Physics Update Course Summer 2013

The Aurora on Earth and Other Planets

Tom Conlon

Mr Conlon is a PhD student within the Radio and Space plasma physics group in the department of Physics and Astronomy The University of Leicester.

An aurora is a natural light display in the sky particularly in the high latitude (Arctic and Antarctic) regions, caused by the collision of energetic charged particles with atoms in the high altitude atmosphere (thermosphere). The charged particles originate in the magnetosphere and solar wind and, on Earth, are directed by the Earth’s magnetic field into the atmosphere. Most aurorae occur in a band known as the auroral zone, which is typically 3 to 6 degrees in latitudinal extent and at all local times or longitudes.

The human race has been aware of auroras for a very long time.

The book of Ezekiel in the Old Testament likens it to a whirlwind from the north, a great cloud, and a fire infolding itself, and a brightness about it, and out of the midst thereof as the colour of amber, out of the midst of the fire (King James Bible).

Aristotle (384 BC – 322 BC) believed the heat from the sun raised steam from the ground. The steam hit the fire element which ignited and caused the northern lights.

In ancient Roman mythology, Aurora is the goddess of the dawn, renewing herself every morning to fly across the sky, announcing the arrival of the sun.


Aurora, by Guercino, 1621-23: the ceiling fresco in the Casino Ludovisi, Rome,

Seneca (ca. 4 BC – AD 65) speaks of auroras in the first book of his Naturales Quaestiones, drawing mainly from Aristotle; he classifies them as “putei” or wells when they are circular and “rim a large hole in the sky”, “pithaei” when they look like casks, “chasmata” from the same root of the English chasm, “pogoniae” when they are bearded, “cyparissae” when they look like cypresses. He describes their manifold colours and asks himself whether they are above or below the clouds.

In Norse mythology, it is a burning rainbow bridge that reaches between Midgard (the world) and Asgard, the realm of the gods (Bifröst).


The god Heimdallr stands before the rainbow bridge while blowing a horn (1905) by Emil Doepler.

Benjamin Franklin (October 18, 1785 – November 5, 1788) theorized that the “mystery of the Northern Lights” was caused by a concentration of electrical charges in the Polar Regions intensified by the snow and other moisture.

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The images above show the Earth’s Aurora from the Ground

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The images above show the Earth’s Aurora from space. The top left image is Aurora australis (11 September 2005) as captured by NASA’s IMAGE satellite. The top right image is made using data from the POLAR spacecraft. The bottom image is of the Aurora from the International Space Station.

Why do auroras occur in Polar Regions?

It’s all about Magnetism! But why?

Magnets Produce a Magnetic Field! They have invisible lines of magnetic field that flow from North Pole to the South Pole. The poles are the regions where the magnetic field is strongest.


The Earth is a Giant Magnet. It has a huge magnetic field! Which happens to be upside down!

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That is why compasses work.


The Sun also has a magnetic field. But it is more complicated than the Earth’s

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The Sun Can Have Many Pairs of Poles!

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SOHO Satellite Images

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The upper left hand picture is a false colour ultraviolet image of coronal loops. Coronal loops form the basic structure of the lower corona and transition region of the Sun. These highly structured loops are a direct consequence of the twisted solar magnetic flux within the solar body. The right hand picture shows typical coronal loops observed by TRACE.

Plasma is often called the fourth state of matter

Plasma is one of the four fundamental states of matter (the others being solid, liquid, and gas). The Sun can be considered as a ball of plasma.

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Below are examples of the use of plasmas.

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Heating a gas may ionize its molecules or atoms (reduce/increase the number of free electrons), thus turning it into plasma, which contains charged particles: positive ions and negative electrons or ions. Ionization can be induced by other means, such as strong electromagnetic field applied with a laser or microwave generator, and is accompanied by the dissociation of molecular bonds, if present. Plasma can also be created by the application of an electric field on a gas, where the underlying process is the Townsend avalanche.

The Magnetosphere is a protective bubble (full of trapped plasma) around the Earth. It is actually formed by the impact of the solar wind on the Earth’s magnetic field. It is the area of space near an astronomical object, like the Earth, in which charged particles are controlled by that object’s magnetic field It forms an obstacle to the solar wind, diverting it, at an average distance of about 70,000 km.. Near the surface of the object, the magnetic field lines resemble those of an ideal magnetic dipole. Farther away from the surface, the field lines are significantly distorted by external currents, such as the solar wind. When speaking about the Earth, magnetosphere is typically used to refer to the outer layer of the ionosphere.

Below is a picture of the solar wind flowing down the Earth’s magnetic field lines.




When charged particles have velocities perpendicular to a magnetic field, they undergo a circular gyro motion, whose direction may be determined by the right-hand rule. This explains the path taken by the charged particles.

Lorentz force

In physics, particularly electromagnetism, the Lorentz force is the force on a point charge due to electromagnetic fields. It deflects charged particles around the Earth rather than bombarding the atmosphere or surface.


Solar wind flows down the Earth’s Magnetic field


The solar wind is a stream of charged particles released from the upper atmosphere of the Sun. It mostly consists of electrons and protons with energies usually between 1.5 and 10 keV. The stream of particles varies in density, temperature, and speed over time and over longitude. These particles can escape the Sun’s gravity because of their high kinetic energy and the high temperature of the corona. The charged particles cannot jump from one field line to another most of the time. When that stops being the case, then you can have particles enter the Earth’s atmosphere and you get an aurora.

What causes the Aurora?

The Earth’s magnetic field traps these particles, many of which travel toward the poles where they are accelerated toward Earth. Collisions between these ions and atmospheric atoms and molecules cause energy releases in the form of auroras appearing in large circles around the poles.


Electrons in the solar wind hit the air molecules in the atmosphere. The air molecules become excited as their own electrons move to higher energy levels. The molecules give off light (emitted) as they calm down. Their electrons move to lower energy levels.

Electron excitation/de-excitation

Light can be considered to exist as packets of energy called photons and the energy (E) of these emitted photons = hc/λ = hv =  (hf) where h is Planck’s constant, c is the speed of light in a vacuum, v (or f) is the frequency of the light and λ is the wavelength of the light. The energy is equal to the difference in the energy levels. In the picture below on the right the red wave’s energy would be E2 – E1. The n stand for the number of the energy level, not its value. n = 1 is the ground state (the lowest energy state).

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Schematic diagram of spontaneous emission (In physics frequency is given the letter f but chemists use v)

Other Planets Have Aurorae Too

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We can look at them with the Hubble Space Telescope.


Saturn’s Aurora Looks Similar to Earth’s.


Jupiter’s Aurora also looks similar but works very differently.


It is caused by charged particles from the sun and charged particles given off by its volcanic moons. The moons give off charged particles from the volcanic activity via an invisible current of charged particles called a “flux tube. The particles then follow Jupiter’s magnetic field toward the poles. The charged particles then hit the atmosphere and cause the light of the aurora.

The auroral footprint occurs when the particles reach Jupiter’s upper atmosphere.


Aurorae sound weird; right? They create radio waves and listening for an aurora could help us find other planets that can support life.

Kristian Birkeland (1867 – 1917) was a Norwegian scientist and one of the founding fathers of modern space science. He built the Haldde Observatory in Northern Norway where he spent a winter with his research students observing the Earth’s aurora and measuring the Earth’s magnetic field. He hypothesised that the Earth’s aurora were created by electrons from the Sun propagating along the Earth’s magnetic field lines and precipitating into the Earth’s upper atmosphere in the polar regions.

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The above picture on the right shows Kristian Birkeland and his magnetized terrella experiment, which led him to surmise that charged particles interacting with the Earth’s magnetic field were the cause of the aurora. A terrella is a small magnetised model ball representing the Earth that is thought to have been invented by the English physician William Gilbert while investigating magnetism, and further developed 300 years later by the Kristian Birkeland, while investigating the aurora.


An example of an active terrella

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