[Northern lights (with Table
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For the charged particles in space to gain the high velocity required to make auroras, an energy source is needed. That energy source is the Sun. The part of the Sun's energy that makes auroras is transported to the Earth in a special way, with the so-called solar wind. The solar wind mostly consists of protons, positive charged particles, and electrons, negative charged particles. These charged particles have a common name, plasma. The plasma is hurled from the Sun with an enormous velocity, approximately 400 kilometres per second.
Plasma is a kind of gas, but there is an important difference. A gas has no electric charge, all of the protons and electrons are stuck into its atoms. In an atom there is the same amount of negative charged electrons, and positive charged protons. There are also neutrons that have no charge at all. When there is the same amount of positive and negative charged particles the total sum of the atom is no charge.
The picture shows an atom, with its protons and neutrons in the core and the electrons circling.
In a plasma the electrons and protons move freely; because of this the plasma can be electrically charged.
The picture shows the difference between the particles in a gas and those in a plasma.
The Earth is an impediment to the solar wind. Actually it is the Earth's magnetic field that stops the solar wind. The solar wind is pushing, the Earth's magnetic field is pushing back and a bow shock is created. The bow shock is similar to the one created by an aeroplane flying faster than sound. The main part of the solar wind plasma is diverted from its path and does not interact with the Earth. You can compare it to a rock in a stream. An essential difference is that the Earth's bow shock arises without any particles colliding.
The solar wind encloses the Earth's magnetic field in a bubble. The bubble is the Earth's magnetosphere. The magnetosphere acts as a shelter to the Earth: it makes sure that no particles from space hit the Earth. The magnetosphere is a limited area in space where forces from magnetic fields control the movements of the plasma in the area. Magnetospheres are found around planets with magnetic fields, but even other astronomical objects that are pulsating, and whole galaxies, can have magnetospheres.
The image shows the Sun and the Earth's magnetosphere. On the side turned towards the Sun the solar wind is pushing the magnetosphere together. On the other side the solar wind is pulling it into a tail called the magnetotail which reaches far beyond the orbit of the Moon.
In a period of one second, the outer limit of the magnetosphere, the magnetopause, is hit with the same amount of energy that is produced by all Swedish power plants in three years. Just a small part of the energy reaches the magnetosphere and a small amount of that energy becomes auroras.
A magnetic substorm is a local disturbance in the magnetic field acting in the auroral zone. The storm lasts about 10- 30 minutes and is followed by strong auroras. The substorms are due to changes in magnetic fields and charged particles in the magnetotail, which is why they are also called magnetospheric storms.