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 ProminencesA large looping curtain of gas

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.

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 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.

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