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Transcript
The Sun 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 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 Structure of the Sun 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. 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 ! Atmospheric structure of the Sun • The visible surface of the Sun is called the Photosphere. It emits visible light, infra red and UV radiation 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. Spicules Solar Prominences A large looping curtain of gas Corona • The Corona is the outer atmosphere of the Sun. here radio waves and xrays are emitted. The Corona can reach temperatures of 1,000,000 degrees Kelvin. • The Corona can extend many millions of kilometres into space. 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. Sunspots – produced by the rotating Sun stretching the magnetic field lines 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. Sunspots and Solar flares Sun’s Corona Sunspots Solar flare Emissions from the Sun • Electromagnetic Radiation – Produces a range of radiations from shortwavelength 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. 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. 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. 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. 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 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). The cyclic nature of sunspot activity Radiocarbon levels in tree rings and sunspot activity !! 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. 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. 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. The magnetosphere of the Earth 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. The Auroras