PASJ: Publ. Astron. Soc. Japan 55, 547–551, 2003 June 25
c 2003. Astronomical Society of Japan.
Meteor Storm Activity and Average Size of Meteoroids for the 2001 Leonids
by FM Broadcasting Radio Observation
Hideto YOSHIDA
Department of Earth & Planetary Science, Graduate School of Science, The University of Tokyo, Bunkyo-ku, Tokyo 113-0033
[email protected]
(Received 2002 October 18; accepted 2003 April 15)
Abstract
Radio-wave observations using FM broadcasting frequencies have been performed on the 2001 Leonids. The
observations were designed so that only a long echo could be received. The following results were obtained. First,
the peak activity of the 2001 Leonids was approximately 18h10m (UT) on November 18 and the solar longitude of
this peak corresponded to 236.◦ 45(J2000.0). Second, it is inferred that the peak activity of the 2001 Leonids (1699
and 1866 dust trails) contained meteoroids of a larger average size than those of the 1999 Leonids (1899 dust trail).
Key words: comets: individual (55P/Tempel–Tuttle) — interplanetary medium — meteors — solar system:
general
1. Introduction
The Leonids are famous as a meteor storm observed roughly
every 33 years. Ham-band Radio Observation (HRO) using the
50 MHz band is a forward-scatter observation method, which
is popular for radio-wave observation of meteors in Japan
(Maegawa 1999), because observations are performed using a
stable continuous wave beacon. However, when a meteor storm
occurs, accurate radio observations are difficult because meteor
echoes received due to the large number of echoes overlap.
Actually, HRO with the 50 MHz band became impossible to
count the number of echoes in the 2001 Leonids (Ogawa et al.
2002).
The duration of each meteor echo is proportional to the
second power of the wavelength of the radio wave used for
the observation (Ceplecha et al. 1998). If a radio observation
is performed using an FM broadcasting frequency [FM Radio
Observation (FRO) : Suzuki et al. 1976] with a wavelength
shorter than HRO (50 MHz), the duration of each meteor echo
is shorter, and each meteor can be separately detected. The
long-lasting overdense echo ( > a few seconds) reflects a radio
wave in all directions. This phenomenon is considered to be
due to the “glob” and “glint” (McKinley 1961). Therefore, it
is not necessary to take into account the phenomenon that the
number of meteor echoes rapidly decreases at the time when
the radiant point is near the zenith. Hence, if the long echo is
used in an observation, it is possible to accurately determine
the activity of a meteor storm.
In this paper, the details are reported of the peak activity of
the 2001 Leonids by an FM broadcasting radio observation.
Moreover, the average size of the meteoroids in the 2001
Leonids is compared with that of the 1999 Leonids.
does not have factories, which generate noise, within a radius
of less than 5 km, does not have any high-voltage power lines,
and has little traffic. Because it is located near the center of
a basin, the field of view is open. Therefore, this location is
suitable for radio-wave observations.
The target broadcasting station was AIR-G (88.8 MHz,
transmitting output is 250 W) in Hakodate-city, which is
approximately 600 km north from the observation location
(figure 1). This station broadcasts for 24 hours almost every
day. When this broadcasting station stopped transmitting
2. Observational Techniques
The FM broadcasting radio observation was performed at
Hanawa-town (36◦ 57. 97N, 140◦ 26. 17E, H = 208m) located in
the southern part of Fukushima Prefecture (figure 1). This site
Fig. 1. Map showing the observation location and the target FM
broadcasting stations.
548
H. Yoshida
[Vol. 55,
Fig. 2. Meteor echo detection circuit and the obtained meteor echoes record. (a) Meteor echo detection circuit (see text for details). VF is the forward
direction voltage fall of a germanium-type diode. (b) Meteor echo recorded approximately 18h10m (UT) on 2001 November 18. It notes that each meteor
echo was separated and recorded.
for any reason, NHK-FM (88.0 MHz, 250 W), which also is
in Hakodate-city, was used as a target broadcasting station
(figure 1). Generally, the larger is the transmitting output of
a broadcasting station, the stronger is the intensity of a meteor
echo that we receive. However, in the case of the usual FRO
when a meteor storm occurs, observation becomes impossible,
because of the large number of meteor echoes. Both weaker
broadcasting stations were chosen in order to accomodate the
extremely large numbers of meteor echoes. It was also determined that there was no interference by other broadcasting
waves.
The observation system was as follows. A Yagi antenna
with 5 elements was oriented towards the zenith, and extended
to a height of 3 m. It was connected to a multi-band wide
receiver (YUPITERU MVT-7200) using a coaxial cable of
5C2V. The audio frequency output of the receiver was input
into a circuit shown in figure 2a. The output of the circuit was
connected to a pen recorder by a 2-axis shield line. The meteor
echoes as shown in figure 2b were automatically observed and
recorded. The circuit shown in figure 2a was used for detecting
whether there is any audio output. A squelch circuit was always
open. Even when the receiver detected no modulation wave,
an output appeared on the pen recorder. There was a very short
audio signal generated from the receiver itself at the moment
of a change from the state of white noise to the soundless state.
This “sound” was due to the response to a rapid change of the
received signal intensity. Therefore, a signal also appeared
on the pen recorder. The influence on the observation was
apparently minimal, because it is rare that FM broadcasting
is soundless.
The audio signal intensity and the intensity of a meteor
echo were correlated as follows. Since the amplitude recorded
on a pen recorder depends on the audio signal intensity, the
signal intensity of a meteor echo cannot be expressed directly.
However, the duration of a meteor echo can be confidently
expressed. In this circuit, the time resolution was degraded
and was set as 0.27 s. This was for choosing only long echoes.
A forward direction voltage fall of a germanium-type diode
was used in order to remove any weak echo and/or noise. As
described above, it was possible to discriminate the signal of
a long echo by using this circuit. Consequently, as shown in
figure 2b, each meteor echo could be successfully separated
and recorded.
3.
Observation Results
3.1. Peak Activity of a Meteor Storm
The FM broadcasting radio observation in this study
observed the activities of the 2001 Leonids from 12h00m (UT)
on 2001 November 16 until 4h00m (UT) on 2001 November
20. NHK-FM (88.0 MHz) was used for an observation from
15h00m (UT) through 23h00m (UT) on November 18, because
AIR-G (88.8 MHz) was undergoing maintenance.
Figure 3 shows the observation results. Before the Leonid
radiant point reached the highest altitude, an increase in the
number of meteor echoes was observed at approximately
No. 3]
2001 Leonids by FM Radio Observation
Fig. 3. Time variation of the number of meteor echoes by FM broadcasting wave observation of the 2001 Leonids. The solid squares are
the altitude of the Leonid radiant point at the observation location. The
solid line with open circles is the raw number of echoes in an hourly
rate. The question mark represents a time period whose reliability is
not sufficient due to the bad condition of the pen recorder. See the text
for the employed frequency.
18h00m (UT) on November 18. At the time, when the altitude
of the Leonid radiant point was the highest, there were a few
echoes. An increase in the number of echoes was observed
again, even when the altitude of the Leonid radiant point began
to decline. The number of meteor echoes increased from
0h00m (UT) to 3h00m (UT) every day throughout the observation period. Since the time period was restricted, this may
have been artificial interference, although the meteor echo of
the Leonids may had also been included.
From 17h00m (UT) through 20h00m (UT) on November 18,
the time variation of the number of meteor echoes is shown
in figure 4 at time intervals of 10 min. The number is plotted
with the corrected number of echoes against the altitude of the
Leonid radiant point. The altitude of the Leonid radiant point
of this time period kept rising at the observation location with
time. The corrected number of meteor echoes, however, began
to decrease at approximately 18h20m (UT). This indicates that
the activity of the meteor storm began to abate and the peak
activity of the meteor storm was detected. The estimated time
of the peak activity was at 18h10m (UT) on 2001 November 18
and the solar longitude of this peak corresponded to 236.◦ 45
(J2000.0). This is in good agreement with the HRO radio
observation reported by Ogawa et al. (2002). They, however,
estimated the peak activity by analyzing the FFT (Fast Fourier
Transform) image files obtained from HRO. McNaught and
Asher (1999) and Lyytinen and Van Flandern (2000) predicted
two possible activities on November 18 over the Far East area.
In this observation, however, it is difficult to distinguish these
two peak activities.
3.2. A Minor Peak
The time variation of the number of meteor echoes from
17h00m (UT) on November 18 through 4h00m (UT) on
549
Fig. 4. Time variation of the number of meteor echoes from 17h00m
(UT) through 20h00m (UT) on 2001 November 18 at intervals of
10 min. The solid line with the solid circles is the corrected number
of echoes in consideration of the altitude of the Leonid radiant
point. The corrected number “Nc ” was derived by using Nc =
Nobs sin (h + 7◦ cos h)−1 , where h is the altitude of the Leonid radiant
point (degree), and Nobs is raw number of meteor
echo. The error bars
√
Nc−error were derived by using Nc−error = Nobs sin (h + 7◦ cos h)−1
The dotted line is the altitude of the Leonid radiant point at the
observation location. The heavy line is the regression curve using a
polynomial fit. The heavy dotted line is the result of observations made
when no meteor showers were active (2002 February 11). Observations
were performed under the conditions explained in the text. In the observational setup described in this paper, a meteor echo was hardly recognizable.
Fig. 5. Time variation of the number of meteor echoes from 17h00m
(UT) on 2001 November 18 through 4h00m (UT) on November 19 at
time intervals of 30 min. The dash-dotted line with open diamonds is
the raw number of meteor echoes. The other lines are the same as those
of figure 4.
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H. Yoshida
[Vol. 55,
Fig. 7. Histogram showing the proportion of total meteor echoes for
each duration interval in the 1999 and 2001 Leonids. “n” is the number
of meteor echoes before and after 2 hours of the peak activity.
before the altitude of the Leonid radiant point reached the
highest at the observation location. Although the altitude of
the Leonid radiant point became low after 21h30m (UT), the
number of meteor echoes tended to increase. The peak activity
existed at approximately 22h00m (UT) and continued until
23h00m (UT). This observation indicates that the activity of
the Leonids increased again. This suggests that a minor peak
distinct from the major peak activity at the above-mentioned
18h10m (UT) was present. A similar minor peak has also been
reported by the HRO observation (Ogawa et al. 2002).
4.
Fig. 6. FM broadcasting wave observation results in the 1999
Leonids. (a) Observation from 8h00m (UT) on 1999 November 17
through 20h00m (UT) on 1999 November 19. The peak activity was
observed approximately 2h00m (UT) on November 18. Moreover,
small activity was also recognized at approximately 22h00m (UT).
(b) Time variation of the number of meteor echoes from 1h00m (UT)
through 3h00m (UT) on 1999 November 18 at intervals of 5 min. The
lines are the same as those of figure 4. The filled bar is the peak activity
time reported from IMO (Arlt et al. 1999).
November 19 is shown in figure 5 at time intervals of 30 min.
In this figure, the corrected number of meteor echoes is plotted
against the altitude of the Leonid radiant point as well as raw
data. When the altitude of the Leonid radiant point was low, the
effect of the Leonid radiant point correction was remarkable.
Since this part had a few raw meteor echoes, its error was
large. In addition, after 0h00m (UT) every day, as described
above, there seemed to be included some artificial interference
in the background observation period. Thus, full argument
was impossible after 0h00m (UT). However, this observation
obtained the following observation results. The number of
meteor echoes decreased from approximately 18h00m (UT)
Discussion
The 1999 Leonids was also observed under the same
observation conditions as those in this study, and succeeded
to observe the peak activity of that Leonid meteor storm
(figure 6). Figure 7 compares the duration of a meteor echo
in the peak activity time period in 1999 with that of 2001. In
this figure, the horizontal axis is the duration of the meteor
echo, and the vertical axis is the proportion of the total meteor
echoes for each duration interval. The proportion of total
meteor echoes was taken before and after 2 hours of the peak
activity in both years. The activity of 1999 is distinguished by
a predominance of a duration interval of less than 10 s (95%).
The longest duration interval is about 50 s. On the other hand,
a duration interval less than 10 s of the activity in 2001 is
only about 65%. There are also variations in the proportion of
total meteor echoes for each duration interval, and the longest
duration interval is about 80 s. Generally, the larger is the
size of the meteoroid, the longer is the duration interval of the
meteor echo. Hence, if the radio wave propagation conditions
are similar for both years, the peak activity of the 2001 Leonids
(1699 and 1866 dust trails) would contain larger meteoroids
than that of the 1999 Leonids (1899 dust trail).
The magnitude distribution index obtained from visual
observations in Europe in 1999 is 2.3 (Arlt et al. 1999). On
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2001 Leonids by FM Radio Observation
the other hand, the magnitude distribution index obtained from
the video observation in Japan in 2001 is 1.5 ± 0.3 (Watanabe
et al. 2002). These results mean that the meteor shower in 2001
contained larger meteoroids on average than that in 1999. The
results in the current study support the results reported by these
papers.
551
not predicted, may have been observed from approximately 21h30m (UT) through approximately 23h00m (UT) on
November 18.
Third, it is inferred that the peak activity of the 2001 Leonids
(1699 and 1866 dust trails) contained meteoroids of a large
average size compared to those of the 1999 Leonids (1899 dust
trail).
5. Conclusion
Radio-wave observations using FM broadcasting frequencies were performed on the 2001 Leonids. In this study, when a
meteor storm occurred, the number of echoes could be counted.
The following results were obtained.
First, the peak activity of the 2001 Leonids was approximately 18h10m (UT) on November 18 and the solar longitude
of this peak corresponded to 236.◦ 45 (J2000.0).
Second, a dust trail, which researchers (e.g. McNaught,
Asher 1999; Lyytinen, Van Flandern 2000) previously had
The author would like to thank Dr. Jun-ichi Watanabe for his
helpful advice and discussion throughout this study. He would
also like to thank Dr. Takuji Nakamura for his advice on radio
observation. The author is grateful to Dr. Takashi Mikouchi for
a critical review of the manuscript. He is also grateful to an
anonymous reviewer for helpful and constructive comments,
which improved the manuscript. This study was supported
in part by the Grant (11914011) from the former Ministry of
Education, Science and Culture.
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