Solar wind is rhombic-shaped

The plasma beta represents the ratio of kinetic to magnetic pressure in the cosmic plasma. 

The anisotropy is the ratio of the perpendicular and parallel temperatures to the magnetic field lines. 

The number of measured values is shown in colour (red corresponds to many values, blue to few values). 

Why the measurements take on the characteristic rhombic shape is explained by a new model by Bochum's physicists. Source: Physical Review Letters.

Why the temperatures in the solar wind are almost the same in certain directions, and why different energy densities are practically identical, was until now not clear.

With a new approach to calculating instability criteria for plasmas, Bochum researchers lead by Prof. Dr. Reinhard Schlickeiser (Chair for Theoretical Physics IV) have solved both problems at once. They were the first to incorporate the effects of collisions of the solar wind particles in their model.

This explains experimental data significantly better than previous calculations and can also be transferred to cosmic plasmas outside our solar system. The scientists report on their findings in Physical Review Letters.

Temperatures and pressures in the cosmic plasma
The solar wind consists of charged particles and is permeated by a magnetic field. In the analysis of this plasma, researchers investigate two types of pressure: the magnetic pressure describes the tendency of the magnetic field lines to repel each other, the kinetic pressure results from the momentum of the particles.

The ratio of kinetic to magnetic pressure is called plasma beta and is a measure of whether more energy per volume is stored in magnetic fields or in particle motion.

In many cosmic sources, the plasma beta is around the value one, which is the same as energy equipartition. Moreover, in cosmic plasmas near temperature isotropy prevails, i.e. the temperature parallel and perpendicular to the magnetic field lines of the plasma is the same.

Explaining satellite data
For over a decade, the instruments of the near-earth WIND satellite have gathered various solar wind data. When the plasma beta measured is plotted against the temperature anisotropy (the ratio of the perpendicular to the parallel temperature), the data points form a rhombic area around the value one.

"If the values move out of the rhombic configuration, the plasma is unstable and the temperature anisotropy and the plasma beta quickly return to the stable region within the rhombus" says Prof. Schlickeiser.

However, a specific, detailed explanation of this rhombic shape has, until now, been lacking, especially for low plasma beta.