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Re: (meteorobs) Leonid dust trail age vs. population index
On Fri, 14 Dec 2001, ccmlt wrote:
> Rob :
> > The relevant parameter is not trail age, but the da0 for the trail
>
> It would be very nice if you can explain the da0 parameter or give a link
> where we can get this informations ?
It is really necessary to read the original papers to fully understand the
details, but those who have some basic understanding of what is happening
might be able to understand the following.
The orbital period of the meteoroids in the portion of a dust trail that
meets the Earth, differ from that of the comet (by necessity or they would
return with the comet). Orbital period and semi-major axis are directly
related. The difference in semi-major axis at ejection between these
particles and the comet is "da0". In a standard ejection model the
number and size of particles with a particular da0 would be a function of
mass, mass index, ejection velocity and ejection direction. There are too
many parameters here to predict reliably what might be expected for a
specific trail (and it would be very model dependent). David and I did not
attempt to introduce any ejection model into the ZHR calculations because of
this model dependence, but we do know of a way to derive many quantities like
those given above from different ejection models.
In ejection models, large particles would tend to be ejected at lower
velocities and thus have orbital periods closer to the comet (da0 tends
towards zero). Smaller particles ejected at higher velocities tend to
have da0 rather more distant from zero (both +ve and -ve). The effect of
solar radiation pressure is to skew all particles of a certain mass to
more +ve values of da0, with visual sized particles being skewed towards
da0~0.2. The overall effect is that for each portion of dust trail
encountered there will be a different function for the encountered masses.
This function can probably exhibit effective upper and lower mass limits
and have a variable mass index.
Esko could describe how the satellite model would differ (e.g. I don't
understand how that model can produce -ve da0's and David and I found that
some storms probably require ejection velocities above about 20 m/s which
may be inconsistent with the satellite model). However Esko's ZHR (dust
trail density model) looks excellent and may be quite independent of the
satellite model. I should know these details, but I'm sorry to say I don't.
Cheers, Rob
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