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#18.     The Solar Wind


  (Files in red–history)

           Index

16. The Sun

  16H. Schwabe, 1843

  16a. Schwabe paper

  16b. Carrington, 1859

17. The Corona

18. Solar Wind

  18H.Solar Wind obs.

18A. Interplan. Field

18B. Heliosphere.

19. Magnetopause

    19H.Chapman, 1930

20.Global Structure

21. Lagrangian pts.

22. "Wind" s/c
        The first indication that the sun might be emitting a "wind" came from comet tails, observed to point away from the Sun, whether the comet was approaching the Sun or whether it was moving away. Kepler in the early 1600s guessed that those tails were driven by the pressure of sunlight, and his guess still holds true for the many comet tails which consist of dust.

      Comet Halley, with
      plasma tail kink

         Comets however also have ion tails, shining in their own spectral lines, not just in scattered sunlight. Such tails may point in slightly different directions, and are at times observed to accelerate quite suddenly, causing them to become kinked or bent. Comet Hale-Bopp , a prominent comet which was at its brightest in March-April 1997, clearly exhibited such twin tails. While the dust tail was much brighter, the plasma tail had a different color, tending towards the blue.

       Sunlight pressure cannot explain such behavior, but in 1943 Cuno Hoffmeister in Germany, and later Ludwig Biermann, proposed that apart from sunlight, the Sun also emitted a steady stream of particles, a "solar corpuscular radiation" which pushed the ions. Variations in the speed of the particles would explain the accelerations, and the tail did not point straight away from the Sun because the flow velocity of the particles was not too many times larger than the velocity of the comet itself.


Parker's Theory

    No one gave a good reason why this "particle radiation" should exist, until Eugene Parker of the University of Chicago in 1958 tried to derive the equilibrium structure of the corona.

    The corona, like the Earth's atmosphere, is held down by the Sun's gravity. In any atmosphere, the average velocity of atoms, molecules and ions depends on their temperature. Individual velocities are spread out around this average, and a few particles are always fast enough to "evaporate" and escape the restraining gravity.

    Our Moon's gravity is weak, 1/6 of the Earth's at the Moon's surface, and it is believed that if it ever had any atmosphere, it would have evaporated long ago. The stronger gravity of Earth, on the other hand, has managed to hold on to a substantial atmosphere, on which all terrestrial life depends.

    The Sun's gravity is much stronger, but a million-degree atmosphere is too much for it. If the Sun's corona behaved like the Earth's atmosphere, it would gradually get cooler with increasing distance, with a cool top in equilibrium with the surrounding space. Parker however found that the conduction of heat interfered with such an equilibrium and instead another solution suggested itself, in which the topmost layers of the corona flowed away from the Sun at a velocity like that of Biermann's "corpuscular radiation." The flow was named "solar wind" and its existence was later confirmed by instruments aboard spacecraft.

    The solar wind shapes the Earth's magnetosphere and supplies energy to its many processes. Its density at the Earth's orbit is around 6 ions per cubic centimeter--far, far less than that of the "best vacuum" obtainable in labs on Earth. The distribution of ions in the solar wind generally resembles the distribution of elements on the Sun-- mostly protons, with 5% helium and smaller fractions of oxygen and other elements. (Of course it has electrons too, balancing the positive charge of the ions and keeping the plasma electrically neutral.) All this flows away from the Sun with a mean speed of about 400 km/sec, and as shown by the Voyager 2 space probe, this flow extends past the outermost planets, more than 30 times more distant from the Sun than Earth, and it probably continues much further than that.

The Interplanetary Magnetic Field

    The regions where the solar wind starts are immersed in the Sun's magnetic field (though perhaps in regions where that field is relatively weak). However, plasma outflows from regions of magnetic fields can spread those fields to wherever they arrive. This happens by "field line preservation," a property derived from the equations of an ideal plasma. By those equations, in an ideal plasma ions and electrons which start out sharing the same magnetic field line continue to do so later on, as if the line were a (deformable) wire and the particles beads threaded by it.

    If the energy of the magnetic field is dominant, its field lines keep their shapes and particle motion must conform to them; that is what happens in the radiation belts. On the other hand, if the energy of the particles is dominant--that is, if the field is weak and the particles dense--the motion of the particles is only slightly affected, whereas the field lines are bent and dragged to follow that motion. That is the case with the solar wind.

   Imagine a field line extending from the bulk of the Sun to the upper corona. The particles at its "roots" stay with the Sun, but those in the high corona flow out with the solar wind, to the Earth's orbit and far beyond. All that time (under ideal conditions--a fair approximation) the same field line continues to link both groups. Thus some solar field lines will extend to the Earth and further out, producing the interplanetary magnetic field (IMF). It is the IMF that allows the solar wind to "pick up" the ions in a comet's ion tail, as it also did to an "artificial comet" produced in a 1985 experiment (see positive ions, "clouds of barium ions"). As will be seen, the IMF plays a major role in linking the magnetosphere to the solar wind.

    A graphic exercise on tracing the shape of interplanetary magnetic field lines is presented in the next section.


Questions from Users:
            ***     Use of solar wind for space propulsion
                  ***     Any connection between Solar Wind and Solar Flares?
                      ***     Can anything solid be carried by the solar wind?
                          ***         The Speed of the Solar Wind
                                ***         The solar wind and solar escape velocity
                                      ***         Solar wind effects on our lives
                                            ***         A solar wind contribution to global warming?
                                    ***         Can upper atmosphere atoms join solar wind?

Next Stop: #18H.  The Solar Wind--History

Last updated 8 August 2007
Re-formatted 3-13-2006