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The Sun
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General details about the Sun
• Nearest star to Earth at distance of 152,100,000 km (max) to 147,100,000 km (min)
• A second generation star with composition– 73.4% Hydrogen
– 25% Helium
– 1.6% Heavier Elements (Iron etc..)
• Heavier elements are not produced in big bang but have been produced in a large red-giant star
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Sun Specifications
• Mass – 1.99 x 1030 kgm• Volume – 1.3 million times the Earth’s volume• Diameter – 1,392,000 km (109 times Earth diameter)• Surface Temperature – 5,500 Kelvin (mean)• Gravity – 280 ms-2 (about 28 times Earth’s gravity)• Light takes around 8 minutes to reach Earth from the
Sun
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Structure of the Sun
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Structure of the Sun – Zone 1• Mostly hot and highly ionised plasma• Core (Zone 1) - Inner most layer where Nuclear
Fusion reaction of Hydrogen to Helium occurs.
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Structure of the Sun – Zones 2 to 3• Radiative Zone 2 – Electromagnetic energy transmitted slowly through this layer• Interface Zone – Thin layer that generates Sun’s Magnetic Field• Convection Zone 3 – Region where energy is transmitted to the surface by
convection currents
• Note: It takes millions of years for the heat to reach the surface !
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Atmospheric structure of the Sun
• The visible surface of the Sun is called the Photosphere. It emits visible light, infra red and UV radiation
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Atmospheric structure of the Sun
• The lower atmosphere is called the Chromosphere which is about 100,000 km thick.
• The Chromosphere contains ‘spike like’ prominences, called spicules, that rise like geysers carrying magnetic fields.
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Spicules
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Solar ProminencesA large looping curtain of gas
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Corona
• The Corona is the outer atmosphere of the Sun. here radio waves and x-rays are emitted. The Corona can reach temperatures of 1,000,000 degrees Kelvin.
• The Corona can extend many millions of kilometres into space.
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Sunspots
• Sunspots are associated with magnetic storms. They are regions of intense magnetic activity.
• They appear as dark spots because they are cooler than the surrounding surface. The temperature of sunspots is about 3800 Kelvin.
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Sunspots – produced by the rotating Sun stretching the magnetic field lines
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Solar flares
• Solar flares are sudden explosive outbursts of radiation and matter near sunspots.
• Both sunspots and solar flares cause massive disruption to radio communications.
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Sunspots and Solar flares
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Sun’s Corona
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Sunspots
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Solar flare
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Emissions from the Sun
• Electromagnetic Radiation– Produces a range of radiations from short-
wavelength gamma to long radio.– However, visible spectrum is most intense
wavelength (peak at 460 nanometres).– The visible and infra-red closely matches
theoretical radiation curve of ‘black body’ at a temperature of 6000 Kelvin.
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Emissions from the Sun
• Solar Wind & Interplanetary Magnetic Field– Difference in pressure between corona and
interplanetary space causes outflow of material from corona. This is the solar wind.
– Speeds of 400–500 kms-1 and takes 3 to 4 days to reach Earth.
– Near Earth has density of 5 protons and 5 electrons per cm3.
– Velocity of particles results in temperature of 10,000 to 100,000 Kelvin.
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Solar Wind & Interplanetary Magnetic Field
• In the photosphere a magnetic ‘coupling’ occurs between magnetic field belonging to plasma of solar wind and Sun’s magnetic field.
• This results in Sun’s magnetic field lines becoming locked into the plasma.
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Solar Wind & Interplanetary Magnetic Field
• The out flowing solar wind thus carries the magnetic field lines with it.
• As the Sun rotates it winds the field lines into a giant spiral – as shown opposite.
• At the Earth the field lines are approximately 45 degrees to the direction of the Sun.
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The Solar Cycle
• Sun experiences a cyclic pattern of turbulent activity (about every 11 years).
• Period of peak activity is solar maximum while period of least activity is solar minimum.
• The solar cycle is an 11 year cyclical pattern of increasing and decreasing frequency of:– Sunspots– Solar flares– Prominences– Coronal mass ejections
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The Sunspot Cycle
• Easiest way to observe the solar activity is to look at sunspot activity.
• The sunspot cycle is a cyclical pattern of increasing and decreasing number of sunspots.
• At solar maximum there is usually over 100 sunspots appearing simultaneously on the Sun’s surface.
• At solar minimum there may be no sunspots.• The sunspot cycle on average is 11 years (but can
vary between 7 and 13 years).
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The cyclic nature of sunspot activity
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Radiocarbon levels in tree rings and sunspot activity !!
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Sunspot activity and solar latitude
• Appearance of sunspots changes in solar latitude during a sunspot cycle. They start at around 40 degree latitude and end at around the Sun’s equator.
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The Sun-Earth Connection - Electromagnetic radiation
• The Sun emits a wide range of electromagnetic radiation.
• In the Earth’s atmospheric different wavelengths of radiation get absorbed by different molecules.
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The Sun-Earth Connection - The Solar Wind
• The solar wind carrying the interplanetary magnetic field interacts with the Earth’s magnetic field.
• Most of the wind flows around and past the Earth distorting the Earths magnetic field to create a ‘tail’ shape.
• The magnetosphere is the region surrounding a planet that contains its distorted magnetic field.
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The magnetosphere of the Earth
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The Van Allen Belts• The Van Allen Belts are two zones of radiation (ions) that
wrap around the Earth. They are a result of ions from the solar wind been captured by the Earth’s magnetic field lines.
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The Auroras