These lights in northern parts are called aurora borealis and in southern parts are known as aurora australis. During a geomagnetic storm the auroral zone expands to lower latitudes.
So, what makes aurora and what makes it colourful?
Typically the aurora appears either as a diffuse glow or as “curtains” that approximately extend in the east-west direction. At some times, they form “quiet arcs”; at others “active aurora,” they evolve and change constantly. Each curtain consists of many parallel rays, each lined up with the local direction of the magnetic field lines, suggesting that auroras are shaped by Earth’s magnetic field. Indeed, satellites show that electrons are guided by magnetic field lines, spiralling around them while moving toward Earth.
Formation of aurora
The cause of formation of aurora is the emission of photons in the earth’s upper atmosphere , above 80 kms, from ionised nitrogen atoms returning from a ionised or excited state to ground state. They get ionised or excited by the collision of solar wind and magnetosphere particles being directed down and accelerated along earth’s magnetic field lines.
Excess energy is lost by the emission of a photon, or by collision with an outer atom or molecule.
Diffuse aurora is a featureless glow in the sky that may not be visible to the naked eye even on a dark night.
Discrete aurora, on the other hand, are sharply defined features within the diffuse aurora that vary in brightness from just barely visible to fairly bright.
Charged particles originate in magnetosphere and solar wind and on earth are directed by earth’s magnetic field into the atmosphere.
The ultimate energy source of the aurora is the solar wind flowing past the Earth. The magnetosphere and solar wind consist of plasma (ionised gas) which conducts electricity. It is well known that when an electrical conductor is placed within a magnetic field while relative motion occurs in a direction that the conductor cut across ,rather than along, the lines of the magnetic field, an electric current is said to be induced into that conductor and electrons flow within it.
Factors; solar wind and magnetosphere
The Earth constantly has to face solar wind, a flow of the gas of free electrons and positive ions emitted by the Sun in all directions, a result of the two-million-degree heat of corona, the Sun’s outermost layer. The solar wind usually reaches Earth with a velocity around 400 km/s, density around 5 ions/cm3 and magnetic field intensity around 2–5 nT. These are typical values. During magnetic storms in particular, flows can be several times faster.
Earth’s magnetosphere is formed by the impact of the solar wind on the Earth’s magnetic field. It forms an obstacle to the solar wind, diverting it. The width of the magnetosphere abreast of Earth, is typically 190,000 km (30 Re), and on the night side a long “magnetotail” of stretched field lines extends to great distances (> 200 Re).The magnetosphere is full of trapped plasma as the solar wind passes the Earth. The flow of plasma into the magnetosphere increases with increases in solar wind density and speed, with increase in the southward component of the IMF and with increases in turbulence in the solar wind flow. The flow pattern of magnetospheric plasma is from the magnetotail toward the Earth, around the Earth and back into the solar wind through the magnetopause on the day-side. In addition to moving perpendicular to the Earth’s magnetic field, some magnetospheric plasma travel down along the Earth’s magnetic field lines and lose energy to the atmosphere in the auroral zones. Magnetospheric electrons accelerated downward by field-aligned electric fields cause the bright aurora features. The un-accelerated electrons and ions cause the dim glow of the diffuse aurora.
Type of emissions and their effects on colour patterns
- Oxygen emissions: green or brownish-red, depending on the amount of energy absorbed.
- Nitrogen emissions: blue or red; blue if the atom regains an electron after it has been ionized, red if returning to ground state from an excited state.
Oxygen is unusual in terms of its return to ground state: it can take three quarters of a second to emit green light and up to two minutes to emit red. Collisions with other atoms or molecules absorb the excitation energy and prevent emission. Because the very top of the atmosphere has a higher percentage of oxygen and is sparsely distributed such collisions are rare enough to allow time for oxygen to emit red. Collisions become more frequent progressing down into the atmosphere, so that red emissions do not have time to happen, and eventually even green light emissions are prevented.
This is why there is a colour differential with altitude; at high altitude oxygen red dominates, then oxygen green and nitrogen blue/red, then finally nitrogen blue/red when collisions prevent oxygen from emitting anything. Green is the most common of all auroras. Behind it is pink, a mixture of light green and red, followed by pure red, yellow (a mixture of red and green), and lastly, pure blue.
Auroras are associated with the solar wind, a flow of ions continuously flowing outward from the Sun. 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. Auroras are more frequent and brighter during the intense phase of the solar cycle when coronal mass ejections increase the intensity of the solar wind.
References & Resources
- Image via National Geographic.
- Halliday D., Resnick R., Walker J. (2000) Fundamentals of Physics (6th ed.). Published by John Wiley & Sons.
- Personal knowledge