Magnetic field nomenclature

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Margaret Campbell
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Magnetic field nomenclature Declination trend angle between horizontal projection of F and true North Inclination plunge angle between horizontal and F

1 Magnetic field nomenclature Declination trend angle between horizontal projection of F and true North Inclination plunge angle between horizontal and F Magnetic equator location on surface where field lines have and inclination of zero Magnetic poles location on surface where field lines have an inclination of ±90

2 Earth s magnetic field 90% dipole nearly parallel with rotational axis 10% non-dipole ~60,000 nt strength Gary Glatzmaier

3 Map of inclination (angle in degrees up or down that magnetic field vector is from the horizontal) at

5 Map of total intensity at

of dipole component Antipodal with south pole Magnetic north pole

7 Interesting Trivia about the North Pole Geographic north pole North end of axis around which earth spins Geomagnetic north pole North end of dipole component Antipodal with south pole Magnetic north pole Defined by 90 inclination Not antipodal with south pole Moves around daily

8 Components of Earth s Field Main field Generated by the outer core Varies over time such that new maps are needed every 5 years External field Generated by ionosphere/solar wind interactions Varies rapidly Crustal field Caused by magnetism of rocks including induction and remanent magnetism

9 Main Field Generated by the outer core Varies over human and geologic time Values can be obtained from NOAA calculator (

electrical currents in ionosphere create magnetic fields https://science.nasa.

10 magnetosphere External Field Charged particles from sun and interstellar space interact with ionosphere and magnetosphere Ionosphere (top of atmosphere) ionized by solar radiation Flowing electrical currents in ionosphere create magnetic fields

11 Crustal Field Caused by magnetic properties of rocks in the crust Does not change on human time scale

12 Temporal Variations Secular long term changes Diurnal 24 hour variations Magnetic storms variable timing Generally short lived (minutes to hours) Effects can linger in magnetosphere for days to weeks

Secular variations Changes caused by rapid

13 Secular variations Changes caused by rapid convection in outer core Needs to be observed every 5 years for precise navigation c_north_pole_positions_2015.svg Geomagnetic Observatories

0 Map of predicted annual rate of change of total

14 Secular variations Includes changes in direction and intensity Map of total intensity at Map of predicted annual rate of change of total intensity for

Diurnal variations Caused by convection in the ionosphere

from equator to midlatitudes Moving ions create magnetic

http://www.spaceweather.gc.ca/images/diurnal2.

15 Diurnal variations Caused by convection in the ionosphere Sun heats day side causing ionosphere Atmosphere convects from equator to midlatitudes Moving ions create magnetic field Field is fixed on day-side as earth rotates if

16 Diurnal variations Occur on a time scale similar to a magnetic survey (days) Variations (~20nT) are significant compared to magnetic anomalies of interest Must be corrected for Scheme is similar to correction for tides on gravity Generally use two magnetometers with one at base station

17 Magnetic Storms Caused by efficient exchange of energy between solar wind and magnetosphere Creates large transients in magnetic field Cannot be corrected for 4L0 4:33 XyZD2vQ 0:42

18 Magnetic Storms Magnetic storms variable timing arrive several days to 18 hours after we see them leave the sun Largest ones associated with coronal mass ejections

Magnetometer Device used to measure aspects of magnetic field Orientation Total field strength Field strength of one or all components Mechanical Compass Dip needle Electronic Fluxgate

19 Magnetometer Device used to measure aspects of magnetic field Orientation Total field strength Field strength of one or all components Mechanical Compass Dip needle Electronic Fluxgate magnetometer Proton precession Alkali vapor (Cs, K) Used for Geophysical surveys Treasure hunting & archeology geolocation optically pumped Potassium magnetometer

20 Fluxgate magnetometer Opposing electromagnets Small external fields causes current in secondary coil Measures external field component parallel to cores Resolution nt Does not experience instrument drift

21 Proton Precession Magnetometer Coil wrapped around container with liquid containing protons H 2 O or organic fluid DC current in coil orients spins of protons Oriented protons will precess in Earth s field when current is turned off Precessing protons create measureable AC current in coil

22 Proton Precession Magnetometer Measures total field strength Resolution of ~0.1 nt No instrument drift Orientation does not need to be precise Can t be used if magnetic gradients (>600nT/m) are present

23 Total Field Measurement Combination of Earth s main field Local induced field Regional crustal field External field

24 Total Field Measurement Anomalous field (F a ) small compared to main field (F e ) Total field direction essentially unchanged Therefore total field strength measurement sufficient

Intro to magnetic methods

Environmental and Exploration Geophysics I Intro to magnetic methods tom.h.wilson tom.wilson@mail.wvu.edu Department of Geology and Geography West Virginia University Morgantown, WV Items on the list 6.5