Aurora Borealis, Northern Lights Explained
The Northern Lights, Aurora Borealis, appear in a clear night sky as swirling rivers of greenish-blue light. They move and dance unpredictably; sometimes barely perceptible, then suddenly growing vivid. In simple terms, the auroras can be explained as an interaction of the solar wind and the Earth’s magnetic field. The solar wind consists of charged particles emanating from the Sun. These are captured by the Earth’s magnetic field and drawn towards the poles at a very high speed.
The phenomenon occurs when the particles collide with atoms and molecules in the upper atmosphere (the ionosphere), transforming kinetic energy into visible light.
How the scientists explain Northern Lights
Luckily, the Swedish Institute for Space Physics, located in Kiruna and with decades of scientific research into Northern Lights, can help us explain:
The particles originating from the ionosphere travel at enormous speed, with help from the solar wind. When the particles enter the atmosphere, they collide with atoms. The particle “rubs off” some its energy to the atom, but still has enough to continue moving although at lower speed. Eventually, it collides with another atom.
Every atom that has collided with the particles now have a surplus of energy, as it releases it, a light appears.
The closer you are to Earth, the more atoms in the atmosphere. So the particle collides with more and more atoms, transmitting some of its energy to each and gradually traveling more slowly and finally coming to a stop, usually around 100 km above the Earth.
The factors at play when the Northern Lights appear on Earth, or any planet:
- There needs to be an atmosphere as this is the “screen” on which the Aurora Borealis is projected. No atmosphere – no atoms for the particles to collide with!
- Secondly, there need to be charged particles or “plasma” that can transmit excess energy to the atoms.
- There needs to be a magnetic field that attracts the particles as they would otherwise just continue through space.
- You also need a “battery” or source of energy that can charge the particles. On Earth, that source is the Sun.
- Finally, you need to transfer the energy from the source to the particles in order to charge the particles. It is the solar winds that transport energy from the Sun to the particles.
Solar winds, solar storms
In order for the charged particles in space to reach the high speed that is required to create Northern Lights, they need the energy from the Sun. The solar wind consists of protons and electrons discharged from the Sun at a speed of 400 km per second. These particles are collectively called plasma.
Plasma is kind of gas, but there is one significant difference. Gas has no electric energy whereas plasma does. In gas, all electrons (negative energy) and protons (positive energy) are organized in atoms and there are equally many of both kinds so they balance each other. Also, atoms have neutrons which are not charged at all.
The Earth creates a barrier for the solar wind. Most of the plasma in the solar wind travel around the Earth, kind of how water travels past a rock in a stream.
The solar wind envelops the magnetic fields of the Earth in a bubble, creating our magnetosphere that works as a shield against particles traveling in space. Planets with magnetic fields have magnetospheres, but other space objects and even entire galaxies can have them as well.
Every second, the edge of the magnetosphere, the magnetopaus, is hit by enourmous amounts of energy. Of all this energy, only a small fraction filters through to the atmosphere, and an even smaller fraction becomes visible as Northern Lights.
A magnetic substorm (solar storm), is an interruption in the magentic field. These interruptions take place in the Auroral Oval and are caused by changes in the magnetic field and charged particles within it. The storm lasts about 10-30 minutes and afterwards the Northern Lights are usually extra strong.
What Northern Lights look like – arch, band, coronas
Aurora Borealis can appear in many different shapes. Early at night, it is usually a grand arc stretching across the sky from east to west. Usually it is green and with fuzzy edges. The bottom of the arc is quite sharp while the upper edge is blurred.
Sometimes, the arc can be active and start moving so it looks like curtains. You may see rays of light that appear in the same direction as the Earth’s magnetic field. The length of the arc can reach up to 1000 km or more, while its width can be limited to as little as 100 m.
An aurora band is similar to an arc but crimped or curled into a spiral.
If you stand right underneath the northern lights in the same direction as the magnetic field, the Aurora rays appear to originate from one single point and radiate outwards. This is called a corona.
If you are watching the same Northern Lights, but from a location further south or north, it will appear as an arch.
The most intense Northern Lights, which appear after solar storms, the entire sky can be filled by incredible shapes and colors including green, red, blue and violet. On these occasions, the light can be strong enough to read a text in a book or magazine.
The shades and shapes change very suddenly. The most intense activity may not last more than 10 minutes so the trick is to check the sky often!
When the extra energy transmitted to the magnetosphere from the solar storm is reducing, you may see pulsating Northern Lights. Underneath, you may see pale patches of light appearing and disappearing randomly and with different duration but usually only a few seconds. These types of Northern Lights are most common after midnight.
The most common Aurora is called “Different” Northern Lights. They are difficult to see because they lack shapes and may just appear as a soft glow over big sections of the sky.
In fact, there is always Northern Lights somewhere on the sky even if you can’t see it. Around the Earth’s magnetic poles there is always a ring of Aurora but you may be in the wrong location. Or, you may be in the right location but still can’t see it because of clouds, daylight or bright summer nights.