Double layer (plasma physics)

[1] Ions and electrons within the double layer are accelerated, decelerated, or deflected by the electric field, depending on their direction of motion.

Under most laboratory situations, unlike outer space conditions, charged particles may effectively originate within the double layer, by ionization at the anode or cathode, and be sustained.

The figure shows the localised perturbation of potential produced by an idealised double layer consisting of two oppositely charged discs.

The details of the formation mechanism depend on the environment of the plasma (e.g. double layers in the laboratory, ionosphere, solar wind, nuclear fusion, etc.).

Proposed mechanisms for their formation have included: In a low density plasma, localized space charge regions may build up large potential drops over distances of the order of some tens of the Debye lengths.

It was first proposed by Hannes Alfvén (the developer of magnetohydrodynamics from laboratory experiments) that the polar lights or Aurora Borealis are created by electrons accelerated in the magnetosphere of the Earth.

Since then other mechanisms involving wave-particle interactions have been proposed as being feasible, from extensive spatial and temporal in situ studies of auroral particle characteristics.

A series of such double layers would tend to merge, much like a string of bar magnets, and dissipate, even within a rarefied plasma.

It has yet to be explained how any overall localised charge distribution in the form of double layers might provide a source of energy for auroral electrons precipitated into the atmosphere.

Interpretation of the FAST spacecraft data proposed strong double layers in the auroral acceleration region.

[51][52] The possible role of precipitating electrons from 1-10keV themselves generating such observed double layers or electric fields has seldom been considered or analysed.

[56] Similar whistler wave structures were observed together with electron beams near Saturn's moon Enceladus,[57] suggesting the possible presence of a double layer at lower altitude.

[citation needed] Unlike experiments in the laboratory, the concept of such double layers in the magnetosphere, and any role in creating the aurora, suffers from there so far being no identified steady source of energy.

The electric potential characteristic of double layers might however indicate that, those observed in the auroral zone are a secondary product of precipitating electrons that have been energized in other ways, such as by electrostatic waves.

[58][59] Establishing such a role indirectly is even harder to verify than postulating double layers as accelerators of auroral electrons within the Earth's magnetosphere.