Geomagnetism – diurnal variation of magnetic declination: How does the daily (diurnal) variation of magnetic declination generally change with latitude and season?

Introduction / Context:
Magnetic declination (variation) is not constant: it changes with location, season, and even time of day. Surveyors should understand general trends so they can schedule compass work and apply corrections prudently. This question focuses on the broad behavior of diurnal variation with latitude and season.

Given Data / Assumptions:

• Diurnal variation refers to the cyclic change in declination over a day.
• Solar heating and ionospheric currents influence the Earth’s magnetic field.
• We consider general trends used in classical surveying guidance.

Concept / Approach:
Classical surveying texts note that diurnal variation is commonly more pronounced near the magnetic equator due to stronger ionospheric current systems (e.g., the equatorial electrojet), and that it usually increases in summer daylight hours because of enhanced solar activity and atmospheric conductivity. Hence, it is generally greater at or near the equator than near the poles and is typically larger in summer than in winter.

Step-by-Step Solution:

Verification / Alternative check:
Historical field recommendations suggest avoiding critical compass observations during midday in summer at low latitudes due to higher diurnal disturbances, reinforcing the chosen trend.

Why Other Options Are Wrong:

• Less at equator than nearer poles: opposite of the commonly observed trend for diurnal swings.
• Less in summer than in winter: seasonal relationship is typically the reverse.
• Same at all latitudes/months: magnetic behavior is location- and season-dependent.

Common Pitfalls:
Confusing long-term secular change with diurnal variation; overlooking local magnetic anomalies that may superimpose site-specific effects.

Final Answer:
It is greater at the equator than nearer the poles.