Do the cometary gas ions which make up the plasma tail of a comet reach the velocity of the Sun's solar wind?


Yes, the ions are said to be "swept out" by the solar wind, such that they reach the speed of the solar wind. That speed varies a fair amount, however, because there is both a "fast" (about 600 km/s) and "slow" (about 300 km/s) solar wind. It is really the magnetic fields entrained in the wind that sweep out the charged ions, since each ion has such a low mass it is easily affected by those fields. Note the dust particles from a comet, which is the tail that we see, are much more massive and are not affected by the solar wind, so they are moving out much more slowly and often make a noticeably curved shape.

  • $\begingroup$ Is the curvature due to the path the comet follows, or to the fact that ions in a magnetic field follow curved paths? $\endgroup$ – Carl Witthoft Jun 11 '18 at 14:39
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    $\begingroup$ It's more like the path of the comet, like the curvature of the smoke coming from the smokestack of a ship in motion. It is the pattern that is curved, the particle motion is a bit different-- the dust particles continue to orbit the Sun in ways that depend on the size of the dust grains. The largest grains, those that make meteors in Earth's atmosphere, continue to follow an orbit not so different from the comet itself. Smaller grains get pushed more by sunlight, so their orbits are more eccentric, but still cross the comet path. $\endgroup$ – Ken G Jun 12 '18 at 4:49
  • $\begingroup$ @KenG you’re saying the radiation pressure is stronger than the solar wind for small grains? What about for gas molecules released from the comet? $\endgroup$ – Chappo Hasn't Forgotten Monica Jun 12 '18 at 8:35
  • $\begingroup$ It's a matter of cross section per gram, which peaks somewhere around the wavelength of the light. $\endgroup$ – Ken G Jun 12 '18 at 13:06

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