International Meteor Organization (IMO)

Bulletin 12 of the International Leonid Watch: Final Results of the 1997 Leonids and Prospects for 1998

Rainer Arlt and Peter Brown

Published in WGN, the Journal of IMO 26:4, p. 161-165 (1998)

Abstract: Visual reports of the 1997 Leonid shower are used to calculate an activity profile for the Leonid stream's 1997 return. Despite the Full-Moon conditions, it was possible to derive a fairly consistent ZHR profile; the flux profile is much more noisy, as it strongly depends on the population index profile which is less reliable due to the limited number of meteor magnitude estimates available. Increased activity began near lambda=235.0° (all solar longitudes refer to equinox 2000.0) and persisted until at least lambda=236.0°. The peak ZHR value is near 100±15 at circa lambda=235.22°±0.04°, corresponding to November 17, 1997, 12h15m UT. High activity of ZHR>80 persisted until lambda=235.5°, but it is argued that this continued activity is an artefact due to underestimated limiting magnitudes as judged from sporadic rates. The population index shows an increase from 2.0 to 2.5 at lambda=235.15°±0.02 (November 17, 10h30m UT), as occurred in 1996, but the statistical significance of the increase is marginal in 1997. Strong lunar interference precludes any definitive statement concerning the visual activity of the shower in 1997. The outlook for the 1998 return based on available information is summarized.

Introduction

As discussed in the previous International Leonid Watch ( ILW ) Bulletin [1], the 1997 return was well covered by observers, but the nearly Full Moon severely hampered most observations for the nights around the peak. Nevertheless, as so many observers did attempt observations, enough data are available to attempt some cautious analysis. In 1997, a total of 73 observers observing 2623 Leonids in 237.30 hours reported their observations. They were from 14 countries: Belgium, Canada, Croatia, Finland, Germany, India, Italy, Japan, Jordan, the Netherlands, Spain, the United States, Venezuela, and Yugoslavia. We thank the many observers listed below who contributed to the 1997 analysis:

Sana'a Abdo (ABDSA, 2h02), Mohammad Al-Alwanew (ALAMO, 4h50), Ramez Al-Mualla (ALMRA, 3h20), Ahmad Al-Niamat (ALNAH, 4h67), Joseph D. Assmus (ASSJO, 2h42), Lance Benner (BENLA, 4h17), Orlando Benítez Sánchez (BENOR, 2h10), Nikola Biliskov (BILNI, 1h67), Matthew Collier (COLMA, 0h96), Hani Dalee (DALHA, 4h27), Mark Davis (DAVMA, 4h00), Peter Detterline (DETPE, 1h08), German Dominguez Delmas (DOMGE, 1h42), Yosinori Fuyube (FUYYO, 0h50), Slaven Garaj (GARSL, 0h67), George W. Gliba (GLIGE, 2h00), Roberto Gorelli (GORRO, 2h67), Lew Gramer (GRALE, 7h94), Robin Gray (GRARO, 0h75), Wayne T. Hally (HALWA, 2h55), Joost Hartman (HARJS, 4h75), Takema Hashimoto (HASTA, 10h55), Roberto Haver (HAVRO, 2h59), Robert Hays (HAYRO, 1h00), David Hernandez (HERDA, 3h15), Dave Hostetter (HOSDA, 1h54), Oomi Iiyama (IIYOO, 3h99), Daiyu Ito (ITODA, 4h71), Kiyoshi Izumi (IZUKI, 1h00), Carl Johannink (JOHCA, 2h08), Niladri Kar (KARNI, 7h41), Kevin Kilkenny (KILKE, 2h03), Marco Langbroek (LANMA, 6h14), Vladimir Lukic (LUKVL, 1h00), Robert Lunsford (LUNRO, 9h49), Katuhiko Mameta (MAMKA, 13h00), Pierre Martin (MARPI, 0h58), Takuya Maruyama (MARTA, 3h43), Antonio Martinez (MARTI, 2h34), Koen Miskotte (MISKO, 8h51), Hidekatu Mizoguchi (MIZHI, 0h73), Sirko Molau (MOLSI, 4h03), Koji Naniwada (NANKO, 1h33), Jos Nijland (NIJJO, 5h40), Markku Nissinen (NISMA, 1h04), Mohammad Odeh (ODEMO, 4h89), Ibrahim Odwan (ODWIB, 4h09), Masayuki Oka (OKAMA, 5h84), Kazuhiro Osada (OSAKA, 10h00), Toru Sagayama (SAGTO, 1h72), Mitsue Sakaguchi (SAKMI, 3h64), Javier Sanchez (SANJA, 2h22), Koetu Sato (SATKO, 1h83), Tomoko Sato (SATTM, 0h50), René Scurbecq (SCURE, 1h23), Miguel Serra Martin (SERMI, 2h93), Hiroyuki Sioi (SIOHI, 4h00), James N. Smith (SMIJN, 3h92), Enrico Stomeo (STOEN, 0h46), Máximo Svárez Tejera (SVAMX, 2h03), Richard Taibi (TAIRI, 3h55), Kazumi Terakubo (TERKA, 0h50), Masayuki Toda (TODMA, 3h00), Robert Togni (TOGRO, 2h41), Michael Toomey (TOOMI, 2h96), Josep M. Trigo Rodriguez (TRIJO, 5h09), Anne van Weerden (VANAE, 2h43), Frans van Loo (VANFA, 1h50), Maarten Vanleenhove (VANMT, 1h75), Ilkka Yrjölä (YRJIL, 1h05), George Zay (ZAYGE, 5h48), Goran Zgrablic (ZGRGO, 2h40)

Population index profile

The average reported limiting magnitudes near the time of the Leonid peak in 1997 were between 4.5 and 5.5, typically resulting in large correction factors due to the limiting magnitude alone (between 2 and 5). This underscores the necessarily large uncertainties which follow in every quantity discussed (and further emphasized by the large error margins present in the figures).

[r-profile of 1997 Leonids]

Figure 1: Population index r versus solar longitude for the 1997 Leonid return. From 235.0°-235.3° the magnitude data were binned in intervals of 0.06° and shifted in steps of 0.03°. From 235.3° onwards, the binning windows were 0.2° wide and shifted by 0.1°.

Since sufficient magnitude estimates were made in 1997, a complete r-profile can be constructed during the principle activity period of the shower; the graph is shown in Figure 1.
The initial values for r are consistent with the longer-term average for the shower near 2.0. The values between lambda=235.0° and lambda=235.15° (equinox 2000.0 throughout this paper) are in the range 1.8-2.1, and are very similar to the profile from 1996. The large increase at lambda=235.17° is at precisely the same location as a similar (though smaller) increase recorded in 1996 [2]. However, the large error margin associated with this particular datum implies that this is only a probable concordance with the 1996 profile. Within error, however, the value of r does clearly increase between lambda=235.08° and lambda=235.17° as in 1996; it is the magnitude of the increase which is most uncertain.
Unfortunately, the remainder of the Leonid interval is only modestly covered by magnitude estimates, particularly as no magnitudes are reported from eastern Asian longitudes, and the most consistent value for r from lambda=235.3° onward is near 2.3. Curiously, these are higher (within error) as compared to the same intervals in 1996 and the longer-term average. It might be argued that, on the one hand, observers were able to estimate a resonable limiting magnitude under the Full-Moon conditions (as can be seen from the reasonable ZHRs), but systematically underestimated meteor magnitudes on the other hand (i.e., making them fainter). This may occur since observers do not always compare a meteors appearance with a star of similar brightness. Instead, judgements like ``relatively faint'' might have been converted into a magnitude estimate as if under better sky conditions, making a magnitude +3 or +4 meteor a full magnitude fainter.

ZHR profile

The ZHR activity profile is shown in Figure 2. The build-up in activity beginning near 235.0° is apparent, and, between lambda=235.1° and lambda=235.3° (November 17, 9h30m-10h UT), a clear peak with a maximum ZHR of 96±13 is reached. The peak at lambda=235.22°±0.04 (November 17, 12h15m UT) is based on reports from a dozen observers and is quite reliable (excepting the everpresent large error margins due to the Moon). Additionally, it comes after the large peak in the population index at a point where this is a large drop in r and is thus not a simple artefact of the sudden jump in r. The activity seems to decline after the peak with about the same steepness as the increase, yet is soon followed by another maximum at lambda=235.45°±0.05 (November 17, 17h30m UT) with ZHR=85±13. It should be noted that the error margins do overlap during large parts of the Leonid maximum. We cannot exclude that the maximum furnished a plateau activity between solar longitudes lambda=235.15° and lambda=235.5°.
However, the sporadic HR during the second, less prominent enhancement of activity is very high, the values being definitely too high (compared to normal sporadic rates for this time of the year) by factor of about 3. We assume that the few observers contributing to these ZHR values underestimated their limiting magnitudes significantly-a typical effect under moonlit sky conditions. Upon changing the limiting magnitude by +1 to reduce the sporadic HRs by a factor of 3, Leonid ZHRs decrease by a factor of roughly 2, i.e., the ZHR graph turns into a gradually declining curve matching the reliable value of about 35 after a solar longitude of sol=235.8°.
When comparing the ZHR-graph with that of the preliminary analysis in [1], one finds the most striking difference in the high value of about 150. This value was based on very few individual counts and is now smoothed out by additional data.
We do not present values for flux, as the large errors in r and ZHR make flux values virtually indeterminate. We can only say that near the time of the early peak (sol~ 235.2°), the shower flux was somewhere in the interval 0.01-0.05 meteoroids/km-2 hour-1 to a limiting absolute magnitude of +6.5.

[ZHR profile of 1997 Leonids]

Figure 2: The ZHR versus solar longitude for the 1997 Leonid return. Observations were binned in windows of 0.06° size from lambda=235.0° to lambda=235.3° and smoothed in steps of 0.03°, while from lambda=235.3° to lambda=235.6°, the binning intervals were 0.2° wide and the steps used were 0.1°. For all intervals after sol=235.6°, the bins were 1.0° wide and stepped at 0.5° intervals.

Prospects for 1998

As all visual data have now been analyzed in the lead-up to 1998, we may attempt to use the features from the last few years, plus model and other general considerations to estimate the time of the peak of the shower in 1998. The magnitude of the peak ZHR in 1998 is much more difficult to divine, but we present some of the most recent predictions in Table 1.

Table 1: Recent predictions for the Leonid meteor storm expected for 1998. The ZHRs are estimates for the anticipated activity.

Author Peak Time (UT) Peak ZHR
Jenniskens, 1996 [7] 235.34° 21h30m 10 000
Yeomans, 1998 [8] 235.26° 19h40m 200-5000
Brown et al., 1998 [6] 235.24° 18h50m 1000-10 000
Kresak, 1993 [9]234.8°08h30m 10 000

From the analysis of each of the returns in 1995, 1996, and 1997 [2,3], a period of transient activity has been noted in each of these years. The peak ZHR values and location in 1995 are most uncertain (ZHR~50 and peak near sol=235.0°), while a clear outburst feature was noted in 1996 near sol=235.16° with a peak ZHR of 90. The present analysis suggests another ``early'' peak near sol=235.22° with a peak ZHR approaching 100. In all cases, the trend appears to be for the peaks to be shifting closer to the nodal longitude of 55P/Tempel-Tuttle (Omega=235.26°) in the few years immediately before the comet reaches perihelion. The most reliably determined of these peaks (that from 1996) is also at the same longitude as the 1966 meteor storm.
On general dynamical grounds, it is expected that any meteor storm in 1998 will occur near the time of the comet's nodal passage [4]. From the recorded Leonid meteor storms over the last 200 years, there is a clear trend whereby the strongest storms occur closest to the cometary node. Curiously, the 6 largest storms all peaked 0.5-2 hours after the nodal longitude of the comet [5].
The most recent numerical modeling results suggest that, if a storm occurs in 1998, it will likely do so within 0.5 hours of the nodal passage (specifically somewhat before the time of the passage) [6]. The most recent predictions for 1998 are summarized in Table 1, along with estimates of the peak ZHR where these have been given.

The eighth ILW period: November 5-25, 1998

Summarizing all of the above, it appears most probable that any significant enhancement in Leonid activity in 1998 will occur in the interval sol=235.15°-235.3°. This implies the best location is likely to be in the Western Pacific or Eastern Asia. Noting the possible plateau in activity observed in 1997 and similar behavior observed in past Leonid returns near the time of the comet's passage (such as in 1965 [5]), it is quite probable that much higher than normal activity from the extended component of the shower may be visible for as much as 24 hours centered about the cometary node.
As a result, observers are encouraged to exercise special vigilance during their local 0h-6h times on the mornings of November 17 and 18 in particular. The days around the time of the peak will be completely free of lunar interference and ideal for observations. As 1998 has some prospect for producing a meteor storm, observers are reminded that fixed cameras may be most useful during the storm, though disciplined observers should be able to make successful counts should rates reach even as many as several Leonids per second [6]. A detailed description of various observing methods during the Leonid maximum is given earlier in this issue.

References

[1] P. Brown, R. Arlt: Bulletin 11 of the International Leonid Watch: First Results of the 1997 Leonids. WGN 26:1, 1998, pp. 11-12

[2] P. Brown, R. Arlt: Bulletin 10 of the International Leonid Watch: Final Results of the 1996 Leonid Maximum. WGN 25:5, 1997, pp. 210-214

[3] P. Brown: Bulletin 8 of the International Leonid Watch. WGN 24:5, 1996, pp. 139-141

[4] D.k. Yeomans, K.K. Yau, P.R. Weissman: The Impending Appearance of Comet Tempel-Tuttle and the Leonid Meteors. Icarus 124, 1996, pp. 407-413

[5] P. Brown: The Leonid Meteoroid Stream: Historical Visual Observations. submitted 1998

[6] P. Brown, J. Jones, K.J. Ellis, W.K. Hocking, A.R. Webster, R.L. Hawkes: Recent Observations and Modelling of the Leonid Meteoroid Stream. AIAA Leonid Storm Hazard Conference Manhatten Beach, CA., April 27, 1998

[7] P. Jenniskens: Meteor Stream Activity. III. Measurement of the First in a New Series of Leonid Outburst. Meteoritics and Planetary Science 31, 1996, pp. 177

[8] D.K. Yeomans: Comet 55P/Tempel-Tuttle and the Leonid Meteors. AIAA Leonid Storm Hazard Conference. Manhattan Beach, CA., April 27, 1998

[9] L. Kresák: Meteor Storms. In Meteoroids and their Parent Bodies J. Stohl, I.P. Williams, eds., 1993, pp. 147-157

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