Possible Detection of GEMINID 2007 Meteor Shower During Day-Time from VLF Radiation Spectra
←
→
Page content transcription
If your browser does not render page correctly, please read the page content below
Earth Moon Planet
DOI 10.1007/s11038-009-9304-0
Possible Detection of GEMINID 2007 Meteor Shower
During Day-Time from VLF Radiation Spectra
Anirban Guha Æ Barin Kumar De Æ Rakesh Roy
Received: 17 March 2008 / Accepted: 7 April 2009
Ó Springer Science+Business Media B.V. 2009
Abstract The results of day-time detection of GEMINID 2007 meteor shower from
dynamic VLF radiation spectra in Tripura (23.50° N, 91.25° E), India, is presented here.
The field experiments were performed during 12–17th December, 2007 inside Tripura
University campus located at a hilly place in the North-Eastern part of India. A well
calibrated software VLF receiver was used to perform the field experiments. Analyses of
data reveal an hourly average rate of the shower around 50. The VLF emissions lie in the
range from 8 kHz to 13 kHz which is 10 to 15 times higher than previous reports. The
mean duration of each VLF emission calculated from dynamic spectra is found to be 6 s
and the mean bandwidth is 3.6 kHz. The temporal variation of VLF emission duration and
bandwidth of VLF radiation is also studied. The results strongly support the fact that VLF
electromagnetic waves are produced during the passage of meteors in the atmosphere. The
experiment also makes the study of dynamic VLF spectra as a strong tool to detect low
intensity meteor shower during daytime.
Keywords Meteors VLF radio waves Electrophonic sounds
1 Introduction
Anomalous sounds known as electrophonics accompanying the passage of meteors have
been well documented over the past two centuries (Keay 1980; Vinkovic et al. 2002). In
the last two decades, a large number of experimental verification of electrophonics had
been done by researchers all over the world. It was verified that meteors entering the
earth’s atmosphere produce electromagnetic waves in the ELF-VLF range (300 Hz to
30 kHz) which propagate and reach the ground at the same instance as the optical signals
(Keay 1995; Beech et al. 1995; Zgrablic et al. 2002; Chakrabarti et al. 2002). The effect of
these waves on subionospherically propagating VLF-LF navigation signals is also studied
by several workers (De and Sarkar 1985; De et al. 2006). These electromagnetic waves
A. Guha (&) B. K. De R. Roy
Department of Physics, Tripura University, Suryamaninagar, Tripura (West) 799 130, India
e-mail: anirban1001@yahoo.com
123
A. Guha et al.
generate electrophonic phenomena at electrically conducting objects near the observer.
Several explanations for their origin have been proposed. Most of the theories suggest that
these are produced by transduction of radio transmissions in the ELF-VLF range generated
in the plasma trails left by the meteors during their passage through earth’s atmosphere
(Bronshten 1983; Keay 1992, 1993).
The production of electrophonics depends solely on the conversion factor of the
transducers present near the observer and sometimes no electrophonics are generated. So,
to detect a meteor shower, electrophonic sounds are always not useful. During daytime and
bad weather conditions at night, it is impossible to get visual confirmation of meteor
shower. ELF-VLF detection of meteor shower eliminates these limitations. But there is an
important issue to note. The natural electromagnetic emission referred as atmospherics or
sferics from thunderstorms all over the world peaks also in ELF-VLF range (Barr et al.
2000). These waves are guided within the spherical waveguide formed between the earth
and the lower ionosphere with little attenuation. The intensity of the subionospherically
propagating ELF-VLF sferics depends sensitively on the electrical activity inside different
clouds, electrical conductivity of the lower ionosphere as well as that of the ground
(Thomson and Clilverd 2000).
In general, the intensity of radiation from meteors in a given spectrum is lower than
sferics generated by thunderclouds (Price and Blum 2000). So, in order to detect the ELF-
VLF signatures originating from meteor shower, it is important to differentiate it from the
spectral character of sferics received at a particular location on earth.
At the Tripura University campus we are actively engaged in the study of sferics under
different atmospheric conditions such as fair weather, local thunderstorm, rain, fog, solar
flare, geomagnetic storm meteor shower etc. (De and Sarkar 1985, 1991, 1995, 1996,
1997). Under normal fair weather conditions, it is observed that the peak intensity of ELF-
VLF sferics originating from thundercloud activity lies in between 3 kHz and 10 kHz
(Volland 1982; De et al. 2005). The identification of signatures of different atmospheric
phenomena in VLF range enables us to precisely detect the presence meteors in the
atmosphere.
2 Experimental Setup
Our experimental setup consists of an inverted vertical L type omni-directional antenna,
a preamplifier with surge protection and SpectrumLab V2.7b14 software VLF receiver.
The effective height of the antenna is 7.85 m and the terminal capacitance is 35.42 pF.
The voltage induced at the antenna is amplified ten times and passed through a VLF
band pass filter having a bandwidth of 30 kHz at the preamplifier. It is then fed to a 24
bit sound card of a Pentium-IV 2.66 GHz computer. The clock of the computer is
synchronized with standard internet time servers. At the input of the sound card, proper
protection is taken to ensure the elimination of surge voltages coming from the CG
lightning discharges. The software VLF receiver collects data at a sampling of 48 kHz.
The maximum frequency that can be recorded is 24 kHz owing to original sampling rate
of 48 kHz, maintaining the Nyquist criteria. FFT of the preamplified signal is done
online at 65,536 points per second using ‘‘Hann FFT window function’’ to get the
dynamic Fourier spectrum. The dynamic spectrum is updated every one second and it
takes three minutes to cover the total screen. A snapshot of the dynamic spectrum is
taken every three minutes in a programmed mode.
123
Possible Detection of GEMINID 2007 Meteor Shower
3 Observational Results
The Geminids are a meteor shower caused by an object named 3200 Phaethon, which is
thought to be an extinct comet. The meteors from this shower can be seen in mid-
December and usually peak around 12–14th of the month. The meteors in this shower
appear to come from a radiant in the constellation called Gemini. The Geminids are now
considered by many to be the most consistent and active annual shower. In 2005, viewing
of the shower was restricted due to a full moon. The 2006 shower had a less full moon,
however the 2007 shower was in a new moon, with the best viewing position being in the
southern hemisphere, with Australia and New Zealand.
The peak activity of Geminid 2007 meteor shower was predicted to occur on 14th
December, 2007 16:45 UT i.e. 22:15 Indian Standard Time (IST). Accordingly, we col-
lected VLF spectrum data from 11th December to 17th December, 2007. The observers all
over the world collected visual data and during predicted peak activity, an hourly rate of
more than 120 was reported by International Meteor Organization.
The local weather condition during the period from 11 to 17th December, 2007 was
hazy and deep fog during night. So, in spite of having no local thundercloud activity, there
was no way to get the visual confirmation of meteor shower. A sample fair weather
dynamic VLF spectrum is shown in Fig. 1. This is the standard spectrum we get in our
location in the absence of any solar flare, geomagnetic storm etc. It can be observed from
the colour code of the spectrum that the sferics intensity is few dB higher in between 5 kHz
and 12 kHz compared to other frequencies.
Interestingly, the analyses of data show no distinctive change is dynamic VLF spectrum
during the predicted peak activity of the meteor shower. But from the morning hours of
15th December, 2007 and up to 11 A.M. the spectra show signatures of VLF emissions of
Time
Frequency (Hz)
Fig. 1 Dynamic VLF Spectrum during Fair-Weather
123
A. Guha et al.
few seconds in the band from 8 kHz to 13 kHz. Such two dynamic VLF spectra are shown
in Figs. 2 and 3. The main features of the VLF emissions are that they are random in
nature, band-limited, and the intensity of emission is few dB more than the overall sferics
activity. The nature of emissions also do not match with other natural VLF phenomena
occurring in the atmosphere like whistlers, tweaks etc. In the morning of December 15th,
2007, there were no local thunderstorm activities, no solar flares, solar particle bursts and
geomagnetic storms and the average Kp value during that period was below 4. The space
weather data were collected from National Geophysical Data Centre, Colorado USA. We
believe that these kinds of VLF emission spectrum signatures are associated with meteor
activities. During fair weather, almost every night, a few emissions having this kind of
spectral signature are found owing to the passage of meteor. In the present case, a total of
252 VLF emission events are recorded within the specified time interval, the hourly rate
being around 50. An hourly rate of 80 is found from the visual confirmation from the
observers all around the world during this period. The possible explanations of non
detectability of meteor shower during predicted peak activity and lower hourly rate is given
under the section ‘‘Discussions’’.
We analyzed our data in terms of the duration of VLF emission and bandwidth of the
spectrum. Figure 4 shows the temporal variation of VLF emission duration. The raw data
was first adjacent averaged for 25 points and then fitted with multiple peak Lorentzian
function. Three distinct peaks around 06:52 IST, 07:55 IST and 09:45 IST are found
having emission duration of almost 10 s. The statistical distribution of emission duration
along with Gaussian fit is depicted in Fig. 5. The average emission duration is found to be
6 s.
Time
Frequency (Hz)
Fig. 2 Dynamic VLF Spectrum 1 during Geminids on 15th December, 2007
123
Possible Detection of GEMINID 2007 Meteor Shower
Time
Frequency (Hz)
Fig. 3 Dynamic VLF Spectrum 2 during Geminids on 15th December, 2007
Fig. 4 Temporal Variation of VLF Emission Duration on 15th December, 2007
For the total observational period, VLF emission lied in the range between 8 kHz and
13 kHz. Following the same analyses method as for duration of VLF emission, the tem-
poral variation of bandwidth of VLF emission is shown in Fig. 6. The initial bandwidth is
found to be around 5.5 kHz followed by two peaks having bandwidth of 4.25 kHz and
3.25 kHz at 06:52 IST and 10:15 IST respectively. It is interesting to note that as time
passes, the bandwidth of the VLF emission decreases except two peaks as stated above.
The statistical distribution of bandwidth of VLF emission along with Gaussian fit is shown
in Fig. 7. The average bandwidth of VLF emission is found to be 3.6 kHz.
123
A. Guha et al. Fig. 5 Histogram of VLF Emission Duration on 15th December, 2007 Fig. 6 Temporal Variation of VLF Emission Bandwidth on 15th December, 2007 4 Discussions The electromagnetic detection of meteors has the major advantage over other methods that it can be done during daytime. Even in nighttime, many meteors may not be visible but the VLF radiation produced by them is easily detectable. The only limitation of VLF method is that the natural noise floor from sferics exactly peaks at VLF frequencies and the strength of sferics is generally more than the VLF emission produced by meteors (Price and Blum 2000). In our observation, interestingly, we did not detect any sign of meteor activity in VLF spectrum, around the predicted peak time 22:15 IST in spite of visible confirmation from other parts of the world. The data of visible detection of Geminids in the predicted peak period from India is given in Table 1. All of the visual observations were taken from Western part of India and the average hourly rate is only 27 compared to average hourly 123
Possible Detection of GEMINID 2007 Meteor Shower
Fig. 7 Histogram of VLF
Emission Bandwidth on 15th
December, 2007
Table 1 Visual Observation Data of Geminids 2007 on 14th December, 2007 from India
Date Name of Location Period of Total number Hourly rate
the observer observation of observation (approx.)
14.12.07 Amol Kankariya Pirangute, India, 18:15–22:45 IST 55 12
18.50 N, 72.71 E
14.12.07 Ashvini Ghadigavkar Vasai, India 19.35 N, 18:00–20:30 IST 29 12
72.80 E
14.12.07 Rohan Shewale Mamnoli, India 16:34–21:16 IST 162 36
19.26 N, 73.30 E
14.12.07 Rushikesh Tilak Sinhagad, Pune, India 20:05–21:10 IST 56 56
18.57 N, 73.97 E
14.12.07 Sarthak Dasadia Mastupura, Gujarat, 19:00–22:00 IST 28 9
India 22.25,
73.33 E
14.12.07 Shishir Deshmukh Mamnoli, India 16:40–20:37 IST 139 35
19.26 N, 73.30 E
14.12.07 Shrikant Vinchurkar Pirangut, Pune, India 19:10–22:50 IST 38 10
18.55 N, 73.74 E
14.12.07 Tushar Purohit Pirangute, India 18:25–20:45 38 19
18.50 N, 73.50 E
rate of 120 reported from observations all over the world. The visual data also reveals a
great variability in observational count in spite of having low great circle distance between
the observation sites. A few observers also reported low shower having hourly rate below
20 (hourly rates marked as bold in Table 1). There is no report of visual observation of
Geminids at that period from North-East India. So it appears that no significant overhead
meteor shower occurred over North-East India at that time. The above data also attributes
to the fact that the intensity of VLF waves at our location coming from Geminids at other
123A. Guha et al.
locations during the peak activity period were below the natural sferics intensity level. As a
result, no signatures of VLF emission from meteors were detectable from our station.
During the morning hours of 15th December, 2007, we detected a daytime hourly rate of
Geminid shower around 50 compared to worldwide visual Geminid activity data which
was reported to be 80 around that time. This can be explained by the fact that meteors are
generally burnt out in the atmosphere at a height around 60 to 90 Km from ground. During
daytime, D-region of the ionosphere exists from around 45 Km height in the atmosphere
and this region is a highly absorptive medium with respect to the propagation of VLF
electromagnetic waves (Thomas and Harrison 1970; Thomson 1993). As a result, VLF
waves generated from meteors above 45 Km get attenuated while passing through the D-
region and become suppressed under the natural noise floor of sferics. As a result, VLF
emission from meteors becomes less detectible since the intensity falls below the intensity
of natural sferics.
Our observation also shows a difference in VLF emission frequency from Geminids
compared to other reports involving VLF emission from Leonids. We observed VLF
emission in a band between 8 kHz and 13 kHz while previous reports from Leonids were
mostly in ELF-VLF bands not exceeding 3 kHz (Garaj et al. 1999; Price and Blum 2000).
It may be recalled that the Geminid meteor shower occurs from an object named 3200
Phaethon, which is thought to be an extinct comet. On the other hand, Leonid meteor
shower occurs from an active comet named 55P/Tempel-Tuttle (Vaubaillon et al. 2004).
The production of VLF waves at different frequencies originating from different meteor
showers can be explained by the production of Kelvin-Helmholtz (K-H) instability
(Landau and Lifshitz 1989; Chakrabarti et al. 2002). Due to K-H instability, a highly
supersonic meteor becomes unstable. The tangential discontinuity separates the evaporated
matter from the meteor head and the shock-compressed matter in between the bow-shock
and the tangential discontinuity. Considering the bow-shock alone, a strong shock would
compress the flow by a factor of q1/q2 * 4, q being the density of different layers of the
compressed matter. The tangential relative velocity difference is expected to be v1 -
v2 * 0–30 km.s-1 depending on the location of the bow-shock which is highest at an
angle h * 30–45° with the propagation axis. The lowest velocity difference is produced at
the stagnation point (h * 0°) and downstream farther away (h * 180°). The frequency t
of the K-H instability is given by,
1 q1 q2
t2KH ¼ ðv1 v2 Þ2 ð1Þ
4p2 ðq1 þ q2 Þ2
If we take q1/q2 * 4, tKH lies anywhere between 0 and 180 kHz.
The possible entanglement of the earth’s magnetic field in the vortices at this K-H
unstable interface can generate E-M waves of the same frequency. It is obvious that
depending on the density of the particles and entry speed, VLF waves at various fre-
quencies are emitted. The speed and density of particles for Geminids are expected to be
different from that of Leonids which might have resulted in the emission of VLF waves in
a different frequency band.
There are two kinds of VLF emission that are documented during a meteor shower. One
is sustained VLF emissions for few seconds and the other is short duration ‘‘busters’’
lasting for a fraction of second. The sustained VLF emissions are believed to be generated
via an interaction between the turbulent plasma column trailing behind an ablating
meteoroid and the Earth’s magnetic field (Keay 1980, 1993; Bronshten 1983). The inter-
action is described as a magnetic entanglement or ‘‘spaghetti’’ model. On the other hand,
123Possible Detection of GEMINID 2007 Meteor Shower
the short duration ‘‘burster’’ emissions are believed to be generated as a consequence of
shock waves propagating along the fireball’s plasma column. It is also to be noted that all
the reports that fall into ‘‘burster’’ category is only 10% of the total observed VLF emission
events during a meteor shower (Beech and Foschini 2001).
Zgrablic et al. (2002) indicated that any theoretical mechanism starting from meteor
alone will have problems explaining this high efficiency of VLF production. They sug-
gested the possibility of triggering of other atmospheric phenomena that could, in con-
junction with the meteor, lead to strong electromagnetic effect.
Spurny et al. (2000) suggested that meteors produced electrophonic sounds approxi-
mately upon entering the E-layer of ionosphere. Based on that, Zgrablic et al. (2002)
suggested a possibility of meteor triggered unidentified atmospheric phenomenon at the E-
layer boundary. One triggering effect of meteor was demonstrated by Suszcynsky et al.
(1999). They observed a sprite phenomenon triggered by a meteor. These reports
strengthen the fact that the interaction of meteors with the ionosphere cannot be ignored
during the process of production of VLF electromagnetic waves.
Keay (1980, 1993) predicted that the VLF signal generated by the meteoroid interaction
with the geomagnetic field will be sustained, possibly observable over an extended period
of time, and it will be distinct from background atmospheric sources. McKinley (1961)
reported long lasting meteor trains of more than 10 s producing radar echo. He also pointed
out that the required energy for these long lasting trains might be latent in the atmosphere
and released by catalytic agents introduced by the meteors. Chakrabarti et al. (2002) even
observed VLF emission in 19 kHz lasting for almost 3 s. Beech and Foschini (2001)
pointed out one important issue that unless the meteor generated VLF signal has charac-
teristics that clearly distinguish it from background sources, then one cannot simply claim
that because short-lived VLF transient are observed at the same time that a meteor is seen
that the two observations are causally related.
We believe that our observation is unique and the signature is different from any other
known atmospheric phenomena. It is almost unlikely that the type of VLF emission we
observed occurred from any other source in the atmosphere especially coinciding during
the peak activity of Geminid 2007 meteor shower.
There are several proposed theoretical mechanisms regarding the generation of VLF
waves from meteors. But none of the theory satisfactorily explains all the phenomena such
as the explanation of production of VLF waves from extremely faint meteors. Some reports
show that the VLF emissions from meteors generally persist for a few milliseconds to a
fraction of seconds. On the other hand reports also exit showing VLF emission duration
more than a second. In the present case we get an average duration of emission around 6 s.
As pointed out earlier, investigating the atmospheric plasma dynamics, a few workers
pointed out that there might be some secondary emissions in the atmosphere due to the
interaction of meteors with upper atmosphere, especially in the E-region of the ionosphere
(Zgrablic et al. 2002). We also believe that coherent investigation from different locations
especially in daytime can reveal more information regarding the process of coupling of
meteor trails with atmospheric plasma.
5 Conclusions
The process of detecting meteors with the help electromagnetic spectrum has now become
a widely accepted tool. This methodology has the prime merit over visual detection
technique that it is applicable at any time and in almost all weather conditions. We would
123A. Guha et al.
like to mention that our results are not conclusive proof of daytime detection of meteors by
VLF emission and more experimental results are required to strengthen the observation.
Future work in this field might include the calculation of threshold intensity of VLF
emission over natural sferics. More experiments at different locations synchronized with
visual data might open up a new era of meteor detection technique not only in earth but in
other planets also. In this way, the basic understanding of interaction of meteoroids with
planetary atmosphere would become more clear.
References
R. Barr, D.L. Jones, C.J. Rodger, J. Atmos. Sol. Terr. Phys. 62, 18–1689 (2000). doi:10.1016/S1364-6826
(00)00121-8
M. Beech, L. Foschini, Astron. Astrophys. 367, 1056 (2001). doi:10.1051/0004-6361:20000436
M. Beech, P. Brown, J. Jones, Earth Moon Planets 68, 181 (1995). doi:10.1007/BF00671507
V.A. Bronshten, Sol. Syst. Res. 17, 70 (1983)
S.K. Chakrabarti, S. Pal, K. Acharya, S. Mandal, S. Chakrabarti, R. Khan, B. Bose, Indian J. Phys. 76B,
6–693 (2002)
B.K. De, S.K. Sarkar, Ann. Geophys. 3, 113 (1985)
B.K. De, S.K. Sarkar, Ann. Geophys. 9, 597 (1991)
B.K. De, S.K. Sarkar, Meteorol. Atmos. Phys. 55, 235 (1995). doi:10.1007/BF01029830
B.K. De, S.K. Sarkar, Meteorol. Atmos. Phys. 61, 107 (1996). doi:10.1007/BF01029715
B.K. De, S.K. Sarkar, Indian J. Phys. 71A, 369 (1997)
B.K. De, M. Pal, S.S. De, R. Bera, S.K. Adhikari, A. Guha, S.K. Sarkar, Indian J. Radio Space Phys. 34, 408
(2005)
S.S. De, B.K. De, A. Guha, P.K. Mandal, Indian J. Radio Space Phys. 35, 396 (2006)
S. Garaj, D. Vinkovic, G. Zgrablic, D. Kovacic, S. Gradecak, N. Biliskov, N. Grbac, Z. Andreic, FIZIKA A
Zagreb 8, 91 (1999)
C.S.L. Keay, Science 210, 11 (1980). doi:10.1126/science.210.4465.11
C.S.L. Keay, Meteoritics 27, 144 (1992)
C.S.L. Keay, J. Sci. Explor. 7, 337 (1993)
C.S.L. Keay, Earth Moon Planets 68, 361 (1995). doi:10.1007/BF00671527
L.D. Landau, E.M. Lifshitz, Fluid Mechanics (Pargamon, New York, 1989). Chap.VII
D.W.R. McKinley, Meteor science and engineering (McGraw-Hill, New York, 1961), pp. 137–144
C. Price, M. Blum, Earth Moon Planets 82, 545 (2000). doi:10.1023/A:1017033320782
P. Spurny, H. Betlem, J. van’t Leven, P. Jenniskens, Meteorit. Planet. Sci. 35, 243 (2000)
D. Suszcynsky, R. Strabley, R. Roussel-Dupre, E. Symbalisty, R. Armstrong, W. Lyons, M. Taylor, J.
Geophys. Res. 104, 31361 (1999). doi:10.1029/1999JD900962
L. Thomas, M.D. Harrison, J. Atmos. Terr. Phys. 32, 1 (1970). doi:10.1016/0021-9169(70)90158-3
N.R. Thomson, J. Atmos. Terr. Phys. 55, 173 (1993). doi:10.1016/0021-9169(93)90122-F
N.R. Thomson, M.A. Clilverd, J. Atmos. Terr. Phys. 62, 601 (2000). doi:10.1016/S1364-6826(00)00026-2
J. Vaubaillon, E. Lyytinen, M. Nissinen, D. Asher, WGN 32:5, 125 (2004)
D. Vinkovic, S. Garaj, P.L. Lim, D. Kovacic, G. Zgrablic, Z. Andreic, WGN 30, 244 (2002)
H. Volland, Handbook of Atmospherics (CRC Press, Boca Raton, 1982), pp. 179–250. Fla
G. Zgrablic, D. Vinkovic, S. Gradecak, D. Kovacic, N. Biliskov, N. Grbac, Z. Andreic, S. Garaj, J. Geophys.
Res. (Sp. Phys.) (2002). doi:10.1029/2001JA000310
123You can also read
