|The Large Scale Solar Wind|
What happens when the solar wind collides with obstacles such as planets or comets?
Since the solar wind flows past the planets and other sizeable objects supersonically, the slowing and deflection of the flow about the obstacle is frequently accomplished by means of a shock wave. Normally, waves would propagate upstream of the obstacle to communicate its existence to the incident gas, but with supersonic flow the waves aren't fast enough. The supersonic flow sweeps waves downstream, so the wind does not know about an obstacle until it "collides" with it. We therefore need a bow shock to deflect the supersonic flow and divert it around the obstacle. A bow shock is similar to a bow wave about a rock in a fast moving stream.
Unlike the shock wave ahead of a supersonic jet plane, the bow shock does not propagate, but is stationary. Also, the bow shock is "collisionless," owing to the collisionless nature of the solar wind (a). Collisions of charged particles in the solar wind are so rare that they do not affect the formation of the shock or the dissipation of the solar wind's kinetic energy. In the solar wind, the particle collisions that would normally take place in a collisional shock, are replaced by wave-particle interactions. When the solar wind passes through the shock, it is slowed, heated, compressed, and diverted.
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The existence and nature of a bow shock depends very much on the nature of the obstacle. For planets such as the Earth(b) and Jupiter(c), with substantial magnetic fields, the incident solar wind, which has an embedded interplanetary magnetic field, compresses the upstream side and bounds the magnetic field inside a current sheet, known as the magnetopause, through which the solar wind does not flow. A bow shock forms ahead of the magnetopause, which deflects the wind about the obstacle (d,e).
(b): SOHO representation of the solar wind colliding with the Earth. The white lines represent the solar wind, the purple line the bow shock, and the blue lines surrounding the Earth represent its protective magnetosphere.
(c): Illustration of the interaction of the solar wind with the Jovian magnetosphere
(d,e): Dynamic modeling of the Earth's bow shock and magnetopause using real time data from the ACE spacecraft. In the figure to the right, the Earth is in the center, and is illuminated from the left by the Sun (not shown). In this view, we are looking down upon the North pole; thus the figure represents the equatorial plane The ACE spacecraft monitors the solar wind from a position about 200 Earth radii (RE) sunward of the Earth. The real time solar wind data from this spacecraft allows us to predict what will happen at the Earth many minutes before the solar wind actually reaches us. Important solar wind values obtained from the ACE observations include the z-component of the interplanetary magnetic field (Bz) measured in units of nano-Tesla, and the dynamic pressure of the solar wind, measured in units of nano-Pascal.
Planets with no, or a very weak, magnetic field but with an atmosphere, such as Venus(f), develop an ionospheric current sheet that, like the magnetosphere, excludes the solar wind and leads to the formation of an upstream bow shock.
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Cartoon illustration of the bow shock and ionopause created by the interaction of the solar wind with the Venusian atmosphere. Adapted from Zeleny and Vaisberg, 1981
If there is no atmosphere, such as with the moon, the solar wind will crash directly on to the surface, gently eroding it and being absorbed by the surface. In this case there is no shock.
Finally, comets(g) too have can have a bow shock, but it is rather different in character. Close to the Sun, the comet nucleus begins to melt, causing the emission of large quantities of neutral particles, which can travel upstream ahead of the comet into the supersonic solar wind. Photoionization, that is ionization by solar radiation, or charge exchange with solar wind ions creates a new population of slow-moving ions that are accelerated by the solar wind motional electric field.
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This color photograph of the comet Kohoutek was taken by members of the lunar and planetary laboratory photographic team from the University of Arizona, who photographed the comet from the Catalina observatory with a 35mm camera on January 11, 1974. Figure courtesy of NASA.
This communicates the existence of the comet to the solar wind at distances far from the comparatively small nucleus. Momentum imparted to the cometary pickup ions by the solar wind leads to the slowing down of the solar wind as it approaches the comet, which results in the formation of a weak bow shock or even no shock at all (h).
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Representation of the interaction of the solar wind with a comet, courtesy of Fran Bagenal, Department of Astrophysical and Planetary Sciences at the University of Colorado. The emission of neutrals and the subsequent creation of pickup ions creates an extended obstacle, which can then sometimes lead to the formation of a cometary bow shock.
The Voyager spacecraft flew through the bow shocks of Jupiter and Saturn on several occasions and measured the interaction of their environments with the solar wind. Voyager 2 also encountered Uranus(i) and Neptune(j). From this figure we can see the striking differences that Voyager discovered in the magnetic fields of the outer planets.(k)
(i): Animation of Uranus's magnetosphere, courtesy of Ralph McNutt et al., APL.
(j): Neptune's magnetosphere, courtesy of Ralph McNutt et al., APL
(k): Magnetic fields and axes of rotation of the four giant outer planets, Jupiter, Saturn, Uranus, and Neptune
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