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First direct evidence of cavitating Langmuir turbulence occurring naturally in any space or astrophysical plasma

9 March 2012


Patrick Guio and co-workers

Cavitating Langmuir Turbulence in the Terrestrial Aurora

Phys. Rev. Lett. 108, 105003 (2012)


Radar observations of the Earth's aurora reveal, for the first time ever, the natural and spontaneous formation of warm, low density 'bubbles' of plasma in the ionosphere: the hallmark of strong Langmuir turbulence. This process has previously been observed only in an artificial context: the driving effects of high-frequency radio waves emitted by powerful transmitters used in ionospheric modification experiments. 

The universe is known to consist overwhelmingly of high-temperature ionised gases known as plasmas.  Langmuir waves are electric waves present in a plasma due to the interaction of electrons, like ocean waves are the product of wind blowing on the ocean surface.  These waves, in a quiescent plasma, are unorganised like the swell on the sea. However, a beam of energetic electrons, like a wind storm, can dramatically increase the level of wave activity leading to Langmuir turbulence.

Understanding turbulence is one of the last frontiers of classical physics.  Such interaction between waves and a beam of electrons is known to occur in controlled laboratory and space plasma experiments.  It is also thought to occur naturally in space and astrophysical plasmas as varied as pulsar magnetospheres and the Earth's ionosphere.  In its ultimate form, Langmuir turbulence contains waves that are trapped in electron density depressions just like voids or bubbles in a liquid would occur in pumps, propellers, impellers, and in the vascular tissues of plants.  Cavitation is the term used to describe the behaviour of such bubbles. 


Related links:

American Physical Society (APS) Synopsis: Watch Those Cavities


Figures:

Figure 1:

The strong central peak in this spectrum is a signature of cavitation -- the creation of "bubbles" in the Earth's turbulent ionosphere in response to "driving" by a beam of electrons -- the same electrons which produce the Earth's beautiful auroral displays.

Radar observations of plasma (ion-acoustic) waves scattered from the Earth's ionosphere.


Figures 2a and 2b:

Simulations (and predictions) of radar observations for different operating frequencies (wavenumbers), for the cases with (Figure 2b) and without a beam of electrons (Figure 2a). The horizontal white lines at the bottom of each plot indicate that "cut" of the wavenumber-frequency space for our radar observations. The strong central peak at zero-frequency is clearly seen for the beam case in the middle panel (ion-acoustic waves) over a wide range of wavenumbers.

Figure 2a

radar observations of plasma (ion-acoustic) waves scattered from the Earth's ionosphere.


Figure 2b:

Simulations (and predictions) of radar observations for different operating frequencies (wavenumbers), for the cases with and without a beam of electrons.