The Grün IFM34 is an empirical compilation of many sources of data and is used as the standard reference for meteoroid flux in near-Earth space. It is based on in situ dust detector fluxes from missions prior to the mid-1980s, lunar microcratering, measurements of zodiacal brightness, and early measurements of meteor flux. The IFM ignores the directionality in the sporadic meteoroid flux and uses a mean velocity (rather than a velocity distribution) in computing flux values. It also ignores temporal variations in the background sporadic meteoroid flux, which have been measured as having an amplitude near a factor of 2 throughout the year.35 For masses of less than 100 µm the IFM is well validated by numerous (and varied) sources of data. Recent (after the mid-1980s) meteoroid flux measurements have been found to be in general agreement with the Grün curve within uncertainty bounds applying at these smaller masses. However, for meteoroids with larger masses, order-of-magnitude disparities exist (see Figure 4.2). At sizes greater than 0.1 mm, fluxes can be derived only from meteor data, and the large uncertainty in mass-brightness-velocity conversions between earlier data and more recent measurements has become apparent.
The SSP 30425 meteoroid model uses as its basis the Grün IFM but makes assumptions about the velocity distribution that are incompatible with the velocity assumptions in the IFM. This inconsistency results in a roughly factor-of-2 underestimation of the flux relative to the IFM (which is the input to SSP 30425). This result is independent of the uncertainty in the flux caused by uncertainty in derived meteoroid mass from measurements, which produces an additional error factor of approximately 3 at submicrogram masses.36 SSP 30425 also assumes isotropy in impact directions, which is at variance with observations.37 The meteoroid bulk density distribution used in SSP 30425 incorporates estimates guided by early radar and photographic measurements of bulk density and chosen to vary as a function of mass from 2 g/cm3 for mass less than 10–6 g, to 1 g/cm3 for mass between 10–6 and 10–2 g, and 0.5 g/cm3 for mass greater than 10–2 g,38 although the actual variation in bulk density has been shown to correlate strongly with orbit type rather than mass alone.39
A physics-based model, the Meteoroid Environment Model (MEM) starts with an assumed parent source population (comets and asteroids) and propagates released meteoroids forward in time in a Monte Carlo manner until they encounter Earth. This distribution of Earth-impacting particles is calibrated in its directionality and velocity distribution by radar measurements and adopts the IFM for flux.40 It has several variants (see Table 3.1 in Chapter 3) but includes Earth-shielding and gravitational focusing where appropriate, and it adopts a single meteoroid density of 1 g/cm3 to provide compatibility in final flux values with the IFM, given the difference in mean velocity between MEM and the IFM. Validation for MEM has come only from data from near-Earth space, while the Grün IFM (implicitly used in MEM) provides meteoroid flux at 1 AU. Although some in situ measurements of very small meteoroids, together with measurements of zodiacal light, provide limited data on meteoroid populations at other heliocentric distances, much greater uncertainties exist in the measurements of meteoroid flux at distances other than 1 AU, with contradictory results particularly evident in measurements in the outer solar system.41 Confident extension of MEM to other heliocentric distances and larger particle sizes likely will require additional measurements beyond those that exist at present.
Recently published work offers the prospect of a higher-fidelity evolutionary model of sporadic meteoroids,42 although such newer work does not entirely agree on source populations and their relative strengths. Development of more physically realistic meteoroid models that fit the available constraints from all data sources remains to be done.
34 Grün et al., Icarus, 1985.
35 M. Campbell-Brown and J. Jones, Annual variation of sporadic radar meteor rates, Monthly Notices of the Royal Astronomical Society 367:709-716, 2006, available at http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2966.2005.09974.x/pdf, accessed August 16, 2011.
36 Anderson and Smith, Natural Orbital Environment Guidelines for Use in Aerospace Vehicle Development, 1994.
37 Jones and Brown, Royal Astronomical Society, Monthly Notices, 1993.
38 Anderson and Smith, Natural Orbital Environment Guidelines for Use in Aerospace Vehicle Development, 1994.
39 Ceplecha et al., Space Science Reviews, 1998.
40 McNamera et al., Earth, Moon, and Planets, 2004; Jones and Brown, Royal Astronomical Society, Monthly Notices, 2003.
41 C. Leinert and E. Grün, Interplanetary dust, pp. 207-282 in Physics of the Inner Heliosphere I1 (R. Schewenn and E. Marsch, eds.), Springer-Verlag, Berlin, Germany, 1990.
42 See Wiegert et al., Icarus, 2009; D. Nesvorny, P. Jenniskens, H.F. Levison, W.F. Bottke, D. Vokrouhlický, and M. Gounelle, Cometary origin of the zodiacal cloud and carbonaceous micrometeorites: Implications for hot debris disks, Astrophysical Journal 713:816-836, 2010.