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Comets2 - factor of 10 3 one could see the speed with which...

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Comets' tails The other piece of pre-space exploration indirect evidence for the solar wind came from the study of comets' tails. Much of this work is due to Biermann (1951-7). It had long been known that comet's sometimes had two tails, and that comets' tails were of two or even three types. Type I tails were straight, and pointed more or less radially away from the Sun. Type II and type III were curved, angled to the sun-comet direction, and were known to be dust (< 1 micron pieces) from the fact their spectrum was that of scatterum was that of scattered sunlight. (Where types II and III were taken to be distinct, II was where large lumps of matter seemed to be given off, and III a more "uniform" flow.) Tail I is ionised, mainly CO + , with some N 2 + and CO 2 + (see later in the course). The Type II tail could be explained by the pressure of sunlight moving small dust particles radially away from the sun. (Accelerations are of the order of solar gravity - about 0.6 10 -2 m s -1 at 1 AU.) Applying radiation pressure to the type I tail, however, showed it to be too small by a
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Unformatted text preview: factor of 10 3- one could see the speed with which the particles in the tail moved by tracking discontinuities in it (type I tails display filaments, kinks and knots at times). Biermann postulated that the motion of the ions was due to their being entrained in a flow of particles being given off by the sun and moving radially outwards. There would be momentum transfer from the proton/electron stream which accelerated the cometary ions according to: where sigma, the conductivity, is evaluated approximately, and N e is the electron density on the "wind" stream. By looking at the motions in the tail we can get an estimate for v e of about 10 3 km s-1 ,P> km s-1 , and this gives an estimate of the solar wind density of about 600 electrons per cubic meter. This is too large, but the error is due to our having ignored the "frozen-in" magnetic field (which we shall return to below)....
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