bow shock

General Physics

Bow shock is a term used to describe something that may occur during the creation of a bow wave: the bowed shape of oncoming waves around an impenetrable object. A bow shock only occurs when matter is traveling through a medium faster than that medium's speed of wave propagation.¹ One of the most familiar examples of bow shock is the way water in a stream moves around a boulder. The same predictable shape occurs in all states of matter. When an airplane moves through the sky, air forms a bow shock around the nose of the plane. And when the plane reaches supersonic speeds, the sound waves form a similarly bowed shape around the nose, as well.

Astronomy

Bow shock is also a term used to describe the phenomenon produced during the interaction between the sun's solar wind and the earth's magnetic field. Because of our planet's tendency to act as a massive magnet, astronomers can map a giant spherical field around the earth, called the magnetosphere. When solar wind leaves the sun, it is comprised of plasma, or super hot electrically charged ions, and is traveling between one million and two million miles per hour. When the solar wind reaches the earth's magnetosphere, the magnetic field deflects the solar wind, inadvertently shielding the earth from the solar particles. The solar wind slows drastically, creating a shock wave. The wind does not dip below the speed of sound, but a shock wave is created nonetheless. Without the magnetosphere, and the resulting bow shock effect, the earth would be bombarded with deadly solar rays and life as we know it could not exist on earth. This is the basis for the old quip about how life could not exist on earth without its magnetic field.

Further from the sun than the bow shock, but before the surface of the earth, exists a region called the magnetopause. Here, solar wind that has passed the bow shock reaches a state of equilibrium with the magnetosphere. Almost no solar wind passes the boundary of the magnetopause. The magnetopause outlines earth's own magnetic domain.

             O  O  O  O  O  O -Bow Shock
         O       
      O       O  O  O  O  O  O -Magnetopause
    O      O
  O      O
 O      O
 O     O   O -Earth
 O      O
  O      O
    O      O
      O       O  O  O  O  O  O
         O        
             O  O  O  O  O  O

<-Sun not shown

Astronomers measure distances around the order of bow shocks in terms of the Earth's radius. One earth radii, or 1 RE is equal to one radius of the earth. The nose of earth's bow shock is 15 to 17 R_E from the earth, and the earth's magnetosphere is approximately 10 to 12 R_E from the earth.

The earth is not the only planet with a bow shock. In fact, any planet in the solar system, or the universe for that matter, that has a magnetic field and orbits around a star that emits solar activity, would have a bow shock of some sort or another. Even stars have bow shocks. A massive bow shock exists around our sun from the interaction of particles coming from nearby parts of the Milky Way galaxy. The Hubble Telescope recently revealed images of bow shock around stars in the Orion nebula. In that case, the star-forming gasses and plasma interacted with the magnetic fields of already formed stars and dissipated in the familiar bow shock shape.

Planetary and stellar bow shock is essentially the same phenomenon as seen in general physics, however there is an important distinction between the bow shock seen around the earth and around a boulder in a stream. The bow shock created by the interaction between the water and boulder consists of a solid-liquid interaction. In the case of the earth's bow shock, we see a similar general shape of a bowed deflection when viewed from afar. However, the reaction between the plasma solar flare and the field of the magnetosphere involves a plasma-plasma interaction. For physicists and astronomers, the earth's bow shock is an exciting plasma playground, an easily accessible breeding ground for odd plasma interaction situations.

Physicists Sir Ian Axford and Paul Kellogg first predicted the bow shock phenomenon in 1962. Their theories were confirmed by data from Explorer 18 (Interplanetary Monitoring Platform IMP-1). The IMP program featured eight spacecraft, from IMP-1 to IMP-8, between 1963 and 1973. IMP spacecraft were designed to study the earth's magnetosphere and the effects from the interaction with solar wind.

¹I can't really get my mind around this last sentence. Thanks to IWhoSawTheFace for correcting me on the definition of bow shock as a general physics term. /msg him for more information on wave phenomena.

Changed "it's" to "its" thanks to generous advice from MALTP

http://www.oulu.fi/~spaceweb/textbook/bowshock.html
http://www.agu.org/revgeophys/omidi01/omidi01.html
http://hubblesite.org/newscenter/archive/2002/05/
http://pixie.spasci.com/DynMod/
Axford, W. I., The interaction between the solar wind and the earth's magnetosphere, J. Geophys. Res., 67, 3791, 1962.
Kellogg, P. J., Flow of plasma around the earth, J. Geophys. Res., 67, 3805, 1962.