Eisse Pieter Bus Summaries of Radio Observations by Forward Scattering are given for the Alpha Monocerotids 1995, Perseids '94, '95, '96, '97 and 1998, Leonids '94, '95, '96, '97 and 1998 and Draconids 1996 and 1998. (All solar longitudes refer to equinox 2000.0) |
Results of Radio Meteor Scatter observations
Since 1993 meteors were detected by receiving forward-scattered VHF radio waves at a
frequency of 66.89 or 72.11MHz. The receiver used was a Bearcat UBC 177XLT scanning radio with
a RF sensitivity of 0.5 microvolts for a signal to noise ratio of 12dB and an IF selectivity
of 50dB at approximately 25 kHz. The transmitters, Polish broadcast stations are located in
Krakow (66.89 MHz) and Wroclaw (72.11 MHz), the receiver is located in Groningen, The
Netherlands.
The path length between Groningen and Krakow is about 1000 km and between Groningen and Wroclaw
about 740 km. A three-element Yagi antenna with folded dipole was used at the receiving station.
The antenna was directed to azimuth 106º (ESE) with an elevation of 9º towards Krakow and 13º
towards Wroclaw. The main lob of the antenna was directed towards the 100-km level, vertically
above the mid-point of the transmitter-receiver path. Because of the long distances between
transmitters and receiver, there is no aircraft interference. Also there was no noticeable
interference from other sources like nearby transmitters or lightning. Some interference was
caused by sporadic-E, aurora, atmospheric inversion, or nearby computers, but these interferences
were easily recognisable.
During the Leonid campaign in November 1996 and 1998, radio observations were made on
location in Spain and P.R. of China.
Results of Radio Meteor Scatter observations
"Sporadic" activity was observed by listening and counting in 5-minute intervals on days
with no (or low) shower activity. Also on days with shower activity, the total meteor activity
was observed by listening and counting in 5-minute intervals. The numbers are corrected for
"dead-time". Dead-time marks the period in which a certain signal of amplitude may mask other
signals of lesser amplitude. The dead-time corrections were applied according to the
"Geiger-counter method".
However, if there are to many long-duration reflections, even with corrections for dead-time
taken into account the total number of radio meteors can drop below sporadic activity.
This phenomenon was first observed with the Leonids in 1994, and also found in the radiodata
of the alpha Monocerotids in 1995 and Perseids 1994, 1995 and 1996.
Results of Radio Meteor Scatter observations
Hines [1] has developed the theory of the variation in the number of shower meteors observed
by forward scattering of radio waves. In his publication he gives an expression for the number
of shower meteors counted in a given observation period for a given meteor radiant position at
the mid-point of a transmitter-receiver path lengths of 1000 km. The calculated values of this
"observability function" were normalized for the given observation period.
However, this observability function seems only valid for underdense reflections because
the numbers of strong overdense signals, like the long-lasting signals caused by Leonids,
seems more or less proportional with the sine of the elevation of the radiant.
Results of Radio Meteor Scatter observations
The net values of the shower meteors were calculated by subtracting the mean "sporadic"
meteor counts as observed during the same observation periods. For each period, this net shower
value was divided by the value of the normalized observability function to obtain the estimated
true shower activity.
Wrong conclusions may be drawn if only raw uncorrected counts are used and no corrections are
applied for the observability function. For instance, the most favourable antenna geometry for
detecting Leonids for many world-wide radio-observers is around 7h-8h Local Time. Since wake-up of
the Leonids took place in 1994, many raw uncorrected world-wide radio-observations always show
a peak around this time.
Results of Radio Meteor Scatter observations
Radio observations of the alpha Monocerotids on November 22, 1995, clearly show an outburst with a maximum at 1h 29m UT. The activity started between 1h 05m - 1h 10m UT with 6 long-duration reflections. The "sporadic" background level is less than 1 long-duration reflection per 5-minute interval. No secondary or multiple peaks are observed. After 1h 55m UT the activity dropped below the "sporadic" background level.
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Figure 1: Hourly alpha Monocerotid rates (red squares) on November 22, 1995. The data are corrected for dead-time, sporadics, and observability function. The bars represent one-sigma errors with the errors of sporadic activity taken into account. Also the mean sporadic activity is given as recorded on 24/25 and 25/26 November 1995 (blue squares). |
Results of Radio Meteor Scatter observations
Because of the unfavourable antenna geometry in 1998 for the "traditional" peak, only
the "new" peak was observed. Since my observations started in 1994, only the peak of
long-duration reflections (more than 1 second, which indicates the visually bright Perseid)
shifted to later longitudes. That peak shifted gradually from about 139º.65 in 1995, to 139º.67
in 1996, 139º.69 in 1997 and 139°72 in 1998.
The peak visual observed in 1993 and visual and by radio in 1994 at 139º.50 has probably
a different origin and is probably not directly related to the peak observed since 1995.
However, in 1996 the "nodal" peak around solar longitude 139º.50 and the second peak around
solar longitude 139º.63 in 1996 and 1998 were still present, both of weaker activity than in 1994.
The "traditional" peak is in 1997 observed at solar longitude 140º.09, the same position
as observed in 1996.
It is still interesting to note that the solar longitude of the 1996, 1997 and 1998 peak
of the long-duration reflections coincides with the position found by Lindblad and Porubcan [3]
based on bright Perseids in the period 1937 - 1985.
Perseid prospects for 1999
Because there is no indication for a great shift to later longitudes, maximum of the visual
bright Perseids is expected on August 12 around 21h 15m UT ( 139°.73 ± 0°.02)
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Figure 2: Uncorrected hourly radio meteor rates as recorded on August 11 and 12, 1996 (filled squares). Also, the mean sporadic activity is given as recorded between July 13 and August 4, 1996. The bars represent one-sigma errors. |
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Figure 3: Hourly Perseid radio rates of the "new" and the "traditional" peak on August 11 and 12, 1996, corrected for dead-time, sporadics, and observability function. The bars represent one sigma-errors with the one sigma-errors of sporadic activity taken into account. |
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Figure 4: Hourly Perseid radio rates on August 11 and 12, 1996. The red squares represent reflections of less than 1 second. The blue diamonds represent long-duration reflections (>1 second). This figure shows clearly the "nodal" maximum for short-duration reflections (less than 1 second) around solar longitude 139º.50 and the high maximum for long duration reflections (larger than 1 second) around 139º.69. At the end of the observation period the short-duration reflections (less than 1 second) dropped to zero. Probably this is an artefact because of the unfavourable antenna-geometry at that moment, and probably saturation of the signals caused due to the long duration-reflections. |
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Figure 5: Uncorrected hourly radio meteor rates as recorded on August 12, 1997. Blue dots represent long-duration reflections >1 second. Blue squares represent all recorded reflections. Also, the mean sporadic activity is given as recorded between July 5 and August 3, 1997. Red dots represent long-duration reflections >1 second. Green squares represent all recorded reflections. The bars represent one-sigma errors. |
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Figure 6: Hourly Perseid radio rates of the "new" peak in 1997 (blue dots, long-duration reflections >1 second and only corrected for dead-time and sporadics) and the "traditional" peak in 1997 (blue squares, corrected for dead-time, sporadics and observability function). The bars represent one-sigma errors with the error of sporadic activity taken into account. Also the results of 1994 (green triangles) and 1996 (red dots) are given. |
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Figure 7: Hourly Perseid radio rates of only long-duration reflections of more than 1 second of the "new" peak on August 12, 1998 (red dots), only corrected for dead-time and sporadics. Also the results of 1995 (squares) and the results of the "new" and "traditional" peak for 1996 (green diamonds) and 1997 (blue triangles) are given. |
Results of Radio Meteor Scatter observations
My radio observations of the Leonids started in 1993, and the observations show evidently
that the new season of the Leonids started in 1994 with a maximum at solar longitude 235º.82.
In 1995 the Leonids were already active on November 17 at 22h UT and still active on November
18 at 10h UT. Maximum activity was observed at solar longitude 235º.32. In 1996 a double peak
was observed, the first one at solar longitude 235º.16. This position is the same as observed
for the outburst in 1966. The second peak, higher than the first, was observed at solar
longitude 235º.27.
It is very interesting to note that the position of the second maximum coincides with the
predicted time that the Earth crossed through the orbit plane of comet 55P/Tempel-Tuttle
computed by Yeomans, Yau and Weissman [4].
Probably this was the first appearance of the "nodal" peak of the Leonids in this new Leonid
season. Radio observations at 72.11 MHz of the Leonids in 1997 showed strong activity the
fourth year in a row. Only one peak was monitored around 10h 50m UT on November 17 at solar
longitude 235º.16. This is exactly at the same position as monitored for the first peak in 1996.
Activity around solar longitude 235º.27, observed as a narrow peak of high activity in 1996,
was not observed in 1997 because of the very unfavourable antenna-geometry after 12h UT.
1998
The Leonids 1998 were observed in P.R. of China at a frequency of 96.7 MHz using the
transmitter at Zhengzhou. The observation period on 16/17 November 1998 was between
15h30m and 4h UT. Between 15h30m UT and 16h20m UT, 10 long lasting reflections were
observed. In this period a Radio Hour Rate of 48 ± 15 is calculated. Close after 16h20m UT,
until about 3h20m UT almost completely saturation of the signals occurred. Therefore no
individual reflections were counted during this period. Between 3h20 and 4h UT on November 17, 21
long lasting reflections are monitored. For this period a Radio Hour Rate of 52 ± 9 is
calculated. Examination of these and world-wide radio observations show a flat high
activity for about 20 hours with no (!) distinguish peak. However, there are in all
world-wide observations hints of peak activity on 16 November 1998 between 22 and 24h UT
(around Solar Longitude 234º.41 ± 0º.05). The peak mentioned by IMO around 17.1
November is certainly not visible in any of the radio of radar observations.
The observation period on November 17, between 15h and 22h UT show lower activity than
in the previous period. There is also a flat activity with no distinguish peak observed.
World-wide radio and radar observations show almost the same results.
The 1995, 1996, 1997 and also the 1998 observations suggest a minimum around solar
longitude 235º.20
Notes: In 1994 only long-duration reflections >10 seconds were counted. The 1995 and 1997
observations showed there are about 1.35 times more reflections of 7 seconds compared with
10-second reflections. The 1994 counts are not corrected for this factor.
In 1994, 1995 and 1997 the frequency was 72.11 MHz. In 1996, I was in Spain using the
transmitter at Lousa in Portugal at a frequency of 87.9 MHz. Literature [5] gives an inverse
quadratic relation between duration-time and frequency: a 7-second reflection at 72 MHz is
equivalent with a 5-second reflection at 88 MHz.
Prospects for 1999
For planning observations this year consider that the moment of maximum activity could
be later than the predicted time of passage of the Earth through the orbital plane of
comet 55P/Tempel-Tuttle on November 18, 1999 at 1h 48m UT [4]. In 1833 the maximum
activity was about 0.4 hours later, in 1866 about 1.6 hours later and in 1966 about 1.2
hours later than the predicted time of passage through the comet's orbital plane [6,7,8].
In 1999, probably the time of maximum activity is on 18 November around 3h30m UT
(Solar Longitude 235°.35 ± 0°.1).
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Figure 8: The corrected counts for 60-minute periods of the 1994 Leonids (purple dots), the 1995 Leonids (blue dots), the 1996 Leonids (red dots) and the 1997 Leonids (green dots). All radio observations are corrected for dead-time, sporadics and observability function. |
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Figure 9: The corrected counts for 60-minute periods of the 1998 Leonids observed at 96.7 MHz in P.R. of China. Between about 234°.13 and 234°.58 no individual reflections were counted because the almost completely saturation of the signals caused by the visual very bright Leonids and their persistent trains. The bars represent one-sigma errors. |
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Figure 10: The similarity of the visual uncorrected counts of Leonids (brighter than mvis + 2) and the uncorrected radio counts of Leonids per 20-minute intervals between 4h and 9h UT on November 17, 1996 for reflections >5 seconds. The results are in close agreement with the theory [5]. There are two maxima: the first at 4h 38m UT (solar longitude 235º.16), and the second maximum at 7h 18m UT (235º.27), with a minimum at 5h 50m UT (235º.21). Dots are observations by radio; squares are visual observations by Koen Miskotte en triangles by Marco Langbroek. |
Results of Radio Meteor Scatter observations
Since 1993, around the time the Earth crossed the orbit plane of comet
21P/Giacobini-Zinner around Solar Longitude 195°.40 [6], the number of meteor reflections
was slightly higher than in the day's before or after. In 1993, 1994 and 1995 this
activity never exceeded the "sporadic" background level with more than 2 sigma's.
Probably this (weak) annual activity was caused by particles of the comet.
In 1996 on October 8, after about 7h 30m UT the total activity rose clearly above
"sporadic" background level (about 3 sigma's). This weak activity on October 8 is
probably caused by the Draconids.
1998
Because the results from other observers in 1985 [10] and the expectation by E.D.Reznikov [11],
I started my observations on October 8 1998 at 7h UT.
The long-lasting reflections > 1 second (red dots) and the total number of all reflections
(blue dots in figure 13) show evidently that Draconid activity rose significantly (>3 sigma)
above background level after about 9h UT. The duration of the long-lasting reflections was in
the order of 2-4 seconds and reflections lasting longer than 10 seconds were very rare.
Between about 12h55m and 13h05m UT saturation of the signal occurred. Therefore no individual
reflections were counted during this period. Between 13h05m and 13h25m UT almost every
12 seconds a reflection was counted.
Corrected for "dead-time", "sporadics" (about 1 per minute) and "observability function"
for this period a peak rate is calculated of about 1020 ± 84 for all reflections and
about 315 ± 17 for the long-lasting reflections. Because of saturation of the signals the
results in figure 13 around 13h UT are probably to low.
However, a half-wide maximum is calculated around 13h10m UT (at 195°.075) for all the
Draconids and for the long duration reflections > 1 second (visual brighter Draconids).
After 13h25m UT a sharp drop in the activity is monitored and after 14h40m UT interference
obscured the observations until about 17h UT probably caused by nearby computers.
Between 17h UT until 21h UT, if the Draconids were still active, it was below detection
level. The rising of activity in figure 13 after 20h UT is almost certain artificial and
probably also caused the higher activity before 8h30m UT.
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Figure 11: Raw hourly radio meteor rates as recorded on October 8, 1996 (filled diamonds). Also, the running mean of the sporadic activity is given as recorded on September 22, October 6, October 12, and 13, 1996 (filled triangles). On some days, only a part of the whole observation period of October 8 is monitored. |
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Figure 12: Hourly radio meteor rates (probably caused by Draconids) on October 8, 1996, corrected for dead-time, sporadics and observability function. The bars represent one-sigma errors with the errors of sporadic activity taken into account. |
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Figure 13: Hourly radio Draconid rates as recorded on October 8, 1998, corrected for dead-time, sporadics and observability function. The blue dots represent all Draconids per 10-minute intervals and the red dots Draconids with reflection-time >1 sec per 30-minute intervals. The grey line represents the preliminary Japanese ZHR results. On the x-axis: Time in UT on Oct 8. On the y-axis: Radio Hour Rates of Draconids/Giacobinids. Note: The "dip" around 12h30m UT is probably artificial. |
Prospects for the 2011 Draconids
If in 2011 the orbit of the particles is still the same as in 1998 and 1985, maximum of
the Draconids is expected on October 8 around 16h 43m UT ± 5m at Solar Longitude 194°.894.
Results of Radio Meteor Scatter observations
The author would like to thank Ton Schoenmaker for his helpful comments and Marco Langbroek and Koen Miskotte for their Leonid 1996 observations.
For the Leonids 1998 the author would like to thank all the members of the Sino-Dutch Leonid
Expedition, The Chinese Academy of Sciences especially dr. Li Guanyou, dr. Xu Pingxin,
prof. Hu and dr. Tang (PMO), dr. Zhu Jin (BAO), Peter Jenniskens (NASA/Ames Research
Center) and the people of Lin Ting Kou.
This Sino-Dutch Leonid Expedition was supported by Koninklijke Nederlandse Academie van Wetenschappen,
het Kerhoven-Boscha-Fonds, Prins Bernardfonds, Canon, Kodak, 3-M, U-Freight. For the
logistics we thank VNC (Utrecht) and CITS (China).
Finally, I would like to thank Casper ter Kuile for all his work as WEB-master for these pages.
Results of Radio Meteor Scatter observations
- Hines, C.O., Can. Journ. Phys. 22, pp. 493-503, 1955
- I.P.Williams and Z.Wu, Mon. Not. R. Astron. Soc. 269, pp. 524-528, 1994
- Lindblad, B.A. and V.Porubcan, Planet. Space Sci., Vol. 42, No2, pp. 117-122, 1994
- Yeomans, D.K., K.K. Yau and P.R.Weissman, Icarus 124, pp. 407-413, 1996
- McKinley, D.W.R., Meteor Science and Engineering, New York, Toronto, London, 1961
- Yeomans, D.K., Icarus 47, pp. 492-499, 1981
- Kresák, l., Astron. Astrophys. 279, 646-660,1993
- Jenniskens, P., Astron. Astrophys. 295, 206-235, 1995
- Nakano, S., MPC 25184, May 1995
- IAU Circular 4120 and 4124, (October 1985)
- Reznikov, E.D., E-mail (September 1998)
Results of Radio Meteor Scatter observations
- Global Meteor-Scatter Network by Dr. P. Jenniskens
- Radio Observations of Meteors by the International Meteor Organization (IMO)
- The RAMSES project by URANIA, the Public Observatory of Antwerp
- Radio Meteor Data, Japan by Kazuhiro Suzuki
- Astronomical Observatory of the University of Ghent
- Genesis Software WWW Server
- Jordanian Astronomical Society (JAS)
