Aurora Borealis in the Artic (northern lights), and the Aurora Australis in the Antarctic (southern lights). Auroras are produced when solar explosions, known as coronal mass ejection, generate highly charged particles, that flow within the solar winds. These are able to disturb the Earth's magnetosphere sufficiently to enter into the lower regions of the atmosphere, by flowing down the magnetic field lines at the north and south magnetic poles. Within the atmosphere, the solar particles and the disturbed magnetic fields release particles already trapped near earth within the Radiation Belts, which triggers the reactions with gases such as Oxygen (O2 and O) and Nitrogen (N2). Once these charged (excited) gases return to their normal, lower energy state, they give off light (photons), which is the oval band that can be seen from Earth and from Space; Green (high energy) and red (lower energy) light from Oxygen, and blue and purple from Nitrogen. The Auroras also have an effect on the ionosphere, within the Earth's atmosphere.
Earth's Geodynamo Theory
It is thought that the Earth has at its' centre, an inner core, a solid mass roughly the size of the moon, which is mostly composed of iron (Fe) and some other elements such as Nickel (Ni), and it's temperature is similar to that of the surface of the Sun. Surrounding the inner core, is the outer-core, that is also believed to be of a similar composition, but is a rotating fluid (rotating due to the rotation of the Earth - the coriolis effect), which moves in spiral currents, across weak magnetic fields, creating an electric current (the dynamo effect). The spiralling of the currents is thought to be driven by convection, where hot, buoyant iron in the lower areas of the outer core (heated by latent heat released by the expanding and cooling inner core), float up to the surface (like a lava lamp), where they cool and therefore get denser, and sink down again. This continuous convection process, also releases heat from the core, into the mantel, which helps drive plate-tectonics.
Structure of the Earth
The dynamics of the structure of the Earth, linked to plate-tectonics will be discussed in a page to come: this is a brief overview of the main structures in the Earth.
The structure of the Earth can be described chemically (separating layers that have different chemical compositions), or mechanically (separating layers that have different physical properties - i.e., solid, or liquid).
The chemical way to describe the Earth’s Structure, starts with the outermost layer - the Crust. It is the thinest layer, and there are two main types: Continental Crust, and Oceanic Crust. The Continental Crust (land), is thick (about 30-60km), relatively light, and is mainly granite. And the Oceanic Crust (the rock under the worlds oceans) is thinner (about 5-10km), heavier, and is mainly basalt. Both float on the layer below, the silicate Mantle, which is about 2900km thick, and below this is the Earth’s Core (as mentioned above is mostly iron). In general, during the development of the early Earth, the densest elements sunk to the centre (the core), and the lighter elements and gases bubbled up to the surface, and some out into the atmosphere.
The mechanical description, combines the crust and the coolest mantle layer, and is called the lithosphere (10-200km thick - although closer to the higher thickness). It is rigid, cool and brittle. The layer below is not a liquid, but is more liquid (plastic) than the layer above, due to the higher temperature, and in terms of the mantle, is still within it (going down to around 660km), and is called asthenosphere. The next lower layer has pressures and temperatures so high, it is rigid, and is called the mesosphere (ending at the bottom of the mantle - so 2900km depth). The core is divided into two layers, the Outer Core, which is liquid (around 5100km deep), and the Inner Core, which is solid.