Researchers at the University of Warwick, UK, have found
what could be the signal of ideal wave "surfing" conditions for
individual particles within the massive turbulent ocean of the solar wind. The
discovery could give a new insight into just how energy is dissipated in solar
system sized plasmas such as the solar wind and could provide significant clues
to scientists developing fusion power which relies on plasmas.
The research, led by Khurom Kiyanai and Professor Sandra
Chapman in the University
of Warwick's Centre for
Fusion, Space and Astrophysics, looked at data from the Cluster spacecraft
quartet to obtain a comparatively "quiet" slice of the solar wind as
it progressed over an hour travelling covering roughly 2,340,000 km.
In space, on these large scales, and quiet conditions,
nature provides an almost perfect experiment to study turbulence which could
not be done on Earth in a laboratory. This plasma energy does eventually
dissipate. One obvious way of understanding how such energetic plasma could
dissipate this energy would be if the particles within the plasma collided with
each other. However the solar wind is an example of a "collisionless plasma".
The individual particles within that flow are still separated by massive
distances so cannot directly interact with each other. They typically collide
only once or twice with anything on their journey from the Sun to the Earth.
The University
of Warwick Centre for
Fusion, Space and Astrophysics led team drilled down into the data on this
2,340,000 km zooming down to see how the turbulence works on these different
length scales which might provide some clue as to how the plasma was able to
dissipate energy.
When the researchers were able to make observations all the
way down to about 1 km they could resolve the behavior of individual particles
within the total 2,340,000 km slice of solar wind. These regions, which held
just one particle of the plasma, were themselves almost a kilometer in size.
The researchers were surprised to see a new kind of turbulence on these small
scales.
At this particular scale they saw that the levels of
turbulence switched from being mutlifractal to single fractal pattern. This
single fractal pattern turbulence appears just right to create and sustain
waves that can interact with the individual particles in the solar wind.
University of Warwick astrophysicist Khurom Kiyani said: "The particles in
this "collisionless plasma" may too spread out to collide with each
other but this could indicate that they can, and do, interact with waves and
surfing these ideal waves is what allows them to dissipate their energy."
University of Warwick astrophysicist Professor Sandra
Chapman said "We have been able to drill down through a vast ocean of data
covering well over two million kilometers to get an insight in to what is happening
in an area about the size of a beach, and on all length scales in between. We
believe we are seeing waves on that beach that are providing the ideal surfing
conditions to allow plasma particles to exchange energy without
collisions."
Professor Sandra Chapman also said "These results are
not just an interesting piece of astrophysics as the work has been led by a
'Centre for Fusion, Space and Astrophysics' the results have also immediately
come to the attention of our colleagues working to increase the stability of
plasmas involved in the generation of fusion energy. Turbulence is a big
problem in keeping the hot plasma confined long enough for burning to take
place to generate fusion power."
The research entitled “Global Scale-Invariant Dissipation in
Collisionless Plasma Turbulence” has just been published in Physical Review Letters and was
conducted by Khurom Kiyani, and Professor Sandra Chapman of the University of
Warwick in the UK; Yu.V. Khotyaintsev of Swedish Institute of Space Physics,
Uppsala, Sweden; M.W. Dunlop, Rutherford Appleton Laboratory, United Kingdom;
and F. Sahraoui of 4NASA Goddard Space Flight Center US and the Laboratoire de
Physique des Plasmas, CNRS-Ecole Polytechnique, France.
Study abstract
Centre for
Fusion, Space and Astrophysics
SOURCE: University of
Warwick