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Fractal Solar Wind


Massive solar flare starting and ending on earth’s surface. Click to enlarge.Sunspot cycles are hot right now - literally and figuratively. After just posting about Cycle 24 - the about-to-begin 11-year cycle of sunspot activity, I now see a lot of references to scientists at Warwick University reporting on observed fractal nature of the solar wind, and the ramifications for prediction and understanding of sunspot cycles

The articles announcing the findings have been very exuberant about this latest finding, but so far they are short on some crucial facts. They are also woefully inadequate when it comes to reporting previous work.

Most web sites are just reproducing the press release from the University of Warwick:

The researchers, led by Professor Sandra Chapman, have also been able to directly tie these fractal patterns to the Sun's 'storm season'. The Sun goes through a solar cycle roughly 11 years long. The researchers found the fractal patterns in the solar wind occur when the Sun was at the peak of this cycle when the solar corona was at its most active, stormy and complex - sunspot activity, solar flares etc. When the corona was quieter no fractal patterns were found in the solar wind only general turbulence.

From this description it is not clear what the fractal pattern is. The New Scientist site provides more details, including a possible reason for the fractal pattern:

Sandra Chapman and colleagues at the University of Warwick, UK, used sensors on the ACE, Wind and Ulysses spacecraft to measure the sun's magnetic field strength in the solar wind as it fluctuates over time. When plotted on a graph, the team found that this pattern becomes fractal when the sun is in the most stormy phase of its 11-year cycle, they report in an upcoming issue of Physical Review Letters. "The field describes a wiggly line, just like a crinkly coastline," Chapman says. She says the fractal pattern could be created by energetic "ropes" of swirling magnetic flux, which cross over in an ordered and organised way during the stormy season. The observation could provide an insight into the driving force that accelerates the solar wind to such high speeds away from the sun, something that is poorly understood, Chapman says.

This is exciting news, but I wonder what is the major difference between what the Warwick group has observed and what was noted way back in 1993 when Russian physicists published a paper titled "Fractal and Multifractal Structures in Solar Wind. This work, by L.M. Zeleny and A.V. Milovanov, was published in Geomagnetizm i Aehronomiya. From the abstract:

Results of a study of interplanetary magnetic field turbulence at heliocentric distances of 1-30 AU are analyzed. It is shown that the fractal distribution of magnetic force tubes over the sun surface, discovered at distance scales of 400-40,000 km, leads to the formation of magnetic clouds at heliocentric distances of about 10 solar radii, with the spatial distribution of the magnetic clouds in solar wind characterized by a fractal dimension close to 3/2. The value of the spatial fractal dimension that determines the fine structure of the magnetic cloud as a fractal cluster of magnetic force tubes is obtained. The relationship between the spatial fractal dimension and the turbulence spectrum of interplanetary plasma is examined.

On the surface, this sounds remarkably like the current findings. I have not yet seen the article by the Warwick group. Titled "Self- similar signature of the active solar corona within the inertial range of solar wind turbulence", the work was published on May 18th 2007 in Phys. Rev. Lett. 2., by K.Kiyani, S. C. Chapman, B. Hnat, and R. M. Nico. I assume that they refer to the Russian work. If anyone has access to this particular issue of Phys Rev. Lett., please post a comment describing whether this is true.