Results

The corresponding orbital elements for L97 and L98 datasets are displayed in Figure 1 for the events which resulted in particle radii greater than 0.5 microns. We excluded smaller particles because the calculated possible magnetospheric forces exerted on them due to dust charging is of the same order as the earth attraction term or larger. Also it is unclear how particles of these sizes have survived such large deceleration, reaching relatively low altitudes, and it is possible that they are the result of larger meteoroids which have an across-the-beam velocity component. For a more detailed description on these results, the reader may refer to Janches et al. [3].

Panels (a) and (b) in Figure 1 give the perihelion times of the entire IDP set showing nearly equal pre- and postperihelion numbers in the case of L97 and a lack of preperihelion numbers, due to the bias introduced by using only 6 hours of observation, in the L98 case. Panels (c) and (d) give the semimajor axis (a) versus the eccentricity (e). In addition, the Whipple K and Pe criteria for particle asteroid/comet origin and the evolutionary path due to drag effects are shown for various values of the constant C [7]. As can be noted, most of the particles detected at AO are concentrated in asteroidal type orbits of a<2 AU (i.e. above the criteria curves) at the time of earth interception. The comparison between our results and the evolutionary paths or drag contours show that the a/e of the AO particles have evolved (downward along the curves) from orbits within the distance of Saturn, with the majority coming from within the orbit of Jupiter. This evolution is produced under non-gravitation influences such as Poynting-Robertson effect and radial solar corpuscular radiation pressure [8]. These diagrams also suggest that the particle orbit semi-major axes are reduced to ~1 AU by radiation pressure evolution followed by eccentricity increases (at nearly constant a). We interpret this increase as produced in a manner similar to the electromagnetic resonance (with interplanetary magnetic field sector boundary crossings) mechanism predicted by Morfill and Grün [9]. The meteoroid orbital inclinations displayed in panels (e) and (f) show that while both prograde and retrograde ecliptic concentrated particles are present for the preperihelion case, the postperihelion IDPs are dominated by the presence of retrograde orbits with a peak at ~ 140 degrees. Finally the perihelion distances (q) are displayed in panels (g) and (h). A dramatic reduction of "sungrazers", for the case of L97, in the postperihelion sample can be seen. For the L98 case a weaker lack rather than a pronounced reduction is indicated because of the low preperihelion particle orbits number. This reduction (or lack) is presumed in both years to be due to solar evaporation or other thermal destruction of these particles. Both the L97 and L98 datasets show a concentration of orbits with q between Mercury and Venus. The large remaining fraction of postperihelion orbits with q within the orbit of Mercury provides again a strong indication that the AO micrometeors are particularly durable compared with classical cometary meteoroids.

D. Janches et al.

Irtclinotion

0 50 100

0 50 100 150

Perihelion Distance

-100 0 too 200

At (tioya)

Pefihilioo Distance Total - 515 ntnil

L98 Orbital Results

Time to Perihelion

Tel ol - 551 Mftta . Bin *tr*«5 Hoy*

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