science

Geomagnetic secular variation

Geomagnetic secular variation is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Geomagnetic secular variation rather than just read about it. In short: Geomagnetic secular variation is the changes in the Earth's magnetic field on time scales of about a year or more. These changes mostly reflect changes in the Earth's interior, while more rapid changes mostly originate in the ionosphere or magnetosphere.

Key takeaways

Geomagnetic secular variation belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Geomagnetic secular variation to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Geomagnetic secular variation from memory before moving on to harder problems.

Reference excerpt

Geomagnetic secular variation is the changes in the Earth's magnetic field on time scales of about a year or more. These changes mostly reflect changes in the Earth's interior, while more rapid changes mostly originate in the ionosphere or magnetosphere. The geomagnetic field changes on time scales from milliseconds to millions of years. Shorter time scales mostly arise from currents in the ionosphere and magnetosphere, and some changes can be traced to geomagnetic storms or daily variations in currents. Changes over time scales of a year or more mostly reflect changes in the Earth's interior, particularly the iron-rich core. These changes are referred to as secular variation. In most models, the secular variation is the amortized time derivative of the magnetic field B {\displaystyle \mathbf {B} } , B ˙ {\displaystyle {\dot {\mathbf {B} }}} . The second derivative, B ¨ {\displaystyle {\ddot {\mathbf {B} }}} is the secular acceleration.

Recent changes

Secular variation can be observed in measurements at magnetic observatories, some of which have been operating for hundreds of years (the Kew Observatory, for example). Over such a time scale, magnetic declination is observed to vary over tens of degrees. A movie on the right shows how global declinations have changed over the last few centuries. To analyze global patterns of change in the geomagnetic field, geophysicists fit the field data to a spherical harmonic expansion (see International Geomagnetic Reference Field). The terms in this expansion can be divided into a dipolar part, like the field around a bar magnet, and a non-dipolar part. The dipolar part dominates the geomagnetic field and determines the direction of the geomagnetic poles. The direction and intensity of the dipole change over time. Over the last two centuries the dipole strength has been decreasing at a rate of about 6.3% per century. At this rate of decrease, the field would reach zero in about 1600 years. However, this strength is about average for the last 7 thousand years, and the current rate of change is not unusual. A prominent feature in the non-dipolar part of the secular variation is a westward drift at a rate of about 0.2 degrees per year. This drift is not the same everywhere and has varied over time. The globally averaged drift has been westward since about 1400 AD but eastward between about 1000 AD and 1400 AD.

Paleomagnetic secular variation Changes that predate magnetic observatories are recorded in archaeological and geological materials. Such changes are referred to as paleomagnetic secular variation or paleosecular variation (PSV). The records typically include long periods of small change with occasional large changes reflecting geomagnetic excursions and geomagnetic reversals. The Levantine Iron Age anomaly was a fast and spatially localized geomagnetic positive anomaly which took place in the Levant, with maxima at about 950, 750 and 500 BCE.

See also Geomagnetic jerk Secular variation

Notes

References

Illustrations

Worked examples

Example 1 — a first encounter with Geomagnetic secular variation

Start with the simplest possible case. Write down what Geomagnetic secular variation claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Geomagnetic secular variation before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Geomagnetic secular variation ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Geomagnetic secular variation

In research: Geomagnetic secular variation appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Geomagnetic secular variation in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Geomagnetic secular variation is common in secondary-school and first-year university syllabi. It links to neighbouring topics Geomagnetism, so understanding it makes those chapters shorter.
In everyday life: Look for Geomagnetic secular variation outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

Ask Teacher Smith questions about this articleOpens your AI tutor with a question about “Geomagnetic secular variation” →

Affiliate

Preply — study more efficiently by working with a personal tutor. 50% off.

How to study Geomagnetic secular variation in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Geomagnetic secular variation means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Geomagnetic secular variation out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Geomagnetic secular variation in simple terms?

Why does Geomagnetic secular variation matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Geomagnetic secular variation?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Geomagnetic secular variation.