Geology 5660/6660 Applied Geophysics 28 Mar 2014 © A.R. Lowry 2014 For Mon 31 Mar: Burger (§7.4–7.6) Last Time: Earth’s Main Magnetic Field Earth’s Main Field derives from “core dynamo” … convective flow of Ni-Fe outer core + coriolis forces (  + feedback )  electrical current flow  magnetic field (  + feedback ) + single-crystal Fe inner core (  + feedback ) Mostly dipolar, oriented ~ (and precesses around) rotation axis; varies nonlinearly through time… Express the vector field at a point as either intensity H E, inclination i, declination or local H x, H y, H z components:

500 yrs before reversal mid-reversal500 yrs after reversal Glatzmeier modeling: Solid inner core magnetized opposite main field; forced to rotate by applied torque  precession (~0.2°/yr for real Earth) Inner core stabilizes field dipole; long time required to diffuse outer core field to inner core controls reversal timescale

Problem : If both Earth’s main field and crustal field are determined from surface measurements, how do we separate? Core field dominates long wavelengths (small n of spherical harmonics). We describe core field only out to n = 14–15 where it dominates the total field.

Measurement: Fluxgate magnetometer : Wire coils wound in opposite directions; these cancel & produce zero current in secondary coil in absence of external magnetic field, but if aligned with a field, one core reinforces, other counteracts external field resulting in a current. Gives intensity in the orientation of the coils. (Direction?)

Proton precession magnetometer : Bottle containing a hydrogen-rich fluid (distilled water or hydrocarbon) is surrounded by a wire coil. Current through the coil produces a strong magnetic field; protons align with field… Current is shut off & as protons realign with Earth’s magnetic field they precess at a frequency determined by magnetic field strength ( Hz/nT). So, measure frequency of the induced AC current and convert to a total field strength. (Lots of other types but these two are most commonly used for terrestrial geophysics!) Measurement:

Data Reduction: Remove/avoid all metal objects when collecting data!!! Keep magnetometer high off the ground to reduce “noise” Sensitive to variations in ionosphere, magnetosphere: Perform looping and correct for drift; don’t bother measuring during solar storms Correct for elevation if > a few hundred m (~0.03 nT/m) Horizontal position correction: Use WMM if latitude change is > a few hundred m (correction here ~6 nT/km) From ComponentField ValueSecular Variation Declination degrees -8.1 arcmin/year Inclination degrees -1.6 arcmin/year Horizontal Intensity10887 nT-15.8 nT/year North Component (x) nT -5.1 nT/year East Component (y) 4392 nT nT/year Vertical Intensity (z) nT nT/year Total Intensity nT-98.7 nT/year