J.2.1.1. Meteoroids directionality
The meteoroid flux model given in 10.2.4 assumes an isotropic flux with respect to the Earth surface. For an orbiting spacecraft the Earth shielding and the spacecraft motion both introduce a directional dependence.
The directionality caused by the spacecraft motion leads to increased fluxes on forward facing surfaces and to reduced fluxes on trailing surfaces.
Combining the two factors approximate flux ratios for meteoroids are found for 400 km and 800 km altitudes, given in Table J-1.
As resulting effects such as penetration depth or impact plasma generation also depend on parameters such as impact velocity and angle, the directional ratios for these effects can be considerably different from those given in Table J-1.
J.2.1.2. Mass density of meteoroids
The mass density of meteoroids varies widely from about 0,15 g cm-3 to 8 g cm-3.
According to reference [RD.21] the average density of micrometeoroids larger than 0,01 g is assumed to be 0,5 g cm-3. Meteoroids smaller than 10-6 g are thought to have a higher mean density of 2 g cm-3. The recommended value for masses between 10-6 g and 0,01 g is 1 g cm-3.
The reference meteoroid models given in 10.2.3 and 10.2.4 are based on a mass density of 2,5 g cm-3.
However, there is still a considerable uncertainty about these densities.
J.2.1.3. Flux enhancement from gravitational attraction
The method described in C.1.2 to account for a modified velocity distribution and enhanced fluxes due to Earth’s gravitational attraction modifies the range and width of each velocity bin given in Table C-1. It is acceptable to account for this re-binning by simply oversampling the original velocity distribution, e.g. using 0,1 km s-1 steps in velocity instead of 1 km s-1 steps, and ignoring the changes in bin width but simply adding the contribution to the appropriate velocity bin. This gives approximately the same result. For example, the n(v∞) velocity distribution at 13,5 km s-1 is 4,83E-02 (from Table C-1) where this value represents the contribution from 13 to 14 km s-1. We can oversample n(v∞) by a factor of 10 such that we can assign 4,83E-03 to 10 bins between 13 and 14 km s-1. (alternatively, interpolate the n(v∞) values to produce a new 0.1 km s-1 step distribution, and renormalize it). So for example, a value of n(v∞) (or rather, nk) at 13,95 km s-1 (i.e. representing values between 13,90 to 14,00 km s-1) is 4,83E-03. Using expression (C-3), a value of v∞ = 13,95 km s-1 becomes v = 17,64 km s-1 (assuming vesc = 10,8 km s-1 which is true for LEO). Thus our n(v∞) value of 4,83E-03 gets added to the 17,6 to 17,7 km s-1 bin. Once all values of n(v∞) have been considered, the 0,1 step distribution can be converted to the final 1 km s-1 step distribution (i.e. this is then n'k).
J.2.1.4. Meteoroid streams
At peak activity stream fluxes can exceed the sporadic background fluxes by a factor five or more. Occasionally, very high fluxes (meteoroid storms, the visible meteor background flux can be exceeded by a factor 10 000 or more) can be encountered for short periods (1-2 hours). Examples of such storms are the Leonid streams in 1998, 1999 and 2001.
Meteoroid streams consist of relative large particles only (mass > 10‑8 g) with low density (0,5-1,0 g cm-3).
J.2.1.5. Interplanetary meteoroid model
New interplanetary meteoroid flux models were presented in [RD.106], [RD.110] and [RD.108]. These models are based on different types of meteoroid populations whose relative contributions depend on the particle size range and the distance from the Sun.
The models include directional distributions of the populations.
For Earth orbits the meteoroid models predict similar total fluxes as the reference model in clause 10.2.4. In addition they include directional effects.
The interplanetary meteoroid models are still in the development stage. At present no specific reference model is defined as standard.