Mathematics – Probability
Scientific paper
May 1998
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998icar..133...36b&link_type=abstract
Icarus, Volume 133, Issue 1, pp. 36-68.
Mathematics
Probability
34
Scientific paper
Four major models of cometary meteoroid ejection are developed and used to simulate plausible starting conditions for the formation of the Perseid stream. In addition to these physical variants, three different choices for initial meteoroid density (100, 800, and 4000 kg m^-3) are used to produce a total of 12 distinct initial models. The development and evolution of the stream are simulated for each model by ejecting 10^4 test meteoroids at seven distinct mass categories over the full arc of 109P's orbit inside 4 AU at each perihelion passage from 59 to 1862 AD. All test meteoroids are followed to their descending nodes for times closest to the recent perihelion passage of 109P (1992). In addition to these integrations, we have also performed long-term integrations over the interval from 5000 to 10^5 years ago using two plausible sets of starting orbits for 109P over this interval. We find that the choice of cone angle and precise cutoff distance for ejection make only minor modifications to the overall structure of the stream as seen from Earth. The assumed density for the meteoroids has a major influence on the present activity of the stream as radiation pressure moves nodal points further outside Earth's orbit and hence decreases the probability of delivery for lower density meteoroids. The initial ejection velocities strongly influence the final distributions observed from Earth for the first ~5 revolutions after ejection, at which point planetary perturbations and radiation effects become more important to subsequent development. The minimum distance between the osculating orbit of 109P at the epoch of ejection and the Earth's orbit is the principal determinant of subsequent delivery of meteoroids to the Earth. The best fit to the observed present flux location and peak strengths are found from models using Jones (1995) ejection velocity algorithm with an r^-0.5 dependence and densities between 0.1 and 0.8 g cm^-3. The recent activity outburst maxima observed for the Perseids from 1989 to present show a systematic shift in location from year to year, which is explained by changing ages of the primary component of the meteoroids making up the outbursts. Specifically, it is found that from 1988 to 1990 ejecta from 1610 and 1737 are the dominant population, while 1862 and 1610 are the primary material encountered in the outbursts from 1991 to 1994. From 1995 to 1997 the most prevalent populations are ejections from 1479 and 1079. The older populations tend to shift the locations of the maximums to higher solar longitudes. A discrepancy which is present for both the 1993 and 1994 peak locations of 1-2 h between the observed and modeled flux profiles is most likely the result of emissions from 1862, which were observed to have a large component of their velocity out of the cometary orbital plane. The cause of Perseid activity outbursts is found to be direct planetary gravitational perturbations from Jupiter and Saturn that shift the nodes of stream meteoroids inward and allow them to collide with Earth. The last such perturbations was due to Jupiter in 1991, and this effect combined with the return of 109P in 1992 produced the strong displays from 1991 to 1994. On average, it is found that the Perseids observed each year in the core portion of the stream left the parent comet (25 +/- 10) x 10^3 years ago. From the modeling, the total age of the stream is estimated to be on the order of 10^5 years. From the simulations over the last 2000 years, the progression rate of the node of the stream is estimated at (2.2 +/- 0.2) x 10^-4 degrees/annum. The effect of terrestrial perturbations has been evaluated from the long-term integrations and found to play only a minor role in the stream's development, producing a 5-10% increase in the stream's nodal and radiant spread as compared to an identical simulation without the Earth. The primary sinks for the stream are found to be hyperbolic ejection due to Jupiter (and to a smaller degree Saturn) as well as attainment of sungrazing states. Both the relative and absolute contributions of these two loss mechanisms to the decay of the stream is found to be highly dependent on the assumed cometary starting orbits, with as much as 35% of initially released stream meteoroids removed by hyperbolic ejection after 10^5 years for the smallest Perseids on some starting orbits to less than 1% removed after the same time for larger meteoroids on other potential seed orbits. On average, it requires 40-80 x 10^3 years for a noticeable fraction of the initial population (>0.1%) to be removed by these mechanisms, depending on the chosen starting orbits.
Brown Patrick
Jones Jason J.
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