Origin of Aurora

The origin of aurora is 93 million miles (149 million kilometer) fro earth at the sun. The ultimate energy source of the aurora is the solar wind flowing past the Earth. The magnetosphere and solar wind consist of plasma (ionized 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 cuts across (or is cut by), rather than along, the lines of the magnetic field, an electric current is said to be induced into that conductor and electrons will flow within it.

The amount of current flow is dependent upon a) the rate of relative motion, b) the strength of the magnetic field, c) the number of conductors ganged together and d) the distance between the conductor and the magnetic field, while the direction of flow is dependent upon the direction of relative motion. Dynamos make use of this basic process, any and all conductors, and solid or otherwise are so affected including plasmas or other fluids.

Energetic particles from the sun are carried out into space along with the ever present hot solar wind. This wind sweeps supersonically towards earth through interplanetary space at speeds ranging from 300 to over 1000 km per second, carrying with it the solar magnetic field.

    Aurora australis (September 11, 2005) as captured by NASA's image satellite

In particular the solar wind and the magnetosphere are two electrically conducting fluids with such relative motion and should be able (in principle) to generate electric currents by "dynamo action", in the process also extracting energy from the flow of the solar wind. The process is hampered by the fact that plasmas conduct easily along magnetic field lines, but not so easily perpendicular to them.

So it is important that a temporary magnetic connection be established between the field lines of the solar wind and those of the magnetosphere, by a process known as magnetic reconnection. It happens most easily with a southward slant of interplanetary field lines, because then field lines north of Earth approximately match the direction of field lines near the north magnetic pole and similarly near the south magnetic pole. Electric currents originating in such way apparently give auroral electrons their energy. The magnetospheric plasma has an abundance of electrons, some are magnetically trapped, some reside in the magneto tail, and some exist in the upward extension of the ionosphere, which may extend some 25,000 km around Earth.

                   The Aurora Borealis as viewed from the ISS Expedition 6 team.

Bright auroras are generally associated with Birkeland currents which flow down into the ionosphere on one side of the pole and out on the other. In between, some of the current connects directly through the ionospheric E layer (125 km); the rest detours, leaving again through field lines closer to the equator and closing through the "partial ring current" carried by magnetically trapped plasma.

The ionosphere is an ohmic conductor, so such currents require a driving voltage, which some dynamo mechanism can supply. Electric field probes in orbit above the polar cap suggest voltages of the order of 40,000 volts, rising up to more than 200,000 volts during intense magnetic storms.

Ionospheric resistance has a complex nature, and leads to a secondary Hall current flow. By a strange twist of physics, the magnetic disturbance on the ground due to the main current almost cancels out, so most of the observed effect of auroras is due to a secondary current, the auroral electro jet. An auroral electro jet index is regularly derived from ground data and serves as a general measure of auroral activity.

However, ohmic resistance is not the only obstacle to current flow in this circuit. The convergence of magnetic field lines near Earth creates a "mirror effect" that turns back most of the down-flowing electrons (where currents flow upwards), inhibiting current-carrying capacity.

To overcome this, part of the available voltage appears along the field line, helping electrons overcome that obstacle by widening the bundle of trajectories reaching Earth; a similar "parallel potential" is used in "tandem mirror" plasma containment devices. A feature of such voltage is that it is concentrated near Earth and indeed, as deduced by Evans and confirmed by satellites, most auroral acceleration occurs below 10,000 km. Another indicator of parallel electric fields along field lines are beams of upwards flowing O+ ions observed on auroral field lines.

While this mechanism is probably the main source of the familiar auroral arcs, formations conspicuous from the ground, more energy might go to other, less prominent types of aurora, e.g. the diffuse aurora and the low-energy electrons precipitated in magnetic storms.


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Aurora by Menal Salim is licensed under a Creative Commons Attribution-NonCommercial-NoDerivs 3.0 Unported License.
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