Geomagnetic Field

Audience

Difficulty	Introductory

Summary

The geomagnetic field is the Earth's planetary magnetic field, generated primarily by the geodynamo — convection of electrically conducting iron-nickel fluid in the Earth's outer core. Field intensity at the surface ranges from ~ 25 to ~ 65 μT (25,000-65,000 nT) depending on latitude. The field is approximately dipolar with axis tilted ~ 11° from the rotation axis; substantial non-dipole components (especially the South Atlantic Anomaly) make the field structurally complex.

Structure

• Inner dipole (~ 90% of field): generated by core dynamo, slowly varying on decadal-to-millennial timescales.
• Non-dipole components: complex multipole structure including the South Atlantic Anomaly (depressed field over the South Atlantic / South America).
• External components (magnetosphere, ring currents, ionospheric currents): variable on minutes-to-days timescales, driven by solar-wind interactions.

Magnetosphere

Outside the surface field is the magnetosphere — the region of space where the geomagnetic field dominates over the solar-wind magnetic field. The magnetosphere extends ~ 10 Earth radii sunward (bow shock) and trails out into a long magnetotail on the night side. The magnetosphere shields the Earth from much of the direct solar-wind plasma.

Temporal Variability

The geomagnetic field varies on multiple timescales:

• Diurnal variation (~ 10-100 nT): ionospheric solar-quiet (Sq) current system, daily cycle.
• Geomagnetic storms (10-1000 nT): driven by solar-wind / CME / coronal-hole interactions; quantified by Kp, Dst, AE indices (see Geomagnetic_Indices_Kp_Dst).
• Solar-cycle modulation (~ 11 years): storm frequency and intensity vary with solar activity.
• Secular variation (decades to centuries): westward drift of the field configuration; pole positions migrate measurably year-to-year.
• Geomagnetic reversals (~ 100,000-1,000,000 years): irregular polarity reversals of the dipole, documented in the geological record.

Biological Effects

Biological effects of geomagnetic-field variation are well-established for:

• Magnetic navigation in many migratory species (birds, sea turtles, salmon, certain insects). The mechanisms involve cryptochrome-based radical-pair magnetoreception and / or magnetite-based detection.
• Diurnal and seasonal rhythms in many species correlate with geomagnetic state.

Effects on human physiology and cognition are documented but smaller and more contested:

• Geomagnetic-storm correlations with mortality, accident rates, psychiatric admissions reported in multiple epidemiological studies; effect sizes small, mechanisms incompletely understood.
• Persinger et al. reported correlations between geomagnetic activity and reports of anomalous experiences.

Psionic Relevance

In the psionic framework, the geomagnetic field is one of the principal environmental modulators that could affect ψ-field coupling to biological systems. The framework's empirical predictions include detectable correlations between geomagnetic indices and ψ-related phenomena (consistent with the Persinger / Nelson / etc. correlations).

See Also

• Geomagnetic_Indices_Kp_Dst
• Space_Weather
• Schumann_Resonance
• Earth-Ionosphere_Cavity
• Solar_Cycles

External Links

• Wikipedia: Earth's magnetic field
• NOAA Space Weather Prediction Center (swpc.noaa.gov).

References

• Merrill, R. T., McElhinny, M. W., McFadden, P. L. (1996). The Magnetic Field of the Earth. Academic Press.