The Effect Of Meteoroid Streams

It is clear that the overall annual mean meteoroid influx has a component that comes from recognisable meteoroid streams. As the meteor ionisation efficiency is highly dependent on meteoroid velocity, then a prominent shower does not necessarily mean that the meteoroid flux at a given mass is higher than the sporadic background. Thus to consider streams fully, one must model them in terms of their meteoroid flux distributions. McBride [64] presents a method for determining the contribution of the annual streams to the total annual mean flux, and calculates the instantaneous flux at large Fmax throughout the year due to the 50 most prominent meteor showers, using shower parameters from Jenniskens [65] (note that this method of quantitatively accounting for the major streams has been incorporated into the recent upgrade of ESA's ESABASE environmental software package, referred to as the "Jenniskens-McBride model").

Figure 7 shows the instantaneous flux, as a function of solar longitude, for Fmax=l mm (i.e. this is the flux of meteoroid impacts that would penetrate 1 mm of aluminium — this being particularly applicable to considerations of significant spacecraft damage). In Figure 7, the upper (solid) curve shows the results obtained using a foil detector mounted perpendicularly to all streams simultaneously, and hence gives a (somewhat unrealistic) upper level for any instantaneous exposure. The lower (dashed) curve is for a foil detector mounted on the space face of a gravity stabilised LEO spacecraft (e.g. like LDEF) with its orbit parallel to the ecliptic plane, and hence represents the most realistic scenario. The lower dashed line represents the mean value of this curve, while the the upper dashed line gives the annual mean level derived for the space face detector using the isotropic meteoroid model discussed above (i.e. using the 'Grün flux' within the model). It is seen that only around 10% of the total mean meteoroid exposure, at this size regime, is obtained purely from the 50 streams. However, the instantaneous contributions from particular streams can exceed the annual mean, and so can be of 'interest' to short exposure missions (e.g. a 2 week Shuttle mission) where the total stream fluence would be highly dependent on the time of year.

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