University of Kansas

Space Physics and
Plasma Astrophysics

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Shock Waves

We are surrounded by SHOCK WAVES! Our solar system's planets have shocks in front of them. Shocks can be found in a planet's magnetotails and are formed in the solar corona and solar wind. Wherever plasmas and field energy flow there will be shock waves. Shocks are places where the plasma and field go through intense changes such as density, temperature, field strength and flow speed.

The average person probably associates a shock wave with supersonic aircraft and explosive blasts. The study of plasma shocks surfaced during the 1950s, with interest in fusion plasmas and shocks caused by explosions in the upper atmosphere. Our understanding of shocks at a basic level is built on a group of concepts involving the speed at which information can travel.

We transmit information by making a disturbance that propagates. For example, we clap our hands and a pressure pulse in the air travels outward. Afterwards, at increasingly later times, people farther and farther away will be able to hear the hand clap. The speed of sound is the typical speed at which these vibrations are transmitted, so what happens if, in the process of hand clapping, your hand moves faster than the sound speed? Your hand speed gives energy and momentum to any molecules hitting it, and there must be flows of air around your hand to allow it to move forward. The air in front of your hand knows that a hand is approaching or information is propagating ahead of your hand. There must be information (a wave) moving ahead of your hand faster than the speed of sound. We have just described a shock wave--a wave that travels faster than the speed of sound and changes the state of the medium through which it travels.

Shocks occur in the solar atmosphere (corona) during solar flares and other active events. Flares and coronal mass ejections inject energy and material into the solar wind, causing interplanetary shocks, which are traveling shocks propagating out through the solar system. The solar wind has high-speed and low-speed streams, coming from different source regions on the sun. Shocks can form at the interface between a slow stream being overtaken by a fast stream. This brings us to the research of Juan Gomez, then a doctorate student at the University of Kansas' Physics and Astronomy Department.

Using data from the Ulysses HISCALE project, Juan Gomez studied the interplanetary shocks that form due to a fast plasma stream overtaking a slower one. What happens to some of those charged particles within the shock wave? What is the shape or form of the shock wave--curved or flat?

Gomez found that the charged particles are energized by the shock. He also discovered that the shock can take one of two shapes--flat or curved. In the case of the curved shock, the interplanetary magnetic field intersects the shock in two places. This traps the particles and they bounce back and forth.

Juan Gomez continues his interplanetary shock research, which means that we continue to have a front row seat as discoveries unfold.

Last modified September 26, 2011
C. Graves and T. Hunt-Ward
tizby@ku.edu