Plasma Waves Move Faster Than Shockwaves
July 20, 2012

Plasma Waves Move Faster Than Shockwaves

Lawrence LeBlond for - Your Universe Online

The Sun and the dynamic space weather system that envelops the Earth are what researchers are trying to get a better understanding of with the help of Heliophysics nuggets -- collections of early scientific results, new research techniques and instrument updates.

More than 99 percent of the matter found in the universe resembles nothing like what we have here on Earth, according to scientists from NASA´s Goddard Space Flight Center in Greenbelt, Maryland.

Instead of materials that we, as humans, can touch and see, and motions that we expect to exist like that of gravity, most of the universe is actually governed by rules that react to such things as magnetic force or electrical charge. For example, a cup that we would see sitting on a table on Earth adhering to the force of gravity, elsewhere in the universe this cup might be magnetized and attracted to a metal ceiling above, floating upward, yet resting in the space between the ceiling and the table, balanced by the forces of gravity and magnetism.

This material that saturates the universe, making up the stars and vast interstellar spaces between them, is called plasma. Plasma is much like a gas and is made up of such familiar matter as hydrogen, helium and other heavier elements such as iron. Yet, each of these particles carries electrical charge and the particles tend to move together as they do in fluid.

By understanding how plasma moves under the combined laws of motion we know on Earth, and the less intuitive electromagnetic forces found in the spaces above us, scientists are now understanding events that bring on giant explosions on the Sun as well as changes in Earth´s magnetosphere. However, coming to an understanding of this mysterious plasmatic world is no easy feat. With complexities in the rules of motion, the study of plasma carries intricate details that need to be teased out.

“Which particles are moving, what is the source of energy for the motion, how does a moving wave interact with the particles themselves, do the wave fields rotate to the right or to the left — all of these get classified,” said Lynn Wilson, space plasma physicist at Goddard.

Wilson, lead author of a paper published in Geophysical Research Letters on April 25, used data from the WAVES instrument on NASA´s Wind mission and discovered evidence for a type of plasma wave moving faster than theory has previously predicted could move. His research suggests that a different process than expected, electrical instabilities in the plasma, may be driving the waves. This finding gives Wilson and his colleagues another tool in the understanding of how heat and energy can be transported through plasma.

Wilson and colleagues studied coronal mass ejections (CMEs) -- solar material that explodes off the Sun traveling through space at excessive speed. They found that these CMEs move so much faster than the background solar wind that they create shock waves which are similar to those produced by a supersonic jet when it breaks the sound barrier in our atmosphere.

“A bow shock is a little like a snow plow,” said study coauthor Adam Szabo, a space scientist at Goddard and also the project scientist for the Wind mission. “The wave picks up particles that are traveling more slowly and speeds them up, piling them up in front as it moves.”

This snow plow effect has non-magnetic similarities, and yet, is quite perplexing. With a snow plow it would not be expected that a cloud of snowflakes would magically lift up from the shock and then move ahead, streaming down the street faster than the rest of the snow pile behind. However, in the magnetized gas ahead of the shock of the CME, Wind observed a large wave of plasma ahead moving faster than it should have been able to travel if it was produced by the shockwave.

This type of wave is called a Whistler wave. Since this Whistler wave couldn´t be created by the shock, Wind´s observations suggest the waves may have been created by instabilities in front of the shock. Data by Wind allowed Wilson to measure magnetic field information at 1875 samples per second and new qualities of observations always produce new sights. But the size of the waves really blew the team away.

These waves were massive, noted Wilson. “They are almost as big as the shock itself.” The massive size of the waves means they may play a larger role than previously thought in the quest to understand the ways different types of energy converts from one form to another, he added.

In this context, two forms of energy are of interest to scientists: bulk kinetic energy and random kinetic energy. Bulk relates to the collective movement of a bulk of particles, and random relates to the speeds at which particles move in respect to each other.

Increased random kinetic energy is, in fact, the very definition of heating, since temperature measurements are a characterization of how fast particles are moving within any given material. Large amplitude Whistler waves are known to cause both bulk and random kinetic energy. This suggests that shocks and the instabilities they create may play a larger role in transferring the energy from the plasma´ bulk movement into heat, than previously thought.

Wilson believes the instabilities caused perpendicular ion heating -- a process that increases the random kinetic energy of the positively-charged ions in a direction perpendicular to the background magnetic field. The waves also added energy to the negatively-charged electrons -- with the greatest effects observed not being heating, the random kinetic energy, but bulk acceleration in a direction parallel to the magnetic field.

“The same type of wave-particle interaction is thought to happen in solar flares, the heating of the sun's corona, and supernova blast waves,” said Wilson. “All of these energizations have very similar properties. Now we have evidence that these Whistler-like fluctuations may be causing heating in all these places.”

Wilson said his work may be only a small piece of a much larger puzzle, but together, determining these plasmatic motions will help scientists describe the laws of motion that govern the entire universe.