Class Notes (922,429)
CA (542,813)
Western (51,673)
2123A/B (13)
Lecture 16

Earth Sciences 2123A/B Lecture 16: Lecture 16- Earth Magnetic
Premium

7 Pages
76 Views

Department
Earth Sciences
Course Code
Earth Sciences 2123A/B
Professor
Catherine Neish

This preview shows pages 1-2. Sign up to view the full 7 pages of the document.
Lecture 16 Eath’s Mageti field
History of Geomagnetism
Lodestone mineral that is natural magnet ex: magnetite
o Early Chinese compass spoon points south in 1000 AD
o Compass used for geomancy for a long time before it was used for navigation
Study of geomagnetism has a longer history than other branches of geophysics partly due to its
uses in navigation
o Used for marine navigation by 11th Chinese ships used floating compasses
o Columbus used one notice North Star was not exactly over True North
o Original thought that magnetic field was extra-terrestrial origin but William Gilbert in 1600
solved mystery of the compass
Geomagnetism was discovered by William Gilbert and it was used for navigation
o Personal physician to Elizabeth I
o He published De Magnete in 1600 a 6 volume treatises documenting his experiments
with magnetism
o Proposed that Earth behaves like a giant bar magnet with the magnetic axis approximately
North-South
o Died in 1603 of bubonic plague
o Earth behaves like a giant bar magnet with magnetic axis approximately North -South
o Gauss field generated inside the Earth
Magnetic Dipole pair of magnetic pole of equal strength but opposite sign
o No evidence for magnetic monopole
o Magnetic field lines of force shows the orientation of a small bar magnet paced at that
location in space
To a first approximation geomagnetic field is like the field of a dipole (bar magnet) with
geomagnetic axis titled about 11.5 degree from the geographic axis
Measuig Eath’s Mageti field geeated?
A magnetic need that is free to swing under the influence of a magnetic force will align itself
parallel to the local magnetic field line
Not permanent bar magnetic because of high temperature destroys magnetism it exceeds
Curie temperature
Magnetic mineral iron (Fe) has a Curie temperature of 770
Its more than 3000 in the outer core
o Inclination (i) angle measured from horizontal angle at which point into or away from
earth
find more resources at oneclass.com
find more resources at oneclass.com
Dip angle
o Declination (D) angle measured from geographic north
o Intensity magnetic field strength (Units Nano Tesalas)
The geomagnetic field cannot be generated by a permanent bar magnet at its center because
high temperature destroys magnetism
When ferromagnetic materials exceed a certain temperature known as Curie point they
lose their permanent magnetism
Maell’s Euatio
Changing magnetic field it creates an electric field
Moving electric charges create a magnetic field
o Only when the charges are moving
What auses Eath’s ageti field
If it were not continually generated eath’s field would decay in 20,000 years
Dynamo a machine for converting mechanical energy to electrical current flow
Self-exciting dynamo the earth converts mechanical energy to electrical current flow and
magnetic field
Requires movement of electrically conductive liquid Fe in the core
Geomagnetic field is a dynamic field unlike a bar magnet
o Soued ith Eath’s oute oe
1. Small initial magnetic field is presented
2. Disk begins to rotate- moving an electrical conductor through a magnetic field causes
electrons to flow
3. Electrons flow through a coil of a wire that produces a magnetic field in the same direction
as the original field
4. Magnetic field is enhanced
Thermal convection in the outer core
Heat release inner to the outer core
o Freezing of the inner core
o Radioactive heating may also be contributing
Compositional convection in the outer core
Compositional Convection inner core is made of solid Fe and is growing due to freezing of the
liquid outer core
Pure Fe freezing out to grow the inner core while the lighter elements remain in the outer core
Due to their low density, they rise in the outer core creating convection cells
Dominant cause for convection
find more resources at oneclass.com
find more resources at oneclass.com

Loved by over 2.2 million students

Over 90% improved by at least one letter grade.

Leah — University of Toronto

OneClass has been such a huge help in my studies at UofT especially since I am a transfer student. OneClass is the study buddy I never had before and definitely gives me the extra push to get from a B to an A!

Leah — University of Toronto
Saarim — University of Michigan

Balancing social life With academics can be difficult, that is why I'm so glad that OneClass is out there where I can find the top notes for all of my classes. Now I can be the all-star student I want to be.

Saarim — University of Michigan
Jenna — University of Wisconsin

As a college student living on a college budget, I love how easy it is to earn gift cards just by submitting my notes.

Jenna — University of Wisconsin
Anne — University of California

OneClass has allowed me to catch up with my most difficult course! #lifesaver

Anne — University of California
Description
Lecture 16 – Earth’s Magnetic field History of Geomagnetism  Lodestone – mineral that is natural magnet – ex: magnetite o Early Chinese compass – spoon points south in 1000 AD o Compass used for geomancy for a long time before it was used for navigation  Study of geomagnetism has a longer history than other branches of geophysics partly due to its uses in navigation th o Used for marine navigation by 11 Chinese ships used floating compasses o Columbus used one – notice North Star was not exactly over True North o Original thought that magnetic field was extra-terrestrial origin but William Gilbert in 1600 solved mystery of the compass  Geomagnetism was discovered by William Gilbert and it was used for navigation o Personal physician to Elizabeth I o He published De Magnete in 1600 – a 6 volume treatises documenting his experiments with magnetism o Proposed that Earth behaves like a giant bar magnet with the magnetic axis approximately North-South o Died in 1603 of bubonic plague o Earth behaves like a giant bar magnet with magnetic axis approximately North -South o Gauss – field generated inside the Earth  Magnetic Dipole – pair of magnetic pole of equal strength but opposite sign o No evidence for magnetic monopole o Magnetic field lines of force – shows the orientation of a small bar magnet paced at that location in space  To a first approximation geomagnetic field is like the field of a dipole (bar magnet) with geomagnetic axis titled about 11.5 degree from the geographic axis Measuring Earth’s Magnetic field generated?  A magnetic need that is free to swing under the influence of a magnetic force will align itself parallel to the local magnetic field line  Not permanent bar magnetic because of high temperature destroys magnetism – it exceeds Curie temperature  Magnetic mineral – iron (Fe) has a Curie temperature of 770  Its more than 3000 in the outer core o Inclination (i) – angle measured from horizontal angle at which point into or away from earth  Dip angle o Declination (D) – angle measured from geographic north o Intensity – magnetic field strength (Units Nano Tesalas)  The geomagnetic field cannot be generated by a permanent bar magnet at its center because high temperature destroys magnetism  When ferromagnetic materials exceed a certain temperature – known as Curie point – they lose their permanent magnetism Maxwell’s Equation  Changing magnetic field – it creates an electric field  Moving electric charges create a magnetic field o Only when the charges are moving What causes Earth’s magnetic field  If it were not continually generated – earth’s field would decay in 20,000 years  Dynamo – a machine for converting mechanical energy to electrical current flow  Self-exciting dynamo – the earth converts mechanical energy to electrical current flow and magnetic field  Requires movement of electrically conductive liquid Fe in the core  Geomagnetic field is a dynamic field – unlike a bar magnet o Sourced with Earth’s outer core 1. Small initial magnetic field is presented 2. Disk begins to rotate- moving an electrical conductor through a magnetic field causes electrons to flow 3. Electrons flow through a coil of a wire that produces a magnetic field in the same direction as the original field 4. Magnetic field is enhanced Thermal convection in the outer core  Heat release inner to the outer core o Freezing of the inner core o Radioactive heating may also be contributing Compositional convection in the outer core  Compositional Convection – inner core is made of solid Fe and is growing due to freezing of the liquid outer core  Pure Fe freezing out to grow the inner core while the lighter elements remain in the outer core  Due to their low density, they rise in the outer core creating convection cells  Dominant cause for convection Summary  Geomagnetic field is a dynamic field unlike a bar magnet – which is static/ permanent  Source of the field is in the Earth’s outer core – motion metal o 90% Fe Ni with 10% Si, O generates field is process called Geodynamo  Cannot measure the geodynamo can only model – using magnetohydrodynamic equations and measure its effect on and above the Earth Lecture 2 Computer Models  In 1995 the first computer model was published (the Glatzmaier Roberts Model) which successfully simulated the geodynamo including spontaneous reversal  Snapshot of magnetic field lines produced by G-R’s computer program  Their model gained a lot of credibility since it was the first model to show a Polarity Reversal Earth’s field is the sum of 3 parts 1. Main Magnetic Field – 99.2% 2. External Magnetic Field – sun - <0.6% 3. Induced Magnetic Field - <0.2% External Magnetic Field  Like the Earth, the sun has a magnetic field generated through internal convection  The 11-year sunspot cycle is due to the periodic magnetic polarity reversals whose effects can influence Earth’s climate o Solar wind – charged particles ejected from the sun o Magnetosphere – zone where changed particles are deflected by Earth’s magnetic field o Magnetotail- portion of magnetosphere that is pushed away from the sun by the solar wind  It can affect electronics etc. – astronauts almost Van Allen Belt  Van Allen radiation Belts discovered in 1958 – the first American spacecraft  Main discovery was that there are 2 radiation belts that can destroy spacecraft and harm people- so these orbits must be avoid Aurorae  It also produces the Northern and Southern Lights  Electrons hit air molecules and are excited as the reach earth o The molecules give off light as they calm down o An oxygen rich atmosphere will be green o Nitrogen would have excited atmosphere would be blue or purplish  Saturn also has Aurora because it had a magnetic field – like Jupiter Jupiter and Saturn  Jupiter and Saturn have magnetic fields  The interior of Jupiter and Saturn are most molecular hydrogen and helium  At some depth, under immense atmospheres becomes metallic and conducts electricity  Their magnetic fields are powered by convection generated from internal heat and aligned with the rotation axis Mars and Venus  Neither Mars nor Venus have substantial magnetic fields  This is puzzling since they are like Earth in many ways except neither planet exhibits plate tectonics  Mars is smaller than Earth, so it lost its internal heat more quickly, solidifying any initially liquid core  Venus may lack convection in the mantle – it has no mechanism to dissipate the heat Review Question  Convection is likely happening in the mantle and outer core of the earth Earth’s Magnetic Field Changes Over Time How do we measure changes in the geomagnetic /field?  Since the 1960s the earth magnetic field had been observed intermittently by Satellites (300-
More Less
Unlock Document


Only pages 1-2 are available for preview. Some parts have been intentionally blurred.

Unlock Document
You're Reading a Preview

Unlock to view full version

Unlock Document

Log In


OR

Don't have an account?

Join OneClass

Access over 10 million pages of study
documents for 1.3 million courses.

Sign up

Join to view


OR

By registering, I agree to the Terms and Privacy Policies
Already have an account?
Just a few more details

So we can recommend you notes for your school.

Reset Password

Please enter below the email address you registered with and we will send you a link to reset your password.

Add your courses

Get notes from the top students in your class.


Submit