This is a cautionary lesson on practical skepticism.
My first thought was, "looks like two parallel Birkeland current filaments undergoing normal Lorentz attraction, winding in around each other until opposing forces overcome the attraction and stabilize their distance apart." Then I read the article from which the image was taken (by right-clicking the blue link, selecting Copy Link Address, pasting it into my browser's new window, and deleting the image portion [ /fig_model.gif ] at the end of the link address). This brings up the actual article from the Max Planck Institute, which tells a lot more about the work on this jet.
The article stated that "The MPIfR scientists have shown that this double helical structure is consistent with Kelvin-Helmholtz (K-H) instability developing in a light jet with Lorentz factor of 2 and Mach number of 3.5." I brought that up with Steve Smith, who noted back that, "Tony Peratt told me that it's often tempting to use fluid dynamics to explain plasma but don't do it." Luckily, Steve's wise words set me to thinking, and I got the article back up, and got out Peratt's book to see precisely what he did say. (Measure twice; cut once.)
The Max Planck Institute article says
They also say that, "To determine the interior structure of the jet, the MPIfR researchers have obtained and analyzed 240 profiles of brightness distribution across the jet. In most of the profiles, the presence of at least two distinct features is visible." You can download and see the step by step cross sections as the imaging line is moved along the jet. Only a few of the images look to have two discrete peaks, and one is usually almost buried in noise.Studying the origin and development of plasma instability in extragalactic jets is crucial for understanding the nature of relativistic outflows from AGN. K-H instability is the most likely type of instability to be found in extragalactic jets... Recent theoretical works have predicted that K-H instability should produce complex, three-dimensional ribbonlike and threadlike patterns inside a relativistic jet.
So, the article is theoretical, it uses MHD which leads to the fluid dynamics K-H instability, against the advice in Hannes Alfvén's Nobel Address, and it depends on a perturbation-based model to winkel out the helical flows:
20 years ago, Peratt writes in his Physics of the Plasma Universe, Springer Verlag, 1992, on p.29, 17.3 The Diocotron Instability:To explain the presence of such a peculiar pattern in the jet, the MPIfR researchers have applied linear perturbation analysis of K-H instability, and successfully modelled the observed structure of the jet. Their model explains in detail the internal structure of the jet on scales of up to 30 milliarcseconds (~ 300 parsecs).
Since a galactic jet is an axially aligned plasma current and not a fluid, the Max Plank scientists did not use "best available science 20 years ago" to interpret their beautiful data, but instead used precisely the method that the inventor of magnetohydrodynamics MHD himself developed and warned them will not yield behavioral models that conform to reality and observation.One of the outstanding problems in the propagation of electron beams along an axial magnetic field is the breakup of the beam into discrete vortex-like current bundles when a threshold determined by either the beam current or distance of propagation is surpassed. The phenomenon observed closely resembles that associated with the Kelvin-Helmholtz fluid dynamical shear instability, in which vortices develop throughout a fluid when a critical velocity in the fluid is exceeded, with a large increase in the resistance to flow (Chandrasekhar, 1961).
The link in the TPOD to aurora studies is also borne out by Peratt on p.30:
So be very careful in checking your background references for plasma phenomena whenever you read even the best-looking papers from the most prestigious institutions, because still today they very well may still be unaware of the straightforward experiments and resultant expert advice offered back in the day, and may simply proceed with their MHD theoretical models and mathematical sophistries to generate images that reinforce what they think they are looking for....in the laboratory, well-defined vortices are found to occur over 12 orders of magnitude in beam current. This mechanism [diocotron instability] was first introduced to explain auroral curtains (Figure 1.21) by Alfvén (1950). The diocotron instability is discussed [further] in Section 2.9.8.
The recent excellent paper, "Measurement of the Electric Current in a kPC-scale Jet", arXiv:1106.1397v1, 7 Jun 2011, has as 2 of its 4 authors Phillip Kronberg and S.A. Colgate of the Los Alamos National Laboratory, Sandia, New Mexico. These guys know astrophysics and electrodynamics, and they don't mince words like electric current when that is what they are setting out to measure and describe. Peratt is also associated with Los Alamos Labs, and I bet dollars to donuts that they have well-thumbed copies of Peratt's textbook in their library!
Jim