Draconids 2006: prediction of activity
to the list of predictions
Draconids, also known as Giacobinids, are a meteor shower of the comet 21P Giacobini-Zinner. This comet was discovered in 1900, just after its orbit was moved by Jupiter close to the Earth orbit. In the second half of 1910s some astronomers pointed out that there is possibility of meteor action from this comet in the beginning of October and such activity was actually observed. The meteor shower had its radiant in the head of Draco, which was suitable for 21P orbit. During 20 century and up to the present days the orbit of 21P passed through some perturbations, but was remaining near the Earth orbit. In this period Draconids produced two strong storms in 1933 and 1946 and a number of non-stormy outbursts. The latest activity enhancements occured in 1998 and 1999, when the comet passed its previous perihelion, as well as in 2005. In 1998 ZHR reached 700 meteors and in 1999 - 15-20 meteors. 2005 gave an enhancement up to 30-35 meteors on ZHR scale .
The comet passed its last perihelion in July 2005. In 2006 the period of Draconid activity occurs approximately 1 year and 3 months after this perihelion. Principally, after such period some young trails are still close to the Earth orbit, so even without a conduction of accurate computations we would have substantial reasons to hope for notable activity enhancement. Let look however on the results obtained by modelling of Draconid stream evolution.
The vast majority of Draconid outbursts is traced with the modelling very good (but there are some exceptions, first of all, the 1985 case). Particles ejected by the comet form lengthy trails. One of the reasons is radiation pressure force, which acts parallel with gravitational forces. Gravitational force is dependent on a particle mass, i.e. it is proportional to the third power of particle radius. The outcrying radiation pressuse is defined by the second power of particle radius. So far the influence of radiation pressure is the more the less is size of a particle. Its action is equivalent to the diminishing of gravitational constant G. So it increases the orbital period of particles, and the tinier a particle is, the more it is continuously retarded from larger particles after their ejection be the comet. This process therefore leads to the formation of lengthy comet trails.
Meteor modelling is done through computation of orbital evolution of particles ejected by the comet with different velocities in directions tangential to the comet trajectory at the moment of perihelion. In the reality, of course, particles are ejected not only at the point of perihelion, but also within several months around it. However, comets are close to perihelion during quite a little time comparing to their overall orbital period and main perturbations happen around their aphelions, so when comets are closer to the Sun newly ejected particles are moving very close to them in a compact dust cloud. This is the reason we can take that cloud as completely ejected in the point of perihelion, it doesn't virtualy influence the results of computations.
Speaking of directions in which particles are ejected we can say that, again, in the reality they are ejected far not only in tangential directions, but in all possible ones. However, ejection velocities (from 0 to 100 m/s, and the overwhelming majority of real ejections - from 0 to 20 m/s) are negligibly small comparing to the own comet velocity (from 30 to 40 km/s) near the Earth's orbit), ejected particles have only slightly changed orbits and don't "fly away in all directions". Radial part of ejection velocity defines only thickness of a trail, which usually reaches several hundreds thousands kilometers. The shape of the trail is defined by tangential part of ejection velocity.
And the last. Non-gravitational forces are often not taken into consideration in meteor calculations, as is in our case. However, some of them, say, radiation pressure, can be considered indirectly. As far as this kind of force works as diminishing of gravitational constant G, this is equivalent to increase of ejection velocity which could be easily accounted in the model. So this non-gravitational force, as many others doesn't change the configuration of trails, but leads to shifting of particles with different masses along them.
As already mentioned, Draconid trails modelling allowed to prepare very good predictions of shower activity in the previous years, real maximums differed from predicted ones mostly no more than on several minutes - not very much considering that computations are made for dozens and hundreds years of particles movement. More serious problem is prediction of outburst intensity - how strong the maximum could be. For such predictions special empirical models were elaborated (the single way in this case) but as before for their improvment new observations are very necessary.
The results obtained by the Author for the Draconids 2006 using the modelling of particles ejected by the comet 21P Giacobini-Zinner are presented below. Main characteristics of computations are also described.
I wish to introduce the results of Draconid meteor stream simulation aimed to the prediction of shower activity in 2006. The simulation was made for the trails of latest 28 revolutions, i.e, from the 1817 trail. The Author used the program by S. Shanov and S. Dubrovsky "Comet's Dust 2.0" to calculate orbital elements of ejected meteor particles. To estimate expected ZHRs for different encounters the model by E. Lyytinen and T. van Flandern given in their paper  was used with some Author's alterations made in order to adopt the model for ejection velocity (Vej) instead of da0 (difference in a-semimajor axis) and to convert the model from the Leonid stream (for which it was originally created) to the Draconids. The computation considered only gravitational forces, however, the results are on the whole in good accordance with these of other researchers. The prediction includes all encounters found within interval +/-0.007 a.u. The following parts of trails were computed: the first 15 rev. trails for ejection velocities [-50;100] m/s, 16-28 rev. trails - [-30;50] m/s.
The Fig. 1 below shows the distribution of 21P dust trails in the vicinity of the Earth's orbit within the period of 08.07.2006 - 10.01.2007. The vertical axis shows the minimal distance between trails particles and the Earth's orbit. So far, the Fig. 1 displays the moments of passage of minimal distances to the Earth's orbit for various trails and particles and these distances themselves. The central vertical line corresponds to 8 October 2006.
Fig. 1. Space-temporal projection of Draconid trails parts onto their minimal distance passages (for each trail the year of its formation and ejection velocities of particles, corresponding to the depicted parts of trails, are given)
In 2006 the distribition of Draconid dust trails in the vicinity of the Earth orbit has the following features:
1) We can see parts of young 1959 - 1979 trails formed by the particles with high and very high ejection velocities. No one of these trails approachs the Earth close enough to speak of a direct encounter. However we can see that the Earth is in the space between 1966 and 1972 trails, much closer to the second one. So far, there is a possiblity of activity from non-perihelion particles of 1972 trail. Calculation of these particles parameteres gives the following results: dT(1972)=0.274, fM(fMD)=3.151, Vej=86.4 m/s, sol.long=195.663° (09.10.2006 4:43 UT), theoretical radiant: RA=262.3°, Dec=+55.5°, Vg=21.0 km/s. Principally, perspectives for visual activity are highly unfavorable. In case of ideal direct encounter to a trail with such parameters, expected ZHR is only about 1 meteor. For the parameter dT=0.274, basing of the Draconids 2005 enhancement, activity should be even 50 times lower. Considering that brightness of meteors, produced by particles with ejection velocity of 86 m/s in such a slow shower as the Draconids, should be extremely low, as well as presence of the waning gibbous Moon (phase=0.94) during all 9 October night, the chances of Draconid visual activity for this case have to be excluded.
2) A channel of 19 century trails passes on quite large distance inside the Earth orbit. These parts of trails are formed by particles with mostly low negative ejection velocities. The closest trail from this channel (1880) passes at -0.012 - -0.013 AU, so even not considering low density and obvious perturbation in the structure of the channel, we can't speak about activity from it.Visibility
The region where possible Draconid outburst from non-perihelion particles from 1972 trail - see 1) above - is available for observations is shown on the Fig. 2.
Рис. 2. The Earth as seen from coming Draconid meteors during the outburst at 4:43 UT 9 October. Red line shows the border of hemisphere where the Moon is above horizon (it is shown with light circle in the corner of the Fig. 2.).
At the mentioned time of maximum the shower radiant will lie in zenith for observers in the nothern part of the Pacific ocean, close to Aleutian Islands. Visual observations at this time are possible mostly in Northern America. The best conditions will be in weastern and central parts of the United States (exluding Alaska, where the sky will be still not very dark and of Canada. Closer to the eastern coast of the continent the radiant will already have quite low altitude. Besides this good conditions for visual observations will be in Greenland ind Iceland. For observers in Norway the radiant will be at the margin of availability in the night zone. In the rest of the Northland as well as on the Kola peninsula the sky will be already too light. For most European countries the radiant will lie close to the line of horizon.
Radioobservations are possible on much larger territory. Besides already mentioned regions very good conditions will be in the eastern part of Russia (Siberia and Far East), on most part of Eastern Asia (eapecially in Japan, Korea and China). In western part of Russia the radiant will be much lower, but radioobservations are still very possible, especially closer to Ural. For observer in Indonesia the radiant will be a bit above horizon. Further to the west the border of availability passes through India, the Middle East Countries, Black Sea and to Europe in the night.
I'd like to specially note, that even despite the negative prediction of stream visual activity, its observations are very important. As known, a negative result is also a meaningful result, which could confirm or disprove the prediction. And, on the other hand, the prediction could not consider all the finest features of stream dynamics, so there is always a chance of unexpected activity - from older, not computed trails or due to the possible imperfectness of the model.
In 2006 the Earth will pass trough an area of non-perihelion particles between 1966 and 1972 trails. Visual activity is not expected (moreover, the almost full Moon will be a great disturbance), but the Author does not exclude a radioenhancement at 4:43 UT 9 October. So far radioobservations are recommended during several hours around this time. Theoretical radiant: RA=262.3°, Dec=55.5°, Vg=21.0 km/s.
1. "Comet's dust 2.0" program by S. Shanov and S. Dubrovsky. [Used for computation of stream particles orbital evolution]
2. Information from Gary W. Kronk's page http://www.maa.agleia.de
3. IMO Circular on Draconid 2005.
4. Lyytinen E, van Flandern T. "Predicting the strength of Leonid outbursts", 2000, Icarus, P. 158-160.