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Earth-magneticField.ppt
- 1. Earth’s Magnetic Field • Why does Earth have a global or world- wide magnetic field, while other similar planets either have no magnetic fields or very different kinds of fields? • Why should we care about Earth’s magnetic field? What does it do for us?
- 2. S N • The North Pole of the Earth has a “south” polarity • The Earth behaves as if there was a magnet in its interior. • As a result a compass will always point towards the North
- 3. Earth’s Magnetic Field • Heat makes the outer core convect, which makes the Earth’s magnetic field
- 4. Magnetic field variable Time: Secular Variation Declination, inclination, and intensity change… What causes a Planet’s Magnetic Field?
- 5. The Inner Core: Solid Iron and Nickel The Outer Core: Liquid Iron and Nickel The Mantle: A Dense and Mostly Solid Rock Material The Crust: A Thin Rock Material
- 6. The Iron Core of the Earth is an electromagnet The core is surrounded by liquid Iron and Nickel As electrons flow around the core the magnetic field is produced The Earth’s rotation makes the electrons flow at very high speeds
- 7. Earth’s Magnetic Field • Magnetic fields are made wherever there is an electric current, that is the movement of electrons. • In a regular bar magnet, the magnetic field comes from the electrons orbiting around the nuclei of the iron atoms. All the electrons orbit in the same direction.
- 9. Solid Inner Core 2400 km diameter Iron & Nickel Liquid Outer Core 2270 km thick Iron & Nickel
- 10. Earth’s Magnetic Field • In the earth, electrical currents run through the molten iron core. Friction within the molten, flowing iron knocks electrons off iron atoms. • The molten iron flows at about 0.8 inches per second, but the electrical currents can flow faster.
- 12. Earth’s Magnetic Field • The electrical currents within the earth cause earth to act like a gigantic electromagnetic generator. • This is called the Dynamo Theory of magnetic field generation.
- 13. Earth’s Magnetic Field Global Field
- 14. A little more info… • The earth’s magnetic field isn’t strong enough for us to feel, but many animals can sense it and even use it to navigate. It’s only about 0.4 Gauss, much weaker than a small magnet you can hold in your hand. • A planet’s magnetic field is measured using an instrument called a Magnetometer
- 15. The poles “flip” • Every so often, what was the North magnetic pole suddenly becomes the South magnetic pole.
- 16. The poles “flip” • how long does the process take? thousands of years?
- 17. Hypothesized triggers We used to think that these flips are triggered by external events that directly disrupt the flow in the Earth's core. • impact events •internal events such as the arrival of continental slabs carried down into the mantle •or the initiation of new mantle plumes from the core-mantle boundary.
- 18. The poles “flip” NASA computer simulation using the model of Glatzmaier and Roberts.[28] The tubes represent magnetic field lines, blue when the field points towards the center and yellow when away. The rotation axis of the Earth is centered and vertical. The dense clusters of lines are within the Earth's core.
- 19. Strange Things Going On • Earth’s magnetic field is NOT aligned with its rotational axis. • The magnetic field is tilted 12o to the rotational axis, and doesn’t even pass directly through the center of the earth. • Does this mean that the electrical currents don’t flow evenly and uniformly inside the earth? Is there turbulence inside?
- 20. Magnetic Fields in Space • Earth’s magnetic field extends 7-10 times the earth’s diameter outward from the earth. • The earth’s magnetic field would be spherical, but the solar wind compresses it on the side closest to the sun, and stretches it out into a long tail on the side opposite the sun. • Overall, it’s kind of tadpole shaped.
- 21. Why do we care? • Earth’s magnetic field isn’t just “there” with no purpose. Without it, you and I and every living thing on this planet would be dead (including the cockroaches!) • The magnetic field channels away the solar wind. • It also prevents erosion of the atmosphere.
- 22. Solar Wind • So what is the solar wind anyway? – It’s radiation: extremely hot, high-energy, fast- moving charged particles (ions) given off by the sun. Most of these particles are protons. – If you were exposed to it for just a few hours without protection, your skin and every organ in your body would be burned, and you’d have a fatal dose of radiation poisoning.
- 23. How does the magnetic field protect us? • The magnetic field captures the solar wind and channels much of it into a donut of radiation around the earth. • This donut (actually 2 layers – one inside the other) is called the Van Allen Radiation Belt (V.A.R.B.)
- 25. These particles are deflected by the Earth’s Magnetic Field
- 26. Other Planets in our Solar System have Magnetic Fields Planet Magnetic Field Mercury 100 times weaker than Earth Venus 25,000 times weaker than Earth Earth 30,000 – 60,000 nT Mars 5000 times weaker than Earth Jupiter 20,000 times greater than Earth Saturn 540 times greater than Earth Uranus 40 times greater than Earth Neptune ¼ that of Earth Pluto None that we know of
Editor's Notes
- The Earth and most of the planets in the Solar System, as well as the Sun and other stars, all generate magnetic fields through the motion of highly conductive fluids.[42] The Earth's field originates in its core. This is a region of iron alloys extending to about 3400 km (the radius of the Earth is 6370 km). It is divided into a solid inner core, with a radius of 1220 km, and a liquid outer core.[43] The motion of the liquid in the outer core is driven by heat flow from the inner core, which is about 6,000 K (5,730 °C; 10,340 °F), to the core-mantle boundary, which is about 3,800 K (3,530 °C; 6,380 °F).[44] The pattern of flow is organized by the rotation of the Earth and the presence of the solid inner core.[45] The mechanism by which the Earth generates a magnetic field is known as a dynamo.[42] A magnetic field is generated by a feedback loop: current loops generate magnetic fields (Ampère's circuital law); a changing magnetic field generates an electric field (Faraday's law); and the electric and magnetic fields exert a force on the charges that are flowing in currents (the Lorentz force).[46] These effects can be combined in a partial differential equation for the magnetic field called the magnetic induction equation: ...where u is the velocity of the fluid; B is the magnetic B-field; and η=1/σμ is the magnetic diffusivity, a product of the electrical conductivity σ and the permeability μ .[47] The term ∂B/∂t is the time derivative of the field; ∇2 is the Laplace operator and ∇× is the curl operator. The first term on the right hand side of the induction equation is a diffusion term. In a stationary fluid, the magnetic field declines and any concentrations of field spread out. If the Earth's dynamo shut off, the dipole part would disappear in a few tens of thousands of years.[47] In a perfect conductor (σ=∞), there would be no diffusion. By Lenz's law, any change in the magnetic field would be immediately opposed by currents, so the flux through a given volume of fluid could not change. As the fluid moved, the magnetic field would go with it. The theorem describing this effect is called the frozen-in-field theorem. Even in a fluid with a finite conductivity, new field is generated by stretching field lines as the fluid moves in ways that deform it. This process could go on generating new field indefinitely, were it not that as the magnetic field increases in strength, it resists fluid motion.[47] The motion of the fluid is sustained by convection, motion driven by buoyancy. The temperature increases towards the center of the Earth, and the higher temperature of the fluid lower down makes it buoyant. This buoyancy is enhanced by chemical separation: As the core cools, some of the molten iron solidifies and is plated to the inner core. In the process, lighter elements are left behind in the fluid, making it lighter. This is called compositional convection. A Coriolis effect, caused by the overall planetary rotation, tends to organize the flow into rolls aligned along the north-south polar axis.[45][47] The mere convective motion of an electrically conductive fluid is not enough to ensure the generation of a magnetic field. The above model assumes the motion of charges (such as electrons with respect to atomic nuclei), which is a requirement for generating a magnetic field. However, it is not clear how this motion of charges is created in the circulating fluid of the outer core. Possible mechanisms may include battery-like electrochemical reactions that generate electrical current in the fluid, or a thermoelectric effect[48] (both mechanisms somehow[?] discredited); or a triboelectric effect. More robustly, remnant magnetic fields in magnetic materials in the mantle, which are cooler than their Curie temperature, would provide seed “stator” magnetic fields that would induce the required growing currents in the convectively driven fluid behaving as a dynamo, as analyzed by Dr. Philip William Livermore.[49] The average magnetic field in the Earth's outer core was calculated to be 25 G, 50 times stronger than the field at the surface.[50]
- Changing frequency over time The rate of reversals in the Earth's magnetic field has varied widely over time. 72 million years ago (Ma), the field reversed 5 times in a million years. In a 4-million-year period centered on 54 Ma, there were 10 reversals; at around 42 Ma, 17 reversals took place in the span of 3 million years. In a period of 3 million years centering on 24 Ma, 13 reversals occurred. No fewer than 51 reversals occurred in a 12-million-year period, centering on 15 million years ago. Two reversals occurred during a span of 50,000 years. These eras of frequent reversals have been counterbalanced by a few "superchrons" – long periods when no reversals took place.
- Most estimates for the duration of a polarity transition are between 1,000 and 10,000 years.[9] However, studies of 15 million year old lava flows on Steens Mountain, Oregon, indicate that the Earth's magnetic field is capable of shifting at a rate of up to 6 degrees per day.[19] This was initially met with skepticism from paleomagnetists. Even if changes occur that quickly in the core, the mantle, which is a semiconductor, is thought to act as a low-pass filter, removing variations with periods less than a few months. A variety of possible rock magnetic mechanisms were proposed that would lead to a false signal.[20] However, paleomagnetic studies of other sections from the same region (the Oregon Plateau flood basalts) give consistent results.[21][22] It appears that the reversed-to-normal polarity transition that marks the end of Chron C5Cr (16.7 million years ago) contains a series of reversals and excursions.[23] In addition, geologists Scott Bogue of Occidental College and Jonathan Glen of the US Geological Survey, sampling lava flows in Battle Mountain, Nevada, found evidence for a brief, several year long interval during a reversal when the field direction changed by over 50°. The reversal was dated to approximately 15 million years ago.
- In simulations of planetary dynamos, reversals often emerge spontaneously from the underlying dynamics. For example, Gary Glatzmaier and collaborator Paul Roberts of UCLA ran a numerical model of the coupling between electromagnetism and fluid dynamics in the Earth's interior. Their simulation reproduced key features of the magnetic field over more than 40,000 years of simulated time and the computer-generated field reversed itself.[28][29] Global field reversals at irregular intervals have also been observed in the laboratory liquid metal experiment VKS2.[30] In some simulations, this leads to an instability in which the magnetic field spontaneously flips over into the opposite orientation.