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°
Earth’s magnetic field
 90%
dipole nearly parallel with rotational
axis
 10% non-dipole
 ~60,000 nT strength
Gary Glatzmaier
Map of inclination (angle in degrees up or down that magnetic field vector
is from the horizontal) at 2015.0
http://www.geomag.bgs.ac.uk/education/earthmag.html
Map of total intensity at 2015.0
http://www.geomag.bgs.ac.uk/education/earthmag.html
Interesting Trivia about the
North Pole

Geographic north pole


Geomagnetic north pole



North end of axis around which
earth spins
North end of dipole component
Antipodal with south pole
Magnetic north pole



Defined by 90 inclination
Not antipodal with south pole
Moves around daily
Components of Earth’s Field

Main field



External field



Generated by the outer core
Varies over time such that new maps are
needed every 5 years
Generated by ionosphere/solar wind
interactions
Varies rapidly
Crustal field

Caused by magnetism of rocks including
induction and remanent magnetism
Main Field


Generated
by the outer
core
Varies over
human and
geologic
time
 Values
can be obtained from NOAA calculator
(https://www.ngdc.noaa.gov/geomag-web/)
magnetosphere
External Field
 Charged
particles from
sun and interstellar
space interact with
ionosphere and
magnetosphere
 Ionosphere (top of
atmosphere)


https://science.nasa.gov/heliophysics/focus-areas/magnetosphere-ionosphere
ionized by solar
radiation
Flowing electrical
currents in ionosphere
create magnetic fields
http://solar-center.stanford.edu/SID/activities/ionosphere.html
Crustal Field
 Caused
by
magnetic
properties of
rocks in the crust
 Does not
change on
human time
scale
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
convection in outer core
 Needs to be observed every
5 years for precise
navigation
https://en.wikipedia.org/wiki/North_Magnetic_Pole#/media/File:Magneti
c_North_Pole_Positions_2015.svg
Geomagnetic Observatories
Secular variations
 Includes
changes
in direction and
intensity
Map of total intensity at 2015.0
Map of predicted
annual rate of change
of total intensity for
2015.0-2020.0
http://www.geomag.bgs.ac.uk/education/earthmag.html
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
https://www.windows2universe.org/spaceweather/images/quiet_ionosphere_animated.g
if
http://www.spaceweather.gc.ca/images/diurnal2.gif
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
Magnetic Storms
 Caused
by efficient
exchange of energy
between solar wind
and magnetosphere
 Creates large transients
in magnetic field
 Cannot be corrected
for
 https://www.youtube.com/watch?v=yEYy_nVC
4L0 4:33
 https://www.youtube.com/watch?v=URNXyZD2vQ 0:42
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




Mechanical



Compass
Dip needle
Electronic




Orientation
Total field strength
Field strength of one or all components
Fluxgate magnetometer
Proton precession
Alkali vapor (Cs, K)
Used for



Geophysical surveys
Treasure hunting & archeology
geolocation
optically pumped Potassium magnetometer
http://www.gemsys.ca/ground/
Fluxgate magnetometer





Opposing
electromagnets
Small external fields
causes current in
secondary coil
Measures external field
component parallel to
cores
Resolution 0.5 – 1.0 nT
Does not experience
instrument drift
Proton Precession
Magnetometer

Coil wrapped around
container with liquid
containing protons




H2O 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
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
Total Field
Measurement
 Combination




of
Earth’s main
field
Local induced
field
Regional
crustal field
External field
Total Field
Measurement



Anomalous field
(Fa) small
compared to
main field (Fe)
Total field
direction
essentially
unchanged
Therefore total
field strength
measurement
sufficient