions:

• Secular variation arises from changes in Earth’s core-generated geomagnetic field.
• Both magnitude (range) and timing (period) of oscillations may differ regionally.
• Short-term disturbances (diurnal, storms) are not the subject here.

Concept / Approach:

Secular variation is not globally uniform. Observatories record that the amplitude (range) of declination change and the time between successive extrema (period) vary with latitude, longitude, and epoch. Consequently, correction tables and isogonic charts are published with regional specificity, and survey data must consider the appropriate local curve of variation rather than a single global constant.

Step-by-Step Solution:

Verification / Alternative check:

Magnetic observatory records (e.g., IGRF/WMM models) show location-dependent drift rates and varying quasi-periodic behaviors, supporting the conclusion.

Why Other Options Are Wrong:

Any statement claiming constancy of range or period contradicts empirical records; asserting only one varies is incomplete.

Common Pitfalls:

Confusing secular variation with diurnal variation; assuming a single fixed annual correction applies everywhere.

Final Answer:

Range and period of oscillation both vary