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Re: [comets-ml] Re: About striae, synchrones, syndynes, etc.

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  • "Sebastian F. Hönig"
    Giovanni and Dennis, ... While most people thend to do so, there s actually no reason to distinguish large particles (a ~ lambda) from smaller ones (a
    Message 1 of 64 , Jan 24, 2007
      Giovanni and Dennis,

      > The scattering from molecules and very tiny particles (< 1 /10
      > wavelength) is predominantly Rayleigh scattering. For particle sizes
      > larger than a wavelength, Mie scattering predominates. This pretty
      > much covers particles *smaller* than 5 microns, doesn't it?

      While most people thend to do so, there's actually no reason to distinguish
      large particles (a ~ lambda) from smaller ones (a < lambda) since Mie scattering
      describes them as well.

      But now to the point. Goivanni wrote:
      ------------------------------------------
      the point is that even accounting for the contribution of all particle sizes 5
      microns and below, there is *too much* light contribution visible on that tail
      features. Then the question: where is all the mass that makes such details
      visible? This is the question to answer...
      ------------------------------------------

      I don't know why you want to have more mass.

      Assume we have a dust particle of a certain size Ro (volume Vo) and mass mo.
      This particle fragments into n smaller particles of size r (volume V), where n =
      Vo/V = Ro^3/r^3. What happens if we observe this?

      Along the line of view, the particle column density N is defined as

      N = mc/m

      where mc is the column mass density mc=s*rho=s*m/V (rho ist the (solid state)
      density of the dust, s is the column length). Thus, we obtain

      N = mc/m = (s*m/V)/m = s/V

      Let Adust be the geometrical cross section of the dust particles (Adust~r^2).
      For the intensity of scattering, the scattering cross section (or scattering
      efficiency) A_sca is the important parameter:

      A_sca ~ N*Adust ~ Adust/V ~ 1/r

      The conclusion of this is: If you take one dust grain and fragment it, the
      scattering cross section increases, i.e., the scattering intensifies (I~A_sca).

      (Btw, the basic princliple of the above mentioned is hidden in Sekanina & Farell
      (1980)).

      So, as long as you destroy large particles of the order of 10micron into
      particles of the order of 0.1-1 micron (Mie regime for visual wavelengths), the
      above holds. If the particles are smaller and you run into the Rayleigh regime,
      you probably lose a lot of intensity in the visual and won't see scattered light
      from this dust anymore. This means that you only see the larger particles in the
      striae. Assuming typical collision or break-up power law distributions of grain
      sizes (i.e., number of particles of size r is proportional to size r^-x), I
      would draw the following conclusions:

      * Stiae should be brighter than their "parent synchrones/syndynes" (this is
      laxly formulated but I hope you understand what I mean)

      * a power law distribution for the grain sizes as a result of breakup implies a
      "fade-in" from the position of break-up (Mie regime) and a "fade-out" when the
      particles become to small (Rayleigh regime).

      Cheers,
      Sebastian

      --
      -------------------------------
      Sebastian F. Hoenig
      Max-Planck-Institut fuer Radioastronomie
      Auf dem Huegel 69
      53121 Bonn
      Phone +49 (0) 228 525 188
      Fax +49 (0) 228 525 437
      Skype: sfhoenig
      -------------------------------
    • gvnn64@libero.it
      Dear Sebastian, OK in all points, but you cannot forget, as you do, the syndynes. That of 10 micron is MILLION km far (inside towards the head of the comet)
      Message 64 of 64 , Jan 25, 2007
        Dear Sebastian,
        OK in all points, but you cannot forget, as you do,
        the syndynes. That of 10 micron is MILLION km
        far (inside towards the head of the comet) with
        respect to the locus where all striae clearly start,
        which is a syndyne of ONE-TWO microns!!
        We must take into account all the factors, also those that (unfotunatley) contradict our beloved theory.

        I remind you something similar happened with
        Comet West: Sekanina analysed the striae photometry,
        and was forced to conclude, in order to match the tail
        and striae brightness, that the original parents where
        needles meter long with a section of one micron
        aligned to sun direction, in order to have beta close to
        one (cross section of one micron) and enough mass to
        produce the striae. Here for sure the situation is even
        worse, and who can believe to such needles ??

        Cheers,
        Giovanni



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