A guide to observing the 1998 Leonid activity is given. Expectations of peak time and activity profile are presented, and hints on visual, telescopic, video and photographic observations are given with the intention to derive scientifically useful data about the whole activity range of the 1998 Leonids.
The return of the Leonid meteor shower is no doubt the major astronomical event of 1998. The observing network which has been established within the International Meteor Organization in the last 15 years, provides us with all means for getting a complete picture of the Leonid meteor shower. This guide covers the whole range of activity we are expecting, not just the moment of highest rates, since we should not forget about deriving accurate results for off-peak rates as well.
The Leonid meteoroid stream is linked to the periodic comet 55P/Tempel-Tuttle. The comet has an orbital period of 33.2 years and was rediscovered on March 4, 1997 [1]. For a prediction of the peak time of meteor activity, the time of nodal crossing of the comet is important. The node lies at omega=235.258°, and the Earth will pass the node at lambda=235.29° which corresponds to Nov 17, 20h UT.
Comparing the 1998/1999 Leonid return with past events, we find that the encounter conditions are similar to those of 1866. If we use the 1866 ZHR profile of [2] for a prediction in 1998, we find ZHRs above 1000 between November 17, 19h and 21h UT. The ZHR will have returned to a level of 100 at 23h UT. The background component is fairly broad and lasts for about 12h with ZHRs above 50 according to the 1996 results [3] and for about 10h according to the decay exponent of 1866 given in [2].
Figure 1 shows a sort of visibility function of the Leonids. It will be interesting to know how many hours before the peak time the radiant will be sufficiently high above the horizon. The later limit will be dawn, and the period before the Sun approaches the horizon will be interesting too. We coupled both times by multiplication, since this operation gives only one maximum where both times are equal. Best observability with a minimum radiant elevation of 40° and a minimum depression of the Sun of 12° is in the north-east of China.
![[figure 1]](leofig1.gif)
We may construct a scenario with a peak equivalent ZHR of 10,000 meteors per hour. Given this maximum rate, Figure 2 shows an overview of expected activity at different geographical locations. All positions refer to the same local time - 3h 30m, when the peak is expected in eastern Mongolia and north-eastern China. The activity profile was defined by the exponential-decay constants derived for 1866 in [2]. You can read the geographical longitude as a time axis: Positions east of Mongolia represent times before the peak, positions west of Mongolia represent times after the peak. The radiant elevation at that local time is included as well, giving the visible meteor rate at a limiting magnitude of 6.5. Observers in Japan will see about 1000 meteors per hour in the night November 17-18, shortly before the peak will take place. As it is dark until more than an hour later in Japan, they will observe strongly increasing activity. European observers will see a rate of 100 at best in the same night, that is, after the maximum. American observers will face a low activity of 10-20 on November 17-18. They may have seen, however, higher rates before the peak as shown in the lower part of Figure 2. Visible rates are between 20 and 50 in the night November 16-17. Hawaiian observers are closest to the peak on the western hemisphere with rates of 100. Note that the date now switches to November 17-18 when you consider Japan as above. Again, note that this graph of visible rates is only one of the scenarios possible, the predicted peak activity of 10,000 may well be wrong by a factor of 10 towards both lower or higher rates.
![[figure 2u]](leofig2u.gif)
![[figure 2l]](leofig2l.gif)
Although these predictions look quite accurate, we should definitely not rely on them and be prepared for the full range of activity at any location. It is indeed most unlikely that the peak will be shifted by more than 2 hours or that the background activity is much higher than anticipated. However, if something very unusual happens and we are not properly prepared, we will lose the chance of the first global, scientific monitoring of a Leonid meteor storm.
For everyone who intends to travel to central or eastern Asia, the hints for using astronomical equipment on cold climates [4] are warmly recommended. Night temperatures below -20°C (below -4°F) are very common in Asian desert and prairie areas in November.
The Leonids will cover the whole range of activity from usual major-shower rates up to perhaps several meteors per second within a few hours. It will be very difficult for visual observers to cope with these conditions. We will describe the techniques for which a visual observer should be prepared, depending on the visible rate of meteors which would be recorded if continued for one hour (HR).
In this article, you'll find a modified form for the observing report. The ``Observed shower'' will be LEO, and you can fill in up to 30 observing periods in the form. One period should contain between 10 and 30 meteors. You should, therefore, not forget to give enough time marks on your recording device. If you were not able to discriminate Leonids from sporadics due to high activity, just write TOT in the blank shower field in the header of the table and leave the number of sporadics empty.
Magnitude distributions should be given with 40 to 80 meteors. Please select some of those time marks for the boundaries of magnitude distributions which were already used in the upper table for observing periods.
A tape recorder or the somewhat awkward looking paper-roll technique have proven capable of recording up to 500 meteors per hour. This rate corresponds to 8 meteors a minute. Since meteors are assumed to be randomly distributed, rates may be 15 meteors per minute occasionally. The average rate is thus misleading. Even a usual major shower like the Quadrantids, Perseids, or Geminids can keep you talking or writing continuously for a minute or two which are followed by periods of quiescence.
You should not stop your tape but just speak a magnitude into the microphone whenever you see a meteor. Note that the shower information is not very important for hourly rates beyond 200, since the error caused by the very few sporadic meteors is small. A rate of 200 meteors per hour corresponds to 3 to 4 meteors per minute.
You should not stop reporting magnitudes of the meteors even if you feel uncertain about the quality of your estimates. If all your fellow observers are doing so, meteor quantities will yet be large enough to obtain a good average population index.
The estimation of limiting magnitudes will often be interrupted by meteor sightings. It is suggested to stop the observations for limiting magnitude counts. It's hard to stop recording when many meteors are falling, but it will only be for a short interal of one or two minutes, and remember that the accuracy of the final ZHR highly depends on a reasonable estimate of the limiting magnitude. Don't forget to regularly count two limiting-magnitude fields during your observation.
This range of meteor rates covers 8 to 67 meteors per minute on average. In other words: It will be between `sometimes' and `always' that you are not able to report reasonable magnitudes of the meteors anymore. An activity of 4000 meteors per hour is roughly 1 meteor per second. Again, due to the random temporal distribution of meteors, seconds with three or four meteors will occur as well as quiet seconds.
Try to record magnitudes of the meteors as long as possible. Don't worry if you start feeling less confident in your estimates - the large number of meteors recorded will give your results sufficient statistical significance. You should not stop your tape recorder after each meteor; just speak onto the running tape. Times can be derived afterwards from playing time. Nevertheless, for calibration purposes, it will be useful to record the times of start and end onto the tape. So your recordings will contain an exact start time, then (hopefully) plenty of magnitudes or bleeps, and an exact end time when the tape was stopped.
A rate of one or two meteors per second on average should be recordable by simple `beeps' which you speak onto the tape; higher rates will soon become impossible to record because of the uneven temporal distribution of meteors. You may switch to 10-meteor countings, that is, you `beep' onto your tape when you have the impression that 10 meteors have appeared. The same method of recording the time as in Section 2.2 should be applied here.
Another method was used by observers in 1966 who were completely taken by surprise when they saw many meteors a second. Observer swept their gaze across the sky for one second and estimated how many meteors they saw. A maximum value of 40 was reported. This method bears uncertainties in both the estimation of the number of meteors and the estimation of how long one second is. This year, we have the chance to check visual estimates by video technique (see below), and if we try the same visual method as in 1966, we can calibrate the old activity estimates by comparing our 1998 visual and video results. A powerful software to check your capabilities of monitoring meteors at storm conditions can be found on the internet at ftp://www.imo.net/pub/software/metsim/. Investigations on the reliability of visual observations based on that program were published in [5].
Whereas the observation of very high meteor activity will be most exciting for visual observers, it is the ultimate domain for video systems. A video camera is an emotionless piece of electronics that supplies accurate figures no matter if there is one meteor per hour or one per second. In fact, if a meteor storm establishes this or next year, it will be for the first time that we get reliable quantitative measurements of meteor storms at all.
The main goal for video observers will be the determination of meteor activity followed by meteoroid flux computations. For this purpose, all types of video system (see [6] for a detailed discussion of the different camera types) may be used.
Similar to visual observers, wide angle cameras combine a large field of view with moderate limiting magnitudes. They are able to record a vast number of bright meteors. From the ratio of bright and fainter shooting stars we can derive the mixture of different particle sizes found in the meteoroid stream. Because of their similarity to visual observers, wide angle video systems are the first choice for the calibration of 1966 visual data as explained in Section 2.3.
Normal and tele video systems have successive smaller fields of view, but are also able to record fainter meteors. Thus, they extend the flux profile obtained with wide angle systems to smaller meteoroids causing fainter meteors. With their help we will be able to find out, whether the Leonid acitivity cuts off at a certain magnitude, or if the number of meteors is continues to increase exponentially towards those which cannot be detected by the naked eye anymore.
Finally, a battery of video systems with different lenses gives us the unique chance to study meteor activity over a range of about 15 magnitudes - from fireballs with -7 mag down to the faintest meteors of +7 mag! We suggest that video observers at the same place arrange their activities to gain a large coverage of particle sizes and a maximum of information.
At locations where no video cameras are in operation, photographic equipment can also be supportive in meteoroid flux estimates given very high Leonid activity. When you are lucky enough to experience such rates try to make five minute exposures. Away from the times of highest activity you can increase the exposure time to 10 or 20 minutes to cover the entire night with a single film.
Another observing goal may be the determination of Leonid orbits from the storm filament. For this purpose we suggest the use of photographic equipment. Though video systems will record orders of magnitudes more meteors, the accuracy of meteor photographs is clearly superior. This is caused by the up to 10 times higher spatial resolution of film material compared to the phosphorous screen of an image intensifier. The expected high activity will result in a sufficient number of meteor photographs, which will give the best meteoroid orbits.
Given the large quantities of bright meteors expected, certain special studies may be carried out by means of video and photographic equipment.
High resolution meteor spectra are rare, because the chance of capturing a meteor beeing bright enough is extremely small. Using a high precision grating, the limiting magnitude of the detector is about 3 mag lower for meteor spectra than for meteors. Cheap holographic plastic grating cause another loss of 1 to 2 magnitudes. That is, in the absence of large meteor showers you will have to operate your camera on average in the order of several (video systems) to several thousand hours (photographic equipment) until you have secured a spectrum. Even during the Perseid's maximum average exposure times between several tens of minutes and hours are to be expected. As the activity during a meteor storm surpasses major meteor showers by some magnitudes, you have a fair chance of recording several high quality photographic spectra in one night. Even more, with the help of video systems it will be possible to assess differences in meteor spectra of one meteoroid stream from a large statistical sample.
Another special target for video and photographic observers may be persistent trains. The Leonids are caused by fast meteoroids of cometary origin. They are known to produce a large number of persistent trains, sometimes visible for several tens of minutes [7]. The larger the meteor number, the higher the chance to record bright persistent trains and their deformation by winds in the high atmosphere. Here video systems have the advantage to minutely track all changes. On the other hand, you can use longer exposure times with photographic equipment and thereby follow the train development even after it has become invisible to visual or video observers. If you possess a grating or prims, but no video equipment, you should definitely consider having a camera with your spectral equipment at hand when a very bright fireball appears leaving a train persisting for may tens of seconds. Meteor train spectra are extremely rare, and the Leonid maximum offers a unique chance to capture train spectra.
Last but not least, both video and photographic equipment can present you an unique souvenir from a unique event. Every video observer knows about the excitement of the audience when some recordings of the Perseids are presented. A photograph of the 1966 Leonids showing more than 70 meteors has become famous not only among meteor observers, but among the whole community of astronomy enthusiasts. So, use the chance to produce your own memorable video and photograph! Who will be able to present ``stars falling like rain'' on a video screen in real time? Who will be the first having a hundred shooting stars on a single photograph? We wish you much luck with your experiments!
In these days of video, you would be forgiven for thinking that telescopic observations of the Leonids have minor import. Video systems are still uncommon; many of those will be trained on the maximum in China or Japan, or concentrate on visual meteors. To garner a comprehensive picture of a Leonid outburst, it's imperative to observe meteors across the full spectrum of brightness (mass). Remember that telescopic meteors vastly outnumber their visual counterparts. Telescopic data provide information about the meteors fainter than visual, and is the only means open to amateurs of gathering information for meteors fainter than +9 mag.
The main goals are to determine the meteor flux of faint Leonids throughout the period of activity, not just at the maximum; and to determine the time of peak activity. If you are fortunate to have a selection of telescopes and binoculars a) choose a wider apparent field of view (up to about 70°) to maximise the number of meteors seen, and b) select the largest suitable instrument to detect the faintest Leonids.
Plotting is feasible up to around HR=25-30 based upon experience at a dark site during the Geminid peak. Thus for rates below about 30 meteors per hour, adopt the standard plotting technique, alternating between two fields of view approximately every 30 minutes. Suitable pairs of IMO charts are 123 and 147, 80 and 146, 81 and 145, 103 and 146. Measurement of the deadtime while recording the meteor details and plotting its path is especially important so these data may also be used for flux measurements. Do not forget to record the decay time and distortion of persistent trains if the meteor frequency permits.
A report form for telescopic observations is supplied with this article.
Be prepared to switch to the following technique should rates become too high. Observers are expected to use their judgement as to what is unmanageable.
These rates are too high to plot. At a given time the smaller field of view compared with that of the visual observer will make scientific observations somewhat easier, not least because the observed rate is expected to be lower. However, the onset of higher telescopic rates may occur before the visual rate accelerates.
Select one field. This need not be an IMO chart region, though these are strongly preferred as they will enable limiting magnitude estimates within the field. The important thing is to have a wide range of star brightnesses, be situated 10-20° from and be at a higher elevation than the Leonid radiant. Proceed as if observing with the naked eye, as described in Sections 2 and 2.1. Note that this requires equipment not normally used for telescopic watches. So if you're not familiar with the paper-roll technique or using a tape recorder, practise with them prior to the Leonids so they become second nature. Note that accurate limiting magnitude estimates using several stars in the field are vital, and will need to be estimated regularly. For those using their own fields lacking a magnitude sequence within the field should estimate the naked-eye limiting magnitude. In the case use the standard counting method in two regions in the vicinity of the telescopic field.
Record the magnitude of the meteors seen, and in addition the shower association for non-Leonid meteors; all such meteors are deemed to be sporadic. This will save time if there is a short flurry of activity. It will be obvious which meteors are Leonids as they will dominate the sporadic meteors. As you will need to estimate magnitudes quickly and `on-the-fly', become familiar with the integral magnitudes of selected field stars spanning the range of brightnesses expected. Again it is best to do this before the Leonid activity commences.
Again see the corresponding visual tips in Section 2.2. If the rate goes to one every few seconds to one per second, dispense with the shower discrimination, and just note magnitudes. You can also omit the ``plus'' before the magnitude; a negative magnitude meteor will be a stupendous, but rare sight. In execeptional cases you may wish to pause to allow your eye(s) to recover.
At this point it is going to be very difficult to stay glued to the eyepiece even though you can see meteors continuously. The visual sky will be stunning. If observers can make some measurements at the eyepiece during storm activity, these data will be most valuable, but it would be understandable if you wanted to witness the spectacle of a lifetime across the whole sky. Again adopt the visual technique (Section 2.3) of beeping as meteors appear in the field. There should not be any need to sweep, however. It should be easier to estimate the telescopic count than visually because of the narrower apparent field of view.
[1] B. Marsden, IAU Circular No. 6579,
March 1997.
[2] P. Jenniskens "Meteor stream activity. II.
Meteor outbursts", Astron. Astrophys. 295, 1995, pp. 206-235.
[3] P. Brown, R. Arlt "Bulletin 10 of the International
Leonid Watch: Final Results of the 1996 Leonid Maximum", WGN 25:5,
October 1997, pp. 210-214.
[4] C. Trayner "Using Astronomical Equipment in Cold
Climates", WGN 25:6, December 1997, pp. 236-247
[5] H. Lüthen, S. Molau, "Can Visual Observers
Accurately Estimate Meteor Rates in Meteor Storms?", WGN 26:3,
June 1998, pp. 109-117
[6] S. Molau, M. Nitschke, M. de Lignie, R.L. Hawkes,
J. Rendtel, "Video Observation of Meteors: History,
Current Status and Future Prospects", WGN 25:1, February 1997,
pp. 15-20
[7] S. Molau, G. Volker "Spectacular Leonid Fireball",
WGN 25:1, February 1997, pp. 54-56
