The Sound In Saturns Rings: RUB-Physicists Explain How Nonlinear Dust Acoustic Waves Arise In Dusty Plasmas

 

Dust acoustic waves in a dusty plasma. The upper panel (a) shows how a nonlinear acoustic wave develops in time (horizontal axis) and space (vertical axis). The lower panel (b) shows only the time variation at a certain position (X=0). Over time, the dust acoustic wave transforms into a train of localized spiky wave structures (indicated by arrows) similar to those observed in previous laboratory experiments.

Credit: © Modified from P. K. Shukla and B. Eliasson, Phys. Rev. E, Volume 86, 2012.

Different acoustic phenomena in dusty plasmas

Dusty plasmas are composed of electrons, positive ions, neutral atoms, and dust grains that are negatively or positively charged. Only in plasmas containing electrically charged dust grains, dust sound waves emerge – the so called dust acoustic waves. These waves are supported by the inertia of the massive charged dust particles. The restoring force – causing the particles to oscillate and the wave to propagate – comes from the pressure of the hot electrons and ions. Recently, several laboratory experiments revealed nonlinear dust acoustic waves with extremely large amplitudes in the form of dust acoustic solitary pulses and shock waves, propagating in the plasma with speeds of a few centimeters per second. Padma Shukla and Bengt Elisasson have developed a unified theory explaining under which circumstances nonlinear dust acoustic shocks as well as dust acoustic solitary pulses occur in dusty plasmas.

 

Credit: NASA

Acoustic waves interacting with themselves

Dust acoustic waves with large amplitudes interact among themselves thereby generating new waves with frequencies and wavelengths different from the ones of the original dust acoustic waves. Due to the generation of harmonics (i.e., waves with frequencies that are a multiple integer of the original frequency) and due to constructive interference between dust acoustic waves of different wavelengths, the waves develop into solitary, spiky pulses, or into shock waves. The solitary pulses arise from a balance between the harmonic generation nonlinearities and the dust acoustic wave dispersion. Shock waves, on the other hand, form when the dust fluid viscosity dominates over dispersion. This happens at high dust densities when the dust particles are close enough to interact and collide with neighboring dust particles.

Theory successfully explains data from experiments

The new Shukla-Eliasson nonlinear theory and numerical simulations of the dynamics of nonlinear dust acoustic waves successfully explain observations from laboratory experiments of three different groups world-wide, in the USA (Robert Merlino), Taiwan (Lin I), and India (Predhiman Kaw). These three international groups described the existence of large amplitudes dust acoustic solitary pulses and dust acoustic shocks in their low-temperature dusty plasmas. Applying the new nonlinear dust acoustic wave theory, one can infer the dust fluid viscosity from the width of the dust acoustic shock wave. “Our results may also be important as a possible mechanism for understanding the cause of dust grain clustering and dust structuring in planets and in star forming regions,” suggests Prof. Padma Kant Shukla.

 

Existence of dusty acoustic waves predicted more than two decades ago

More than two decades ago, Prof. Padma Kant Shukla theoretically predicted the existence of linear and nonlinear dust acoustic waves in dusty plasmas, which since then have been observed in many laboratory experiments. His discovery has transformed the field of plasma physics, and has opened up a new interdisciplinary research field at the crossroad between condensed matter physics and astrophysics.

APS Fellowship for contributions to computational and nonlinear plasma physics

For his seminal contribution to computational and nonlinear plasma physics, Dr. Bengt Eliasson was newly elected as a Fellow of the American Physical Society (APS) in September 2012. An APS Fellowship is a distinct honor signifying recognition by one’s professional peers. The number of Fellows that are annually elected is less than one percent of the current number of APS members. Dr. Bengt Eliasson graduated with a Master degree in Engineering Physics from Uppsala University, Sweden, where he also obtained his PhD degree in Numerical Analysis. Since 2003, he works in the Faculty of Physics and Astronomy at the Ruhr-Universität Bochum. 

 

The contributions of Dr. Bengt Eliasson to various fields of space and plasma physics range from large-scale simulations of the Earth’s ionosphere to new theoretical and numerical models of quantum plasmas at nanoscales. The results of his research projects have been published in approximately 150 articles in refereed journals and he was invited to give talks at the European Geophysical Union, European Physical Society, American Physical Society, International Congress on Plasma Physics, and other meetings.

Contacts and sources:
Dr. Julia Weiler