A Universal Planetary Substorm Model

The sun continuously emits a stream of electrically charged particles, known as the solar wind, into space. This wind, travelling at 100s to 1000s km/s, propagates throughout the solar system and all the planets move through it as they orbit the sun. Planets like the Earth are protected from the dangers of the solar wind since the charged particles that it consists of are hindered from striking the surface by a strong internally generated magnetic field. Without this field, the solar wind would strip away the atmosphere and cause enormous damage to living cells, making macroscopic life as we understand it impossible. Although the surface is protected, the solar wind does have important effects, the largest of which is the distortion of Earth's magnetic field, which becomes compressed on the up-stream side of the solar wind and stretched into a long "magnetotail" on the down-stream side. This magnetotail extends beyond the orbit of the moon to a distance over 1,000 times the radius of Earth itself. Events in the magnetotail known as "substorms" accelerate particles from space into Earth's upper atmosphere where they have numerous impacts, the most spectacular of which is the triggering of the Aurora Borealis and Aurora Australis. There are also numerous technological impacts, including electrical charging and increased drag on spacecraft in Low Earth Orbit, which can lead to reduced operational lifetime of the satellites.

Substorms can be described as a simple electrical circuit with a generator in the magnetotail and a resistor in the upper atmosphere linked by "wires" following lines of magnetic force, however the effects of this circuit are not easily predictable because the generator is not creating a steady current. Instead the current varies depending on the "weather conditions" of the solar wind, which provides the energy source for the circuit.

Mercury, Saturn and Jupiter have magnetotails similar in form to that of Earth and also experience substorms. At Mercury the substorm circuit generator is driven by the solar wind, as it is at Earth but at Saturn and Jupiter there is another source of energy for the generator - the planets' own rapid rotation. Jupiter's substorm circuit generator is dominated by this rotational energy source but is still influenced by the solar wind. At Saturn the substorm generator's energy is supplied in significant amounts by both rotational and solar wind inputs.

The aim of this project is to model the substorm electrical circuit for any planet with a magnetotail similar to that of Earth or Jupiter with the intent of understanding the circumstances when substorms are present, when they occur in a predictable, periodic fashion and when they exhibit chaotic, unpredictable behaviour in terms of the energy input from the solar wind and rotational sources. This will allow limits to be placed on the timescale that forecasting of the effects of the substorm circuit is possible for, in the same way that limits can be calculated for the useful timescale of a weather forecast.

Because the model will not be limited to any one set of planetary characteristics (particularly rotational speed, magnetic field strength and orbital distance) or specific set of solar wind conditions, it will not be limited to just one planet but instead can be used for any planet with a magnetotail like Earth's. This means it can be used to make predictions about planets outside the solar system which in turn can be used to help understand the habitability of these planets of from the perspective of protection of the surface from stellar wind particles.