AURORA

Introduction:

A Natural electrical phenomenon characterized by the appearance of streamers of reddish or greenish light in the sky , especially near the northern or southern magnetic pole . The effect is caused by the  interaction of charged particles from the sun with atoms in the upper atmosphere. In northern and southern regions it is respectively called AURORA BOREALIS or Northern lights and AURORA AUSTRALIS or Southern Lights.

An Aurora( plural : auroras or aurorae ) ,sometimes referred to as polar lights , northern lights, southern lights , is a natural light display in the earth’s sky, predominantly seen in the high – latitude regions.

Auroras are produced when the magnetosphere is sufficiently disturbed by the solar wind that the trajectories of charged particle in both solar wind and magnetospheric plasma, mainly in the form of electrons and protons, precipitate them into the upper atmosphere due to Earth’s magnetic field, where their energy is lost.

An Aurora is the impressive end result of a series of events that starts at the sun. The sun constantly emits a stream of charged particles known as the solar wind into the  depths of the solar system. When these particles reach a planet, such as Earth, they interact with the magnetic field surrounding it ( the magnetosphere ), compressing the field into a teardrop shape and  transferring  energy to it.

Because of the way the lines of a magnetic field can change, the charged particles inside the magnetosphere can then be  accelerated into the upper atmosphere . Here they collide with molecules such as nitrogen and oxygen, giving off energy in the form of light . This creates ribbon of colour that can be seen across the sky close to the planet’s magnetic north and south poles – this is the aurora.

Gas giant auroras:

Using measurements from spacecraft, such as Cassini, or images from telescopes, such as the Hubble Space Telescope, space physicists have been able to verify that some of our closest neighbours have their own auroras. Scientists do this by studying the electromagnetic radiation received from the planets, and certain wavelength emissions are good indicators of the presence of auroras.

Each of the gas giants (Jupiter, Saturn, Uranus and Neptune) has a strong magnetic field, a dense atmosphere and as a result, its own aurora. The exact nature of these auroras is slightly different from Earth’s, since their atmospheres and magnetospheres are different. The colours, for example, depends on the gases in the planet’s atmosphere. But the fundamental idea behind the aurora is the same.

Interaction of solar wind:

The solar wind is a constant outflow of electrons and protons from the Sun, always present and buffeting Earth’s magnetic field. The background solar wind flows at approximately one million miles per hour.

Even though auroras are best seen at night, they are actually caused by the sun. The sun sends us more than heat and light; it sends lots of other energy and small particles our way. The protective magnetic field around Earth shields us from most of the energy and particles, and we don’t even notice them.

But the sun doesn’t send same amount of energy all the time. There is a constant streaming solar wind and there are also solar storms. During one kind of solar storm called a coronal mass ejection, the sun burps out a huge bubble of electrified gas that can travel through space at high speeds.

When a solar storm comes toward us, some of the energy and small particles can travel down the magnetic field lines at the north and south poles into Earth’s atmosphere. There, the particles interact with gases in our atmosphere resulting in beautiful display of light in the sky. Oxygen gives off green and red light. Nitrogen glows blue and purple.

A typical aurora display consists of these forms appearing in the above order throughout the night.

• Red: At the highest altitudes, excited atomic oxygen emits at 630nm; low concentration of atoms and lower sensitivity of eyes at this wavelength makes this colour visible only under more intense solar activity. The low number of oxygen atoms and their gradually diminishing concentration  is responsible for the faint appearance of the top parts of the “curtains”. Scarlet, Crimson and carmine are the most often seen hues of red for the auroras.

• Green :  At lower altitudes, the more frequent collisions suppress the 630 nm mode: rather the 557.7 nm emission dominates. Fairly high concentration of atomic oxygen and higher eye sensitive in green makes auroras the most common.

•Blue: At lower altitudes, atomic oxygen is uncommon, and molecular nitrogen and ionized molecular nitrogen take over in producing visible light emission, radiating at a large no. of wavelengths in both red and blue parts of the spectrum, with 428 nm being dominant.

•Ultraviolet:  Ultraviolet radiation form auroras have been observed with the requisite equipment. Ultraviolet aurora have also been seen on Mars, Jupiter and Saturn.

•Infrared: Infrared radiation, in wavelengths that are within the optical window, is also part of many auroras.

•Yellow and pink are a mix of red and green or blue. Other shades of red, as well as orange, may be seen on  rare  occasions.

How does the altitude effect the colour of the aurora?

The strong , green light originates at altitudes of 120km to 180km. Red northern light occurs at even higher altitudes, while blue and violet occur mostly below 120km. When the sun is “stormy”, red colours occurs at altitudes of 90 to 100km. Entirely red Northern lights may sometimes be seen, particularly at low altitudes.

The different colours of aurora at different altitude relates to the varying composition of the earths atmosphere and its decreasing density moving away from the surface.

Auroras of different planets:

Earth’s Aurora:

The resulting ionization and excitation of atmospheric constituents emits light of varying colour and complexity. The form of aurora, occurring within  bands around both the polar region.

Jupiter Aurora:

Jupiter’s main Auroral ring maintains a constant size. This is due to its formation through interaction within its own magnetic environment. Jupiter’s moon are also believed to be able to influence auroras.

Saturn Aurora:

On Saturn, the strongest auroras are in the UV and infrared bands of the spectrum and so would not visible to the human eye. But weaker pink and purple auroras have also been spotted.

Venus Aurora:

Astronomers love a good mystery, and here’s one they’ve pondered for decades. That is, Venus may have green auroras despite the fact it has no magnetic field of its own.

Mars Aurora:

 On Mars, aurora appear near areas of magnetised rock within the planet’s crust rather than near the poles, when charged solar particles concentrate towards them. This type of aurora formation is totally unique to mars as far as scientists are aware.

Summary:

Witnessing an aurora first hand is a truly awe-inspiring experience. The natural beauty of the northern or southern lights captures the public imagination unlike any other aspect of space weather. But auroras aren’t unique to Earth and can be seen on several other planets in our solar system.

An Aurora is the impressive end result of series of events that starts at the sun. The sun constantly emits a stream of charged particles known as the solar wind into the depths of the solar system. When these particles reach a planet, such as Earth, Jupiter, they interact with the magnetic field surrounding it, compressing the field into a teardrop shape and transferring energy to it.

Because of the way the lines of a magnetic field can change, the charged particles inside the magnetosphere can then be accelerated into the upper atmosphere. Here they collide with molecules such as nitrogen and oxygen, giving off energy in the form of light. This creates a ribbon of colour that can be seen across the sky close to the planet’s magnetic north and south poles- this is the Aurora.

SUBMITTED BY:

Priyalatha .M.

Ramyashri.

Preethi.

Sahana Rao L.N.

1st M.Sc. (2^ndSemester)     

SUBMITTED ON: 10^th April 2019

REFERENCE:

Internet

Journal