Scientists Explain The Sound In Saturn’s Rings
October 17, 2012

Nonlinear Dust Acoustic Waves In Dusty Plasma: The Sound In Saturn’s Rings

Alan McStravick for — Your Universe Online

The theoretical prediction of the existence of linear and nonlinear dust acoustic waves in dusty plasmas was first postulated more than two decades ago by Professor Padma Kant Shukla. Since that early prediction, these acoustic waves have been observed in several laboratory experiments. With that discovery, Professor Shukla´s discovery has transformed the field of plasma physics, even opening up a new interdisciplinary research field that works in the overlap of condensed matter physics and astrophysics.

Aside from being found in laboratories, dusty plasmas are also prevalent in space. And due to their special properties, dust acoustic waves can propagate inside these plasmas like sound waves in air. They are observable with both the naked eye and also with standard video equipment. In a new study, Professor Shukla and Dr. Bengt Eliasson, physicists from the Ruhr-Universität Bochum (RUB) working in the Faculty of Physics and Astronomy, have published a model with which they describe how large amplitude dust acoustic waves in dusty plasmas behave. Their study and its findings are published in the journal Physical Review E.

The composition of dusty plasmas includes electrons, positive ions, neutral atoms and dust grains that have either a negative or positive charge. It´s in dusty plasmas that contain electrically charged dust grains that dust sound waves can emerge. The waves, themselves, are supported by the inertia of the massive charged dust particles. The restoring force, which causes the particles to oscillate and propagate, comes from the pressure of the hot electrons and ions.

Of late, there have been several laboratory experiments that have revealed nonlinear dust acoustic waves that have extremely large amplitudes in the form of dust acoustic solitary pulses and shock waves. These pulse and shock waves have been observed moving through plasma at speeds of a few centimeters per second. Shukla and Elliason have developed a unified theory explaining under which circumstances nonlinear dust acoustic shocks as well as dust acoustic solitary pulses occur in dusty plasmas.

Though these large amplitude dust acoustic waves interact among themselves, their activity generates new waves with frequencies and wavelengths that differ from the original dust acoustic waves. These new waves that have frequencies that are a multiple integer of the original frequency are called harmonics. With the generation of the harmonics, and due to constructive interference between dust acoustic waves of differing wavelengths, the waves develop into solitary, spiky pulses. These new pulses are called shock waves.

While the solitary pulses arise from a balance between the harmonic generation nonlinearities and the dust acoustic wave dispersion, shock waves form when the dust fluid viscosity dominates over dispersion. This phenomenon occurs at high dust densities when the dust particles are close enough to interact and collide with neighboring dust particles.

This new Shukla-Elliason nonlinear theory and numerical simulations of the dynamics of nonlinear dust acoustic waves successfully explains observations from a series of differing laboratory experiments that were conducted by three different groups around the world. Robert Merlino in the U.S., Lin I in Taiwan and Predhiman Kaw of India each described the existence of large amplitude dust acoustic solitary pulses and dust acoustic shocks in their low-temperature dusty plasmas. In applying the new nonlinear dust acoustic wave theory, one can infer the dust liquid 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 star forming regions,” according to Shukla.

Dr. Bengt Eliasson, being recognized for his seminal contribution to computational and nonlinear plasma physics, was recently elected as a Fellow of the American Physical Society. Eliasson received his Master´s degree in Engineering Physics from Uppsala University in Sweden. He also received his PhD for his study of Numerical Analysis from Uppsala. He has, since 2003, worked in the Faculty of Physics and Astronomy at the Ruhr-Universität Bochum. His fellowship has been conferred upon him for his contributions to various fields of space and plasma physics, ranging from large-scale simulations of the Earth´s ionosphere to new theoretical and numerical models of quantum plasmas at nanoscales. Elliason has been published in some 150 journals.