GEOGRAPHY: COMPOSITION OF THE SOLAR SYSTEM

GEOGRAPHY: COMPOSITION OF THE SOLAR SYSTEM

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THE PLANETS

1. Mercury
   1. Closest planet to the Sun
   2. Smallest planet
   3. No satellites
2. Venus
   1. Very dense atmosphere
   2. Hottest planet (over 400 C)
   3. Large amount of greenhouse gases
3. Earth
   1. Hydrosphere unique among rock-based planets
      
   2. Only planet where plate tectonics observed
      
4. Mars
   1. Atmosphere made mainly of carbon dioxide
      
   2. Red colour comes from iron oxide
   3. Geological activity such as volcanoes as recently as 2 million years ago
      
5. Jupiter
   1. 2.5 times the masses of all other planets combined
      
   2. Composed mainly of hydrogen and helium
      
   3. Great Red Spot in atmosphere created by strong internal heat
   4. 63 known satellites. Ganymede, Callisto, Io and Europa show similarities to terrestrial planets such as volcanism and internal heating
      
   5. Ganymede is the largest satellite in the solar system. It is larger than mercury
      
   6. Has planetary ring system
      
6. Saturn
   1. Extensive ring system
      
   2. Rings made by small particles of water ice clumped together
      
   3. Rings first observed by Galileo
      
   4. Least dense planet in solar system
      
   5. 60 satellites
      
   6. Titan is the only satellite in the solar system with a substantial atmosphere
7. Uranus
   1. Orbits the sun on its side
   2. Very cold core, radiates little heat
      
   3. 27 satellites
      
   4. Has planetary ring system
      
8. Neptune
   1. Smaller in size but more massive and more dense than Neptune
      
   2. 13 known satellites
      
   3. Has planetary ring system
      

OTHER COMPONENTS

1. Asteroids
   1. Small objects composed of rocky and metallic minerals
      
   2. Small asteroids are called meteoroids
      
   3. Main asteroid belt occupies orbit between Mars and Jupiter
      
   4. Asteroid belt is sparsely populated. Spacecraft routinely pass through belt without incident
   5. Ceres is the largest body in the asteroid belt. Classified as a dwarf planet
2. Comets
   1. Small bodies composed of volatile ices
      
   2. Coma of a comet (tail) is observed when proximity to the sun causes the icy particles to sublimate and ionize
   3. Hale-Bopp comet was visible to the naked eye for 18 months. It was the most widely observed and brightest comet recorded
      
3. Interplanetary medium
   1. It Is the interplanetary atmosphere created by the stream of charged particles emitted by the Sun (called solar wind)
      
   2. Aka Heliosphere
      
   3. Stretches out to 100 AU
      
   4. Space weather is created by geomagnetic storms on the Sun’s surface which disturb the heliosphere
   5. Earth’s magnetic field prevents solar wind from stripping away the Earth’s atmosphere
4. Kuiper belt
   1. Ring of debris, similar to asteroid belt, but composed mainly of ice
      
   2. Present in the area beyond Neptune
      
   3. Pluto is the largest object in the Kuiper belt

GALACTIC CONTEXT OF THE SOLAR SYSTEM

1. Located in the Milky Way galaxy, a spiral galaxy. Milky Way diameter 100000 light years. Contains about 200 billion stars
   
2. Solar System resides in one of the outer arms, called Orion Arm
   
3. Sun lies around 25,000 light years from galactic centre. Completes one revolution every 250 million years (aka Cosmic Year)
   
4. Closest star is Alpha Centauri triple star system
   
5. Largest star close to Sun is Sirius
   
6. Closest sun-like star is Tau Ceti
   
7. Closest extra-solar planet is Epsilon Eridani b, which orbits the star.r Epsilon Eridani
   

IMPORTANT GEOLOGICAL FEATURES IN THE SOLAR SYSTEM

1. Venus
   1. Ishtar Terra and Aphrodite Terra: two continents on Venus
      
   2. Maxwell Montes: highest mountain on Venus
      
2. Mars
   1. Adirondack: first rock chosen to be explored by the first Mars rover Spirit in Jan 2004
      
   2. Gusev Crater: crater on Mars, site of landing of the Mars rover Spirit, Jan 2004
   3. Sleepy Hollow: circular, shallow depression on Gusev crater
      
   4. Olympus Mons: tallest volcano and mountain in the Solar System. About 3 times as tall as Mount Everest (88000 ft)
      
   5. Meridiani Planum: Landing site of second Mars rover Opportunity. May indicate the presence hot springs or liquid water in the past
      
3. Moon
   1. South Pole – Aitken basin: largest crater on the Moon, on the far side
      
   2. Shackleton crater: Site where the Moon Impact Probe from Chandrayaan landed and found water, Nov 2008.
      Located at the South Pole, where the rim gets continuous sunlight while the interior is in perpetual shadow. 
      
   3. Cabeus crater: Site where the LCROSS spacecraft landed and confirmed significant presence of water, Oct 2009.Located at the south pole
      

TIMELINE OF SOLAR SYSTEM EXPLORATION

1. 1957: Sputnik 1, first Earth Orbiter
2. 1961: Vostok 1, first manned Earth orbiter
3. 1966: Luna 9, first Lunar Lander
4. 1969: Apollo 11, first manned lunar landing
5. 1970: Luna 17/Lunokhod 1 – first Lunar Rover
6. 1971: Mars 3 – first Mars Lander
7. 1976: Helios 2 – closest Solar approach
8. 1977: Voyager 2 – first to leave Solar System
9. 1978: International Cometary Explorer – first comet flyby (comets Giacobini-Zinner and Halley)
10. 1996: Mars Pathfinder – first Mars Rover
11. 2001: Genesis – first Solar wind sample return

TIMELINE OF SOLAR SYSTEM ASTRONOMY

1. 2137 BCE: Chinese astronomers record solar eclipse
2. 2^ND millennium BCE: Heliocentric solar system, with Sun at the centre, proposed in the Vedic texts
3. 499 CE: Aryabhata, in his Aryabhatiya, propounds heliocentric solar system of gravitation, elliptical orbits for planets, and suggests that moon and planets shine due to reflected light
4. 500: Aryabhata accurately computes the earth’s circumference, solar and lunar eclipses and length of earth’s revolution around the Sun
5. 620: Brahmagupta recognizes gravity as a force of attraction and describes law of gravitation
6. 628: Brahmagupta calculates motion and position of various planets
7. 1150: Bhaskara, in the Siddhanta Shiromani, calculates longitudes and latitudes
8. 1514: Copernicus states his Heliocentric theory in Commentariolus
9. 1610: Galileo Galilee discovers Callisto, Europa, Ganymede and Io, sees Saturn’s rings
10. 1656: Christiaan Huygens: identifies Saturn’s rings as rings and discovers Titan
11. 1705: Edmund Halley predicts the periodicity of Halley’s Comet
12. 1755: Immanuel Kant formulates the Nebular Hypothesis of Solar System formation
13. 1930: Clyde Tombaugh discovers Pluto
14. 1946: American launch of a camera-equipped V2 rocket provides the first images of the Earth from space

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GEOGRAPHY: STRUCTURE OF THE EARTH

GEOGRAPHY: STRUCTURE OF THE EARTH

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STRUCTURE OF THE EARTH

Depth (in km)	Layer
0-35	Crust
35-60	Uppermost part of the mantle
35-660	Upper Mantle
660-2890	Lower mantle
2890-5150	Outer core
5150-6360	Inner core

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Crust

• Depth varies from 70 km under mountains to 5 km under oceans
• Thin oceanic crust is composed of dense iron, magnesium silicate rocks like basalt
• Thick continental crust is less dense, composed of sodium, potassium, aluminium silicate rocks like granite
• The boundary between crust and mantle is called Mohorovicic discontinuity. Signifies change in seismic velocity and rock composition
  

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Mantle

Schematic view of the interior of Earth. 1. continental crust - 2. oceanic crust - 3. upper mantle - 4. lower mantle - 5. outer core - 6. inner core - A: Mohorovičić discontinuity - B: Gutenberg Discontinuity - C: Lehmann discontinuity Source: Wikipedia

• Thickest layer of the earth
• Composed mainly silicate rocks rich in iron and magnesium
  
• Temperature ranges from 500 C (near the crust) to 4000 C (near the core)
  
• Despite high heat, the mantle is primarily solid due to high pressures
  
• The mantle is slightly ductile and can flow, although only on slow, long timescales
  
• Motion of tectonic plates is an expression of convection in the mantle
• The mantle lies exposed without any crust covering on the floor of the Atlantic Ocean near the Caribbean Islands

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Outer core

• Convection in the outer core gives rise to earth’s magnetic field. The mechanism of the magnetic field is explained by the Dynamo Theory, which was proposed by Joseph Larmor in 1919 
  
• Liquid in composition
  

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Inner core

• Believed to consist of an iron-nickel alloy
  
• Hottest part of the earth. Temperature may reach that of Sun’s surface i.e 5700 K
• Solid in composition
  
• Compressional waves can pass through it but not shear waves
  
• Inner core is younger than the age of the earth. Inner core: 2-4 billion years, earth: 4.5 years
  
• Inner core is cooling slowly (about 100 C per billion years)
  
• The inner core is too hot to hold a permanent magnetic field
  
• It has been speculated that the inner core may rotate slightly faster than the rest of the earth (about 0.3 to 0.5 degrees per year)
  

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Lithosphere

• Includes the crust and uppermost parts of the mantle
  
• Constitutes the hard and rigid outer layer of the Earth
  
• Lithosphere is broken down into tectonic plates
  
• Is rigid and deforms through brittle failure, causing faults
• Lithosphere is thought to float or move around on the Asthenosphere, creating plate tectonics
  

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Asthenosphere

• Lies below the lithosphere
  
• Constitutes the weaker, hotter and deeper part of the upper mantle
  
• Involved in plate movements
  
• Deforms viscously and accommodates strain through plastic deformation
  
• Due to high temperature, rock becomes ductile, leading to convection currents
  
• Boundary between Lithosphere and Asthenosphere is defined by a change in seismic velocity: in asthenosphere seismic waves pass relatively slowly and hence it is called a low-velocity zone
  

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Discontinuities in the Earth’s structure

Discontinuity	Depth	Boundary	Other notes
Mohorovicic discontinuity	30-50 km (continents) 7 km (ocean floor)	crust-mantle	Observed by abrupt change in seismic wave velocity Identified by Andrija Mohorovicic (Croatia) in 1909
Gutenberg discontinuity	2900 km	Core-mantle	Observed by difference in seismic wave velocity
Lehmann discontinuity	220 km		Appears beneath continents but not oceans

PLATE TECTONICS

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Overview

• Plate tectonics is a theory that describes large scale motions of the earth’s lithosphere
  
• Proposed by Harry Hess in 1962. Builds on the concepts of continental drift, proposed by Alfred Wegener in 1915.
• Tectonic plates move because lithosphere has higher strength and lower density than the athenosphere. Thus the lithosphere rides on the athenosphere
• Tectonic plates on the earth move in relation to each other
  
• Movement of plates is typically 50 – 100 mm annually
• Earthquakes, volcanic activity, mountain building and ocean trench formation occur along plate boundaries
• Plate tectonics may exist on other terrestrial planets as well, especially Mars
  

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Types of plate boundaries

• Transform boundaries:
  occur where plates slide past each other along transform faults. Eg: San Andreas Fault in California
  
• Divergent boundaries: occur where two plates slide apart from each other. Eg: Mid-Atlantic Ridge, Great Rift Valley (Africa)
  
• Convergent boundaries: occur where two plates slide towards each other forming either a subduction zone or a continental collision. Eg: Andes (South America), Japan, Himalayas
  
  • Subduction zones:
    occur where an oceanic plate is pushed underneath a continental plate. Eg ocean trenches. The descending end of the oceanic plate melts and creates pressure on the mantle, causing volcanoes
    
  • Obduction zones:
    occur where the continental plate is pushed underneath the oceanic plate. However, this is unusual as the relative densities of the plates favours subduction of the oceanic plate
    
  • Orogenic belts:
    occur when two continental plates collide and push upward to form large mountain ranges. Eg: Himalayas
    

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Examples of Divergent boundaries

• East African Rift (Great Rift Valley), Africa
• Mid-Atlantic Ridge: separates the North and South American plates from the Eurasian and African plates
  
• Gakkel Ridge: a slow spreading ridge in the Arctic Ocean
  
• East Pacific Rise: extends from the South Pacific to the Gulf of California
  
• Carlsberg Ridge in the eastern Indian Ocean
  

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Examples of Subduction zones

• The oceanic Nazca plate being subducted under the continental South American Plate forming the Chile-Peru Trench
• The Pacific Plate being subducted under the Eurasian and Philippine Sea Plates forming the Mariana Trench
• The Philippine Sea Plate subducting under the Philippine Mobile Belt forming the Manila Trench

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Examples of Orogenic belts

• The belt between the Indo-Australian and Eurasian Plates giving rise to the Himalayas. This is the most dramatic Orogenic Belt in the world
• Interaction between the African and Adriatic Plates with the Eurasian Plate giving rise to the Alps
• Andes belt on the western margin of South America

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Examples of Transform boundaries

• The San Andreas Fault in California. This arises due to the northwards movement of the Pacific Plate with respect to the North American Plate
• Motagua Fault between the North American Plate and the Caribbean Plate
• Dead Sea Transform fault which runs through the Jordan River Valley

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Major and Minor plates

Major plates	Minor plates
African plate	Arabian plate
Antarctic plate	Caribbean plate
Australian plate	Juan de Fuca plate
Indian plate	Cocos plate
Eurasian plate	Nazca plate
North American plate	Philippine sea plate
South American plate	Scotia plate
Pacific plate

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GEOGRAPHY: THE ATMOSPHERE OF EARTH

GEOGRAPHY: THE ATMOSPHERE OF EARTH

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COMPOSITION OF THE ATMOSPHERE

Compound	Distribution
Nitrogen	78%
Oxygen	21%
Argon	0.9%
Water vapour	0.4% (around 1% at the surface)
Carbon dioxide	0.03%

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STRUCTURE OF THE ATMOSPHERE

1. Troposphere
   1. Begins at the surface and extends to between 7 km (at the poles) and 20 km (at the equator)
   2. Temperature in the troposphere decreases with altitude i.e. the lowest parts are the warmest
   3. The troposphere contains roughly 75% of the mass of the atmosphere and 99% of its water vapour
   4. The lowest part of the troposphere, where friction with the Earth’s surface influences air flow is called the planetary boundary layer. Usually extends from a few hundred metres to about 2 km
   5. The tropopause is the boundary between the troposphere and the stratosphere
2. Stratosphere
   

   
   

   Layers of the atmosphere
   1. Extends from the troposphere to about 51 km
   2. Temperature increases with height
   3. Restricts turbulence and mixing
   4. Commercial airliners usually fly within the stratosphere (10 km) to optimize jet fuel burn and to avoid atmospheric turbulence
   5. The stratopause is the boundary between the stratosphere and the mesosphere
3. Mesosphere
   1. Extends from stratosphere to about 80 km
   2. Upon entering the earth’s atmosphere, most meteors burn up in the mesosphere
   3. Temperature decreases with height
   4. The mesopause, the end of the mesosphere, is the coldest place on Earth with an average temperature of -100 C
4. Thermosphere
   1. Biggest layer of the atmosphere
   2. Extends from the mesosphere to about 500-1000 km
   3. Thermopause is a temperature boundary contained within the thermosphere
   4. Temperature increases up to the thermopause, then remains constant
   5. The temperature can reach 1500 C. However, despite the high temperature one would not feel warm because the atmospheric density is too low to enable heat transfer
   6. The International Space Station orbits in the thermosphere (320 – 380 km)
   7. The ionosphere is formed in this layer as a result of ionization caused by ultraviolet radiation
   8. The boundary between the thermosphere and the exosphere is called exobase
5. Exosphere
   1. Uppermost layer of the atmosphere
   2. It is a transitional zone between the Earth’s atmosphere and interplanetary space and does not fully fall within the atmosphere
   3. Extends to about 190,000 km. This is half the distance to the Moon, at which the influence of solar radiation becomes greater than the Earth’s gravitational pull
   4. The density is so low that molecules can travel hundreds of km without colliding with each other
   5. Composed mainly of the lightest gases such as hydrogen and some helium

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OTHER LAYERS AND BOUNDARIES OF THE ATMOSPHERE

1. Ozone layer
   1. It is contained within the stratosphere at about 10 – 50 km above the Earth’s surface
   2. About 90% of the ozone layer is present in the stratosphere
   3. The ozone layer absorbs 93-99% of harmful ultraviolet light
   4. Ozone is formed when UV light strikes oxygen in the stratosphere to split the oxygen atoms, which then reform as ozone
   5. The ozone layer was discovered by the French physicists Charles Fabry and Henri Buisson in 1913
   6. British meteorologist GMB Dobson established a worldwide network of ozone monitoring stations between 1928 and 1958 that continues to operate today. He also developed a spectrophotometer (called the Dobsonmeter) to measure stratospheric oxygen from the ground. The Dobson unit, a measure of ozone density is named in his honour
2. Ionosphere
   1. Stretches from the thermosphere to the exosphere (100 km – 700 km)
   2. This is caused due to ionization by solar UV radiation
   3. Responsible for radio propagation by reflecting radio waves back to the Earth’s surface thereby enabling long-distance communication
   4. Plays an important part in atmospheric electricity (like lightning)
   5. Responsible for auroras
3. Homosphere and Heterosphere
   1. Homosphere is the part of the atmosphere where gases are well mixed due to turbulence
   2. This includes the troposphere, stratosphere and mesosphere
   3. Heterosphere is the part of the atmosphere where gases are not well mixed
   4. This usually happens above the turbopause (100 km) where distance between particles is large due to low density
   5. This causes the atmosphere to stratify with heavier gases like oxygen and nitrogen present in the lower layers and lighter gases like hydrogen and helium in the upper layers
4. Planetary boundary layer
   1. Part of the troposphere closest to the Earth’s surface and most influenced by it
   2. Friction with the earth’s surface causes turbulent diffusion
   3. Ranges from 100 m to about 2 km
5. Magnetosphere
   1. A mix of free ions and electrons from solar wind and the Earth’s atmosphere
   2. It is non-spherical and extends to more than 70,000 km
   3. It protects the Earth from harmful solar winds
   4. Mars is thought to have lost most of its former oceans and atmosphere to space due to the direct impact of solar winds. Similarly Venus is thought to have lost its water due to solar winds as well
6. Karman line
   1. Defines the boundary between the Earth’s atmosphere and outer space
   2. Lies at an altitude of 100 km above mean sea level
   3. At this altitude the atmosphere becomes too thin for aeronautical purposes
   4. However, there is no legal demarcation between a country’s air space and outer space
7. Van Allen Belt
   1. It is a region of energetic charged particles (plasma) around the Earth held in place by the Earth’s magnetic field
   2. Extends from about 200 km to 1000 km
   3. Has important implications for space travel because it causes radiation damage to solar cells, integrated circuits, sensors and other electronics

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PHYSICAL PROPERTIES OF THE ATMOSPHERE

1. Pressure and thickness
   1. Atmospheric pressure at sea level is 1 atmosphere (around 14.7 psi)
   2. 50% of atmospheric mass is below an altitude of 5.6 km
   3. 90% of atmospheric mass is below 16 km
   4. 99.99% of atmospheric mass is below 100 km
2. Density and mass
   1. Atmospheric density decreases with height
   2. Density at sea level is about 1.2 kg/cu.m

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OPTICAL PROPERTIES OF THE ATMOSPHERE

1. Scattering
   1. When sun’s rays pass through the atmosphere, photons in light interact with the atmosphere to produce scattering
   2. Eg: on overcast days there are no shadows because light reaching the surface is only scattered, indirect radiation, with no direct radiation reaching the earth
   3. Scattering is responsible for blue appearance of the sky, and for red appearance of sunset
2. Absorption
   1. The atmosphere absorbs radiation of different wavelengths, allowing only certain ranges (UV to IR) to pass on to the earth’s surface
3. Emission
   1. The atmosphere absorbs and emits IR radiation
   2. Earth cools down faster on clear nights than on cloudy nights because clouds absorb IR radiation from the Sun during the day and emit IR radiation towards the Earth at night
   3. Greenhouse effect is directly related to emission, where certain greenhouse gases (carbon dioxide) prevent IR radiation from the earth’s surface to exit back to space

WATER VAPOUR IN THE ATMOSPHERE

• 99.9% of water vapour is contained in the troposphere
• Condensation of water vapour into liquid or ice is responsible for rain, snow etc
• The latent heat released during condensation is responsible for cyclones and thunderstorms
• Water vapour is also a potent greenhouse gas
• Water vapour is most common gas in volcanic emissions (around 60%)

CARBON DIOXDE IN THE ATMOSPHERE

• It is an important greenhouse gas
• Natural sources of carbon dioxide in the atmosphere include volcanic activity, combustion of organic matter, respiration, decay of forests etc
• Current carbon dioxide levels (0.0384%) are around 35% higher than the levels in 1832
• The concentration of carbon dioxide is higher in the northern hemisphere because it has greater land mass and plant mass than the southern hemisphere
• Carbon dioxide concentrations peak in May (just after the end of winter in the Northern Hemisphere) and reach a minimum in October (at the end of summer in Northern Hemisphere, when the quantity of plants undergoing photosynthesis is greatest)

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GEOGRAPHY: ATMOSPHERIC ELECTRICITY

GEOGRAPHY: ATMOSPHERIC ELECTRICITY

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Overview

Aurora Borealis seen over Canada

• The Earth’s surface, the atmosphere and the ionosphere combine to form a global atmospheric electrical circuit
• Free electricity is always present in the atmosphere. It is usually positive
• The intensity of atmospheric electricity is usually greater in the middle of the day than in the morning or at night. Also, it is greater in winter than in summer
• Atmospheric electricity increases with altitude
• The Earth’s surface is negatively charged, while the atmosphere is positively charged
• Benjamin Franklin was the first to prove electrical phenomena of the atmosphere in 1752

Variation of atmospheric electricity

• The primary cause of variation in atmospheric electricity is the thermodynamics of radiation
• Atmospheric electricity is maximum in January and minimum in June
• Humidity increases atmospheric electricity in the cold months but decreases it in hot months

PHENOMENA OF ATMOSPHERIC ELECTRICITY

1. Auroras
   1. Auroras are natural light displays observed in the night sky, especially in polar regions
   2. Auroras occur when the Earth’s magnetic field traps solar wind in the atmosphere resulting in a collision between the solar wind and atmospheric molecules leading to release of energy
   3. They are most prominent closer to the magnetic poles because of longer periods of darkness and strength of the Earth’s magnetic field
   4. The Aurora Borealis refers to auroras in the northern hemisphere. The corresponding auroras in the southern hemisphere are called Aurora Australis
   5. Auroras occur most often near the seasonal equinoxes: from September to October and from March to April
   6. Auroras have maximum intensity during the intense phase of solar cycle when coronal mass ejections increase the intensity of solar wind
2. Static electricity
   1. Static electricity is the build of electrical charge on the surface of objects
   2. The static charge remains on the object until it either bleeds off to the ground or is quickly neutralized by a discharge
   3. Lightning is caused by discharge of static electricity
3. St. Elmo’s Fire
   1. St. Elmo’s Fire is a bright blue or violet glow appearing from tall, pointed objects
   2. It is a phenomenon in which plasma is created when the electric field around the object causes ionization of air molecules
   3. Sharp objects tend to create more plasma because electrical fields are more concentrated in areas of high curvature
4. Lightning
   1. Lightning is an atmospheric discharge of electricity
   2. Occurs during thunderstorms, volcanic eruptions and dust storms
   3. The average lightning bolt can reach temperatures of 30,000 C (about 3 times the temperature of the sun) and carry around 100 million V of electricity
   4. This extreme temperature compresses surrounding air and creates a supersonic shock wave called thunder
   5. In addition to light, lightning has been shown to emit radio waves, X-rays and gamma rays

TYPES OF LIGHTNING

Lightning strikes can carry up to 100 million Volts and reach temperatures of 30,000 C

The lightning that is most commonly observed is called streak lightning. This is just the visible part of the lightning stroke – the majority of the lightning occurs inside clouds, so it is not visible from the Earth.

1. Cloud-to-ground lightning
   1. Second most common form of lightning
   2. b. Poses greatest threat to life and property since it strikes the ground
2. Cloud-to-cloud lightning
   1. Lightning occurring between two clouds is called inter-cloud lightning
   2. Lightning that occurs between two areas of the same cloud that have differing electric potential is called intra-cloud lightning
3. Ground-to-cloud lightning
   1. Lightning discharge between ground and cloud, in the upward direction
   2. Very rare
   3. Occurs when negatively charged ions from the Earth’s surface rise up and meet the positive ions in the cloud
4. Heat lightning: lightning that occurs too far away for the sound of thunder to be heard
5. Dry lightning
   1. Dry lightning is lightning that occurs without precipitation at the surface
   2. This is the most common natural cause of wildfires
   3. Occurs as a result of extreme surface temperatures when convection from the hot surface to cooler atmosphere leads to lightning
6. Positive lightning
   1. Occurs when positive charge is carried on the top of clouds
   2. Very rare
   3. Around 10 times more powerful and longer lasting than regular negative lightning
   4. Very dangerous to life and property
   5. At present, aircraft are not designed to withstand positive lightning
7. Sprite
   1. Large scale discharges occurring high above a thundercloud
   2. Occur about 80 km to 150 km above the Earth’s surface
   3. Reddish-orange or greenish-blue in colour
   4. May account for aircraft accidents at altitudes above thunderstorms
8. Blue jets
   1. Occur at lower altitudes than sprites, but still above thunderclouds
   2. Occur about 40 km to 80 km above surface
   3. Blue in colour
9. Elves
   1. ELVES stands for Emissions for Light and Very low frequency Electromagnetic pulse Sources
   2. Occur in the ionosphere, about 100 km above surface
10. Rocket-triggered lightning
    1. Lightning can be triggered by rockets carrying spools of wire into thunderstorms. When the wire unwinds, it provides a path for lightning to conduct to the surface
    2. Lightning can also be triggered by space shuttle launches and aircraft flight
11. Volcanically triggered lightning
    1. Extremely large volcano eruptions which eject gases and material high into the atmosphere can trigger lightning

LIGHTNING IN EVERYDAY LIFE

World map showing the frequency of lightning strikes. Lightning strikes most frequently in the Congo

Lightning conductor

• It is a metal rod or conductor used to protect a building from lightning
• It is mounted on the top of the building and connected to the ground using a wire
• When strikes, it will preferentially strike the rod and be conducted harmlessly to the ground
• Lightning rods are usually made from good conductors of electricity such as aluminium or copper

Lightning protection on aircraft

• On aircraft, an electrical circuit is established on the aircraft’s outer surface
• Aircraft made from aluminium naturally act as good conductors of electricity. When the aircraft is made of carbon composites, a layer of conductive fibre is embedded to ensure conductivity
• When the aircraft is struck by lightning, current travels on the outer surface of the aircraft, with the interior remaining unaffected
• Proper shielding is provided to ensure lightning does not affect cockpit electronics, fuel tanks and radar and other avionics
• Aircraft also use static dischargers to prevent buildup of static electricity

Trees and lightning

• Trees are natural conductors of lightning. They provide connection for lightning to reach the ground. However, the outer layer of trees (bark) is not a good conductor
• Trees get burnt from lightning because lightning travels on the outer surface of the tree, burning away the bark.
• Usually, trees can recover from damage to the bark. However, sometimes the damage is too severe for recovery.
• Oak and elm are two trees most frequently struck by lightning. Teak provides the best conducting connection for lightning
• By attracting lightning towards them, trees prevent damage to nearby buildings. However, for the same reason, it is not safe to seek shelter under trees during lightning

Shelter from lightning

• To get shelter from lightning, there needs to be an electrical connection through the exterior surface on to the ground. The connection must ensure that people do not get in contact with the electricity
• Best lightning shelters: houses, buildings, closed-roof cars, closed-cabin boats etc
• Worst lightning shelters: trees, tents, open barns, open-roof cars, open boats etc
• It is unsafe to use radios, cellphones etc during lightning strikes

MEASUREMENT OF ATMOSPHERIC ELECTRICITY

1. Electrometer
   1. Simple instrument for measuring atmospheric electricity at ground surface
   2. Developed in 1700s by Alessandro Volta
   3. Consists of a glass jar with a pointed metal rod, whose lower end is attached to two straws. Electricity in the atmosphere cause the two straws to recede from each other, the amount of divergence indicating the intensity of electricity
2. Weather balloons
   1. A balloon which carries instruments aloft to send back information regarding temperature, humidity etc
   2. The device that does the actual measuring is called radiosonde
   3. The radiosonde is an inexpensive device, and it is lost when the life of the balloon expires
3. Lightning rocket
   1. It is a device that measures electrostatic and ionic charge in the atmosphere
   2. Consists of a rocket launcher which is in communication with the detection device on the ground
   3. This system controls the time and location of a lightning strike
   4. Uses solid (cesium salts) or liquid (calcium chloride) propellants to produce exhaust gases that act as a conducting pathway between the clouds and the ground

THANKS

JOHAR..