Indian Journal of Radio & Space Physics
Vol 40 April 2011, pp 67-71
MST radar observations of the Leonid meteor storm during 1996-2007
N Rakesh Chandra1,$,*, G Yellaiah2 & S Vijaya Bhaskara Rao3
1
Nishitha College of Engineering and Technology, Lemoor (V), R R Dist 501 539 (AP)
2
Department of Astronomy, Osmania University, Hyderabad 500 007
3
Department of Physics, Sri Venkateswara University, Tirupati 517 502
$
E-mail: [email protected]
Received 27 October 2008; revised 26 May 2009; re-revised received and accepted 18 March 2011
The Indian MST radar is a powerful atmospheric remote sensing instrument for ionospheric studies operating at
53 MHz frequency. Systematic observations of Leonids were carried out during 16–20 November every year from 1996 to
2007 with radar operating in meteor mode. It has been observed that the presence of strong background component which
resulted from several apparitions of parent comet is the cause for the shower activity every year. The trail component that
was left behind during each apparition is strong enough to cause the meteor shower for two consecutive years. In this regard,
the detailed study of 2007 Leonids is presented in the paper, even though the shower activity was observed to be quite low,
and interestingly it resulted in a double (multiple) broad peak rather than a single peak. The results show that the Leonid
stream composed of several narrow dense trails existed simultaneously and the sharp meteor outbursts were observed when
the Earth encountered a region of high spatial density of particles within the stream.
Keywords: Meteor shower, Leonid meteor stream
PACS No.: 96.30.Za
1 Introduction
A spectacular celestial display of Leonid meteor
stream (LMS) associated with the comet 55P/TempelTuttle was observed globally during the early hours of
18 November 1998. This splendid fiery celestial event
was witnessed by professional scientists, amateurs
and general public. The visual, photographic, radio
and radar observations for the event were carried out
extensively at different geographic locations
throughout the world. Soon after the discovery of the
comet 55P/Tempel-Tuttle in 1865, with similar orbit
and orbital period (33.25 years) of Leonids, it was
established that this comet is most likely the parent of
the Leonid meteors1. The LMS is caused by a swarm
of meteoroids in the vicinity of the comet. Most of the
meteor shower storms appeared when the node of the
orbit of the comet is inside the Earth’s orbit during its
perihelion passage. The perihelion passage of the
comet occurred on 28 February 1998 and spectacular
event of Leonid meteor shower activity occurred
when the Earth reached the closest point. In the past
200 years, four such spectacular events of Leonid
shower were observed, i.e. in 1799, 1833, 1866 and
1966 (ref. 2) with a zenithal hourly rate (zhr) in excess
of 6000.
Since, the Leonid meteoroid stream has not been
subjected to major planetary perturbations as it was
prior to 1898, a storm similar to that of 1966 was
expected to be seen in the years towards the end of
2000 (ref. 3). In view of these observations and
predictions, a systematic observational program of
LMS was undertaken by a group of observers
involving visual, photographic and MST radar
observations at National Atmospheric Research
Laboratory (NARL), Gadanki (13.5°N, 79.2°E) in
India, during the events in the years 1996 to 2007 as
presented in Fig. 1. The intent is to speculate the
structure of the Leonid meteor stream with emphasis
on variation in shower activity observed with Indian
MST radar. In the year 2007, remarkably a double
(multiple) peak was observed, rather than expected
single narrow peak. In view of this, a detailed study
for the year 2007 has been presented even though the
shower activity was low.
2 Observations
The Indian MST radar located at Gadanki (13.5°N,
79.2°E) is a powerful remote sensing tool for studying
the atmospheric dynamics and making detailed
observations of meteor echoes because of its high
pulse repetition frequency (PRF) and high power
narrow-near vertical beams. The MST radar system
details and technical specifications are given by Rao
et al.4. The Doppler spectra of the amplitude
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INDIAN J RADIO & SPACE PHYS, APRIL 2011
variations of meteor trails and sporadic-E layers have
simultaneously
been
recorded
continuously
throughout the nights of Leonid activity using the
beams E20, W20, N20, S20 and Zx followed by N13 for
sporadic-E (subscript here indicates the orientation
angle of the radar beam with respect to zenith). The
observations were taken from 20:00 to 06:00 hrs LT
each night continuously during 16 - 20 November of
every year from 1996 to 2007. The frames containing
the meteor echoes were separated on each night and
the hourly rates of occurrence of meteors were
estimated from the counts of the offline display of the
Doppler spectra of the meteors with signal-to-noise
ratio (SNR) > 10 dB. The shower activity on peak day
of every year from 1996 – 2007 have been presented
with detailed analysis for the year 2007.
3 Results and Discussion
The Leonid meteor shower associated with comet
55P/Tempel - Tuttle had its rich returns during 16-20
November 1998 after its 34th perihelion passage on
28 February 1998, 33 years after its last return in
1966. The activity of Leonids during 1996-1997 was
quite low and centered around 200 only. In the year
1998, it has been found that the Leonid shower is
characterized by a strong background component with
a maximum activity of 1450 zenith hourly rate (zhr)
(Radar) around λ 0 = 234.50 (November 16/17). In the
year 1999 and 2000, the activity was recorded as 450
zhr and as low as 220 zhr, respectively. Again, during
the years 2001 and 2002, the activity was strong with
zhr of ~ 1250 and ~450, respectively. Later on, after
the year 2002, no such remarkable activity was
noticed during the years 2003 – 2007.
During the years 1996 and 1997 (corresponding
trails presented in Table 1), the parent comet
55P/Tempel – Tuttle was coming closer to the sun in
its orbit after a long period of time, the Earth passed
through the less dense sections of the trail and hence,
no such storms or sudden burst of meteors was likely
or observed. But, during the month of February in the
year 1998, as the parent comet 55P/Tempel – Tuttle
had its perihelion passage, it resulted in fresh release
of dust due to the ablation of comet by Sun. The Earth
has encountered this young stream which resulted in
fireballs. The storm component, thus, observed belong
to 1898 trail. This background component of the
stream was resulted from several revolutions of the
comet 55P/Tempel-Tuttle around the Sun. The
gravitational perturbations due to planets and solar
radiation pressure have affected the motion of smaller
particles more than that of large size particles,
resulting the 1998 Leonid fireballs observed at
Gadanki which gave rise to a lower mass
index, s = 1.6. This was resulted due to two or three
latest revolutions of the comet 55P/Tempel - Tuttle
with recent apparition on 28 February 1998. In 1999
and 2000, observers at European longitudes observed
very poor activities of the shower, the storm
component was observed to belong to 1898 and 1866
returns of the comet in 1999 and 1866, 1932 returns
for the year 2000 (Table 1). In 2001, the activities of
the shower once again became more prominent
similar to that observed during 1998 apparition, where
the activity resulted from the trails of comet’s
apparition in the years 1767, 1866, 1965. For the year
2002, shower resulted from the trails left behind by
the comet during the apparitions of 1767, 1866, 1965.
Comparatively low activity, observed in the year 2003
and 2004, caused due to the trails of the comet’s
apparition in the years 1499 and 1733 returns. The
activity of the shower in 2005 resulted from 1167
Table 1 — Apparitions of parent comet (55P/Tempel – Tuttle)
causing the meteor shower in the corresponding year (ref. 7)
Fig. 1 — Variation of Leonid activity during the years 1996 –
2007
Year of observation
Trail caused meteor shower
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
1898
1898, 1866
1866, 1932, 1733
1767, 1866, 1965
1767, 1866, 1965
1499, 1733
1733
1167
1932
1932
CHANDRA et al.: MST RADAR OBSERVATIONS OF LEONID METEOR STORM DURING 1996-2007
trail, whereas the activity in the years 2006 and 2007
resulted due to the trail left by 1932 apparition of the
comet. After 2001, up to 2007 the Leonid activity was
found to be constant at a low level around 200 ± 30,
as the parent comet was receding back to the solar
system and the Earth may not have encountered the
dense trails and might have passed through the less
dense sections of the comet’s debris during its
revolution, hence, the observed activity might be low.
In the year 2007, it can be depicted from Fig. 2 that
the maximum activity of the shower occurred on
17/18 November 2007 ( λ 0 = 235.361) with meteor
flux rate of 260. On the preceding day, i.e. on 16/17
November ( λ 0 = 235.353), the flux rate has been
Fig. 2 — Variation of Leonid activity with respect to solar
longitude in the year 2007
69
observed to be about 175; and on succeeding days of
maximum activity, the flux rate has been 228 and 234,
respectively on 18/19 November ( λ 0 = 236.370) and
19/20 November ( λ 0 = 237.379). It has been
observed that on the day before the peak activity (at
λ 0 = 235.353), the flux rate has been only 67% of
peak activity, and on the days after the peak activity
(i.e. at λ 0 = 236.37 and λ 0 = 237.379), the flux rate
has been around 90% of the maximum activity, thus,
resulting a broad peak. From the past observations, it
can be inferred that the meteor flux rates have fallen
to 15% of the observed during 1998 Leonid activity
corresponding to 34th perihelion passage of the comet
55P/Tempel – Tuttle, when compared with the 1999
Leonid peak activity, where the flux rate has been
only 65%. It has been noticed that during the year
2007, the Earth has passed through the outer regions of
dust trail caused by the 1932 storm corresponding to 2
or 3 revolutions of the comet 55 P/Tempel – Tuttle.
The semi-diurnal variation of Leonid activity in the
year 2007 has been presented in Fig. 3, and height
distribution in Fig. 4, whereas the distribution of
estimated radar echo durations has been presented in
Fig. 5. It is seen in Figs 4 and 5 that the maximum
numbers of meteors were confined to the altitude range
of 100 – 110 km with echo durations 0.2 – 0.5 seconds
and estimated mass index (s) 2.04 ±0.003 (ref. 5). This
reveals that the Leonid stream consists of larger size
Fig. 3 — Semi-diurnal variation of Leonid activity in 2007
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INDIAN J RADIO & SPACE PHYS, APRIL 2011
Fig. 4 — Height distribution of meteors during Leonid in 2007
Fig. 5 — Distribution of echo duration during Leonids in 2007
(brighter) meteoroids at Solar longitude λo = 235.361
(i.e. on 17/18 November) giving an estimated mass
index of 1.89 ±0.003 at altitudes ~100 km and smaller
size (fainter) meteoroids on either side (i.e. on 16/17
and 18/19 November).
It is evident from the altitude distribution of Leonid
activity presented in Fig. 4 that most of the observed
meteors are confined to the height regime of 105 ± 5 km.
Since these meteors, have large geocentric velocities
(>50 km s-1), diffuse faster producing maximum
CHANDRA et al.: MST RADAR OBSERVATIONS OF LEONID METEOR STORM DURING 1996-2007
ionization at these heights, thus, having very short echo
durations (Fig. 5) indicating that these trails are weak
and under dense. Figure 4(a) on 16/17 November
indicates that the stream consists of smaller size
(diffusing at higher altitudes) to medium size particles.
Figure 4(b) on peak activity day, i.e. on 17/18
November indicates that the Earth is at middle of the
stream comprising of medium size to large size
particles. From Figs 4(c-d), it is observed that during
the nights of 18 and 19 November, the number of
meteors diffusing at higher altitude is greatly reduced
indicating the fall in contribution of smaller size
particles for the shower as the Earth moves through the
stream. This can be interpreted as a consequence of
Poynting – Robertson effect. In fact, the stream of faint
meteors wraps up the stream of bright meteors.
After the year 1998, a strong increase in shower
activity was observed once again in the year 2001
because of the Earth’s encounter with three different
streams (debris left) corresponding to three apparitions
of the parent comet in the years 1767, 1866 and 1965,
respectively6. Thereafter, the flux rate was confined
around 200 (zhr) with a very minimum variation until
the year 2007.
4 Conclusions
From the above, it can be inferred that the Leonid
stream is composed of several narrow dense trails
(resulted from past apparitions of the parent comet)
existing simultaneously. The sharp meteor outbursts
were observed when the Earth encounters a region
within a stream of a high spatial density of particles.
The meteor trail exists because of dispersion in the
orbital period among the meteoroids released over a
particular perihelion return of the parent comet.
Meteoroids with shorter or longer orbital periods
gradually get ahead or behind, respectively in the trail
as the time passes. The period of meteoroids in turn
depends on its ejection velocity of the comet, the
point on the comet’s orbit where it was released and
also on the amount of solar radiation pressure it
experienced. All these observations reveal that the
Leonid stream is a dynamic system, the structure of
which is mainly determined by the ejection
71
mechanism of the comet during each perihelion
passage apart from the considerable extent of
perturbations by the outer planets, especially during
the periods of close approaches to the stream. Further,
it is concluded that the Leonid meteor stream
comprised of braided structure of the dense dust trails
within the meteoroid stream.
Although the MST radar suffice the need to study
the meteor showers, it is impossible to locate the
radiant position using this radar for which
interferometer is required. This radar works on backscatter Doppler principle, where wind measurements
and E–region irregularities can be studied accurately
but not the radiant of the shower. MST radar scans the
atmosphere in all the directions (E, W, Z, N) but not
simultaneously, hence, during data collection with
beam pointing in one direction will result in loss of
data in remaining directions.
Acknowledgements
The authors gratefully acknowledge the Director
and the staff of NARL for their support in conducting
the observations with MST Radar. They also thank
the coordinator, UGC SVU Centre, SV University for
providing financial assistance to carry out the
experiment.
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