Earth’s Magnetic Field

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This was my project about Earth's Magnetic Field. Hope you find it useful.

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Earth’s Magnetic Field

  1. 1. Earth’s Magnetic Field
  2. 2. The concept Magnetic fields are produced by the motion of electrical charges. For example, the magnetic field of a bar magnet results from the motion of negatively charged electrons in the magnet. The origin of the Earths magnetic field is notcompletely understood, but is thought to be associated with electrical currents produced by the coupling of convective effects and rotation in the spinning liquid metallic outer core of iron and nickel. This mechanism is termed the dynamo effect. Rocks that are formed from the molten state contain indicators of the magnetic field at the time of their solidification. The study of such "magnetic fossils" indicates that the Earths magnetic field reverses itself every million years or so (the north and south magnetic poles switch). This is but one detail of the magnetic field that is not well understood.
  3. 3. Earth’s Magnetic Field Demonstration
  4. 4. Importance Magnetic fields are produced by the motion of electrical charges. For example, the magnetic field of a bar magnet results from the motion of negatively charged electrons in the magnet. The origin of the Earths magnetic field is notcompletely understood, but is thought to be associated with electrical currents produced by the coupling of convective effects and rotation in the spinning liquid metallic outer core of iron and nickel. This mechanism is termed the dynamo effect. Rocks that are formed from the molten state contain indicators of the magnetic field at the time of their solidification. The study of such "magnetic fossils" indicates that the Earths magnetic field reverses itself every million years or so (the north and south magnetic poles switch). This is but one detail of the magnetic field that is not well understood.
  5. 5. Van Allen Radiation Belts A fundamental property of magnetic fields is that they exert forces on moving electrical charges. Thus, a magnetic field can trap charged particles such as electrons and protons as they are forced to execute a spiraling motion back and forth along the field lines. As illustrated in the adjacent figure, the chargedparticles are reflected at "mirror points" where the fieldlines come close together and the spirals tighten. One of the first fruits of early space exploration was thediscovery in the late 1950s that the Earth is surrounded by two regions of particularly high concentration of charged particles called the Van Allen radiation belts.The inner and outer Van Allen belts are illustrated in the top figure. The primary source of these charged particles is the stream of particles emanating from the Sun that we call the solar wind. As we shall see in asubsequent section, the charged particles trapped in the Earths magnetic field are responsible for the aurora (Northern and Southern Lights).
  6. 6. Van Allen Radiation Belts Demonstration
  7. 7. Origin of the Magnetic Field Magnetic fields are produced by the motion of electrical charges. For example, the magnetic field of a bar magnetresults from the motion of negatively charged electrons in the magnet. The origin of the Earths magnetic field is not completely understood, but is thought to be associated with electrical currents produced by the coupling of convective effects and rotation in the spinning liquid metallic outer core of iron and nickel. This mechanism istermed the dynamo effect. Rocks that are formed from themolten state contain indicators of the magnetic field at the time of their solidification. The study of such "magnetic fossils" indicates that the Earths magnetic field reverses itself every million years or so (the north and south magnetic poles switch). This is but one detail of the magnetic field that is not well understood.
  8. 8. Magnetic polesThe positions of the magnetic poles can be defined in at least two ways. A magnetic dip pole is a point on the Earths surface where the magnetic field is entirely vertical. The inclination of the Earths field is 90° at the North Magnetic Pole and -90° at the South Magnetic Pole. The two poles wander independently of each other and are not directly opposite each other on the globe. They can migrate rapidly: movements of up to 40 km per year have been observed for the North Magnetic Pole. Over the last 180 years, the North Magnetic Pole has been migrating northwestward, from Cape Adelaide in the Boothia peninsula in 1831 to 600 km from Resolute Bay in 2001. The magnetic equator is the line where the inclination is zero (the magnetic field ishorizontal).If a line is drawn parallel to the moment of the best-fitting magnetic dipole, the two positions where it intersects the Earths surface are called the North and South geomagnetic poles. If the Earths magnetic field were perfectly dipolar, the geomagnetic poles and magnetic dip poles would coincide and compasses would pointtowards them. However, the Earths field has a significant contribution from non-dipolar terms, so the poles do not coincide and compasses do not generally point at either.
  9. 9. Earth behaves like a magnet
  10. 10. Magnetosphere (1/2) Some of the charged particles from the solar wind aretrapped in the Van Allen radiation belt. A smaller number of particles from the solar wind manage to travel, asthough on an electromagnetic energy transmission line, to the Earths upper atmosphere and ionosphere in the auroral zones. The only time the solar wind is observable on the Earth is when it is strong enough to produce phenomena such as the aurora and geomagnetic storms. Bright auroras strongly heat the ionosphere, causing its plasma to expand into the magnetosphere, increasing the size of the plasmageosphere, and causing escape of atmospheric matter into the solar wind. Geomagnetic storms result when the pressure of plasmas contained inside the magnetosphere is sufficiently large to inflate and thereby distort the geomagnetic field.
  11. 11. Magnetosphere (2/2) The solar wind is responsible for the overall shape of Earths magnetosphere, and fluctuations in its speed, density, direction, and entrained magnetic field strongly affect Earths local space environment. For example, the levels of ionizing radiation and radiointerference can vary by factors of hundreds to thousands; and the shape and location of the magnetopause and bow shock wave upstream of it can change by several Earth radii, exposing geosynchronous satellites to the direct solar wind. These phenomena are collectively called space weather. The mechanism of atmospheric stripping is caused by gas being caught in bubbles of magnetic field, which are ripped off by solarwinds. Variations in the magnetic field strength have been correlated to rainfall variation within the tropics.
  12. 12. Magnetosphere Demonstration
  13. 13. Short-term variations The geomagnetic field changes on time scales frommilliseconds to millions of years. Shorter time scales mostly arise from currents in the ionosphere(ionospheric dynamo region) and magnetosphere, and some changes can be traced to geomagnetic storms or daily variations in currents. Changes over time scales of a year or more mostly reflect changes in the Earths interior, particularly the iron-rich core. Frequently, the Earths magnetosphere is hit by solar flares causing geomagnetic storms, provoking displaysof auroras. The short-term instability of the magnetic field ismeasured with the K-index. Data from THEMIS show that the magnetic field, which interacts with the solar wind, is reduced when the magnetic orientation is aligned betweenSun and Earth - opposite to the previous hypothesis. Duringforthcoming solar storms, this could result in blackouts and disruptions in artificial satellites.
  14. 14. Short-term variations demonstration
  15. 15. Secular variation (1/2) Changes in Earths magnetic field on a time scale of a year or more are referred to as secular variation. Over hundreds of years, magnetic declination is observed to vary over tens of degrees. A movie on the right showshow global declinations have changed over the last few centuries. The direction and intensity of the dipolechange over time. Over the last two centuries the dipolestrength has been decreasing at a rate of about 6.3% percentury. At this rate of decrease, the field would reach zero in about 1600 years. However, this strength is about average for the last 7 thousand years, and the current rate of change is not unusual. A prominent feature in the non-dipolar part of the secular variation is a westward drift at a rate of about 0.2 degrees per year.
  16. 16. Secular variation (2/2) This drift is not the same everywhere and has varied over time. The globally averaged drift has been westward since about 1400 AD but eastwardbetween about 1000 AD and 1400 AD. Changes that predate magnetic observatories are recorded in archaeological and geological materials. Such changes are referred to as paleomagnetic secular variation or paleosecular variation. The recordstypically include long periods of small change with occasional large changes reflecting geomagnetic excursions and geomagnetic reversals.
  17. 17. Earths core and the geodynamoThe Earths magnetic field is mostly caused by electric currents in the liquid outer core, which is composed of highly conductive molten iron. A magnetic field is generated by a feedback loop: current loops generate magnetic fields (Ampères circuital law) a changing magnetic field generates anelectric field (Faradays law); and the electric and magnetic fields exert a force on the charges that are flowing in currents(the Lorentz force). In a perfect conductor (σ=∞), there would be no diffusion. By Lenzs 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 afinite conductivity, new field is generated by stretching field linesas the fluid moves in ways that deform it. This process could go ongenerating new field indefinitely, were it not that as the magnetic field increases in strength, it resists fluid motion.
  18. 18. Numerical modelsThe equations for the geodynamo are enormously difficult to solve, and the realism of the solutions is limited mainly by computer power. For decades, theorists were confined to creating kinematic dynamos in which the fluid motion is chosen in advance and the effect on the magnetic fieldcalculated. Kinematic dynamo theory was mainly a matter of trying different flow geometries and seeing whether they could sustain a dynamo.The first self-consistent dynamo models, ones that determineboth the fluid motions and the magnetic field, were developed by two groups in 1995, one in Japan and one in the United States. The latter received a lot of attention because it successfully reproduced some of the characteristics of the Earths field, including geomagnetic reversals.
  19. 19. Crustal magnetic anomalies Magnetometers detect minute deviations in the Earths magnetic field caused by iron artifacts, kilns, some types of stone structures, and even ditches and maidens in archaeological geophysics. Using magnetic instruments adapted from airborne magnetic anomalydetectors developed during World War II to detect submarines, the magnetic variations across the ocean floor have been mapped. Basalt — the iron-rich, volcanic rock making up the ocean floor — contains a strongly magnetic mineral (magnetite) and can locally distort compass readings. Thedistortion was recognized by Icelandic mariners as early as the late 18th century. More important, because the presence of magnetite gives the basalt measurable magnetic properties, these magnetic variations have provided another means to study the deep ocean floor. When newly formed rock cools, such magnetic materials record the Earths magnetic field.
  20. 20. Crustal magnetic anomalies demonstration
  21. 21. Future At present, the overall geomagnetic field is becoming weaker; the present strong deterioration corresponds to a 10–15% decline over thelast 150 years and has accelerated in the past several years; geomagneticintensity has declined almost continuously from a maximum 35% above the modern value achieved approximately 2,000 years ago. The rate of decrease and the current strength are within the normal range of variation, as shown by the record of past magnetic fields recorded in rocks (figure on right). The nature of Earths magnetic field is oneof heteroscedastic fluctuation. An instantaneous measurement of it, or several measurements of it across the span of decades or centuries, arenot sufficient to extrapolate an overall trend in the field strength. It has gone up and down in the past for no apparent reason. Also, noting the local intensity of the dipole field (or its fluctuation) is insufficient to characterize Earths magnetic field as a whole, as it is not strictly a dipole field. The dipole component of Earths field can diminish even while the total magnetic field remains the same or increases. The Earths magnetic north pole is drifting from northern Canada towards Siberia with a presently accelerating rate— 10 km per year at the beginning of the 20th century, up to 40 km per year in 2003, and since then has only accelerated.
  22. 22. Project by: Leander Uka
  23. 23. Project by: Leander Uka ...always on top
  24. 24. Project by: Leander Uka End of Slide Show

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