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.
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
Figure 2b:
