Observations of the Geminid meteor shower in 1996-2003 by a

forward-scatter radio system for meteor observation operating simultaneously along two baselines, Bologna-Lecce (Italy) and Bologna-Modra (Slovakia), are analysed and discussed. The activity curves of long-duration echoes (≥ 8 s) and their variations indicate a complex structure of the stream. The width of the stream at a half maximum rate level is about two days. The mass distribution exponent s and its variations in the period of the shower maximum indicate a relatively stable population of meteoroids in the stream with a population of smaller particles preceding

the activity peak composed of larger particles

Bortolotti, G.

Cevolani, G.

Franceschi, C.

Hajduk, A.

Kornoš, L.

Porubčan, V.

Pupillo, G.

Trivellone, G.

Zigo, P.

English

Scientific Open-access Literature Archive and Repository

NOTE BREVIDOI 10.1393/ncc/i2004-10030-2IL NUOVO CIMENTO Vol. 27 C, N. 3 Maggio-Giugno 2004The Geminid meteor shower of 1996-2003 from forward-scatterobservations: Activity and mass distributionG. Pupillo(1)(2)(3), V. Porubcˇan(4)(5), G. Bortolotti(1), G. Cevolani(1),C. Franceschi(1), A. Hajduk(5), L. Kornosˇ(4), G. Trivellone(1) and P. Zigo(4)(1) ISAC-CNR - via Gobetti 101, 40129 Bologna, Italy(2) DET, Universita` di Firenze - 50139 Firenze, Italy(3) Osservatorio di Campi Salentina - 73012 Campi Salentina (Lecce), Italy(4) Department of Astronomy, Physics of the Earth and MeteorologyFMPI Comenius University - 842 48 Bratislava, Slovakia(5) Astronomical Institute of the Slovak Academy of Sciences - 845 04 Bratislava, Slovakia(ricevuto il 27 Ottobre 2004; approvato il 24 Novembre 2004)Summary. — Observations of the Geminid meteor shower in 1996-2003 by aforward-scatter radio system for meteor observation operating simultaneously alongtwo baselines, Bologna-Lecce (Italy) and Bologna-Modra (Slovakia), are analysedand discussed. The activity curves of long-duration echoes (≥ 8 s) and their varia-tions indicate a complex structure of the stream. The width of the stream at a halfmaximum rate level is about two days. The mass distribution exponent s and itsvariations in the period of the shower maximum indicate a relatively stable popu-lation of meteoroids in the stream with a population of smaller particles precedingthe activity peak composed of larger particles.PACS 96.50.Kr – Meteors meteoroids, and meteor streams.PACS 95.85.Bh – Radio, microwave (> 1 mm).1. – IntroductionSince September 1996, there has been operating over Italy and Slovakia a forward-scatter system for meteor observations, with a radio signal transmitted along two mu-tually almost rectangular baselines. The transmitter is located at Budrio near Bologna(44.6◦N; 11.5◦E, Italy) and the receivers are at Lecce (40.3◦N; 18.2◦E, Italy) and Modra(48.4◦N; 17.3◦E, Slovakia). The system was built up for a systematic monitoring ofmeteor activity in order to study meteor flux from different baselines directions andconsequently to study the structure and potential sources of the population in a closesurroundings of the Earth’s orbit.The equipment utilizes a continuous-wave transmitting frequency at 42.77 MHz, afixed modulating tone at 1 kHz and 0.25 kW mean power transmitted in the directionc© Societa` Italiana di Fisica 301302 G. PUPILLO, ETC.of both receiving stations. The baseline distances between the transmitter and receiversare: Bologna-Lecce of 700 km (azimuth 307◦) and Bologna-Modra of 590 km (azimuth224◦). Details about the system and its operation have been published by Cevolani etal. [1].The present paper is a third one in a series of the analyses of the activity and structureof major meteor showers observed by the BLM forward-scatter system and we presentand discuss results of the observations of the Geminid meteor shower acquired by theequipment over eight years, 1996-2003.2. – Observations of the Geminids in 1996-2003The Geminid meteor shower is active for two weeks at the beginning of December,with the maximum at the solar longitude of about 262◦ (December 13-14) and visualpeak frequency up 100. After recognition that the stream is probably associated with theasteroid 3200 Phaethon by Whipple in 1983 [2], the shower was studied more intensively.The first observations of the Geminids by the BLM forward-scatter system were ac-quired in 1995, however, along the Budrio-Lecce baseline only. Regular simultaneousobservations of the shower along both the baselines started in 1996. The shower wasalmost regularly monitored in the period December 8-19.The studies of the echo counts obtained by the system reveal that a shower activitycan be clearly recognized only for echoes of longer duration. In the counts of all echoes,the shower echoes can frequently be overlapped by sporadic background echoes. Thistrend is observed also in almost all years of the observation of the Geminids by thesystem. Therefore, to discriminate the activity of the shower, the activity curves werederived for echoes of duration ≥ 8 s.The shower activity was obtained by subtracting the sporadic background counts fromall echo counts (shower and sporadic), corrected for the radiant elevation in correspondingtime intervals. The observations obtained for radiant elevation below 15◦, due to largecorrections to be applied to the observed echo counts, were not included in the streamactivity curves. The activity curves around the shower maximum represented by theshower echo counts in 30 minute intervals (echoes ≥ 8 s) obtained by combining datafrom both the stations, Lecce and Modra, are plotted in fig. 1. The shower radiant forthe central area between the receivers culminates at about 01:25 UT and is above horizonfor 18 h. The gaps in plots on fig. 1 correspond to the periods when the radiant was forthe stations below horizon.The shower maxima observed by the BLM FS system (reduced for equinox 2000.0) arepresented in table I. The activity curves in all return of the Geminids exhibit a slowerincrease of the activity from the beginning up to the maximum followed by a steepdecrease. This feature of the stream activity is known also from visual and previousradar observations. The width of the shower to the half-maximum activity varies and isabout two days.The observations were carried out continuously in all years. Only for 2001 the datadue to malfunction of the transmitter and the receiving station at Lecce are missing. AtModra, there are only partial obervations close to the maximum and sporadic backgroundis missing. In individual years also pronounced secondary maxima on the shower activityare observed. In 1996 shower maximum was observed on December 14, 07:25 UT, solarlongitude 262.63◦ (equinox 2000.0) and a secondary broader peak appeared on December13 at about 22:15 UT (262.25◦). The 1997 maximum appeared at 262.41◦ and a secondarypeak at 262.01◦. The 1999 maximum was not observed. In table I only the highest peakTHE GEMINID METEOR SHOWER OF 1996-2003 FROM FORWARD-SCATTER OBSERVATIONS 303Fig. 1. – Counts of the Geminid meteor shower echoes of duration ≥ 8 s in 30 minute intervalsobserved by the BLM forward-scatter radio system in 1996-2003. The curves represent the meanvalues of the data from Lecce and Modra.from the observed echo counts is presented (in brackets). In 2002 a very pronouncedsplitting of the maximum is evident, with the maximum at 262.10◦ and another peakjust before maximum at 261.68◦.304 G. PUPILLO, ETC.Table I. – The maxima of activity and mass distribution exponent s derived for the Geminidmeteor shower (sh) observed by the BLM forward-scatter radio system in 1996-2003 and sporadic(sp) meteors (L—Lecce, M—Modra).Year Maximum Interval L-sh M-sh L-sp M-sp(equinox 2000.0)1996 262.63◦ 262.15◦– 262.66◦ 1.81 1.72 2.48 2.26261.14◦– 261.64◦ 1.91 1.921997 262.41◦ 261.90◦– 262.41◦ 1.58 1.80 2.63 2.68260.87◦– 261.38◦ 1.82 1.991998 262.10◦ 261.62◦– 262.14◦ 2.00 1.88 2.46 2.361999 (261.83◦) 261.37◦– 261.88◦ 1.86 1.66 2.22 2.18262.39◦– 262.90◦ 1.76 1.582000 262.62◦ 262.12◦– 262.63◦ 1.78 2.01 2.59 2.402002 262.10◦ 261.60◦– 262.11◦ 1.90 2.02 2.40 2.492003 262.46◦ 262.36◦– 262.86◦ 1.55 2.08 2.36 2.45261.34◦– 261.84◦ 1.76 2.173. – Mass distributionThe mass distribution exponent s was derived from the cumulative numbers of echoduration considering diffusion as dominant process of an echo decay in the form [3]logNc =(−34)(s− 1) log TD + const,(1)where Nc is the cumulative number of echoes with the duration equal to and greaterthan TD.Though at lower heights the duration of the echo is affected also by a second mech-anism of reducing the electron line density of a meteor trail, which was previously at-tributed to attachment of free electrons to neutral air particles [4], later Baggaley [5] andothers have demonstrated that the attachment is not a dominant factor for ionizationloss of meteor trails. More recently Jones et al. [6] have shown that a series of chemicalreactions involving ozone is likely to be more important than the electron attachment.However, in this preliminary study of the mass distribution exponent in the Geminidstream from forward-scatter data no correction for attachment effect was taken intoaccount, though the geocentric velocity of the stream is relatively low (35 km/s). Toavoid the problem a narrower interval of echo duration for the derivation of s has beenchosen. Thus the acquired values of s refer to the interval of echo duration from 1 s to10 s.The values of s in individual years were obtained as the mean values from the period ofthe shower maximum (approximately 12 h) and the corresponding sporadic background.The mass exponent values derived for each station separately (L—Lecce, M—Modra) arelisted in table I.An interesting result of previous backscatter radar observations of the Geminids isconnected with mass separation of meteoroids in the stream, with the peak of smallerparticles preceding the peak of larger particles [7]. To verify the finding, we have extendedthe mass distribution analysis, in the years with sufficient statistics of data, to days beforethe day of the maximum too. The mass exponent s values were derived from the periodsTHE GEMINID METEOR SHOWER OF 1996-2003 FROM FORWARD-SCATTER OBSERVATIONS 30512 hours about the culmination of the shower radiant and are listed in table I with thecorresponding solar longitude intervals. The results are summarized in the second rowsrelatively to the individual years. The s values are consistent with the fact that thepopulation of smaller particles is generally preceding the larger ones which are moreconcentrated into the densest part of the stream.4. – ConclusionsObservations of the Geminid meteor shower by the Bologna-Lecce-Modra forward-scatter system in 1996-2003 were analysed and the activity curves for long-durationechoes (≥ 8 s) and mass distribution exponents were derived. The activity curves confirmthat the Geminds are a concentrated stream and the Earth crosses its central dense partin about two and half days. The shape of the activity curve may change from yearto year and indicate a multiple peak structure of the stream in almost all years andexhibits its filamentary structure. The mass exponent derived for the dense centralpart of the stream evidences that the meteoroids within the stream are not distributedhomogeneously and their distribution may change from year to year. The observationsindicate a mass separation in the stream with smaller particles appearing predominantlyprior to the maximum represented according to lower s values by larger meteoroids.∗ ∗ ∗The authors are indebted for the support of the research to the Institute ISAC CNR,Bologna and to VEGA, the Slovak Grant Agency for Science, grant Nos. 1/0204/03 and2/3024/23.REFERENCES[1] Cevolani G., Bortolotti G., Franceschi C., Grassi G., Trivellone G., Hajduk A.and Kingsley S. P., Planet. Space Sci., 43 (1995) 765.[2] Whipple F. L., IAU Circ., (1983) No. 3881.[3] Kaiser T. R., Meteors (Pergamon Press, London) 1955.[4] Plavcova´ Z., Bull. Astron. Inst. Czech., 16 (1965) 127.[5] Baggaley J. W., Mon. Not. R. Astron. Soc., 159 (1972) 203.[6] Jones J., McIntosh B. A. and Sˇimek M., J. Atmos. Terr. Phys., 52 (1990) 253.[7] Sˇimek M., Bull. Astron. Inst. Czech., 24 (1973) 213.

The Geminid meteor shower of 1996-2003 from forward-scatter

observations: Activity and mass distribution

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The Geminid meteor shower of 1996-2003 from forward-scatter observations: Activity and mass distribution

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