Observations of the Lyrid meteor shower in 1997-2005 by a forwardscatter 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 Earth crosses the stream at a half maximum rate level in about two days. The overall activity peak derived from all observed returns is at solar longitude 32.30o (eq. 2000.0). 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 exhibiting filamentary structure

Bortolotti, G.

Cevolani, G.

Franceschi, C.

Porubčan, V.

Pupillo, G.

Trivellone, G.

Zigo, P.

English

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NOTE BREVIDOI 10.1393/ncc/i2005-10212-4IL NUOVO CIMENTO Vol. 29 C, N. 5 Settembre-Ottobre 2006The Lyrid meteor stream: Activity and mass distributionV. Porubcˇan(1)(2), P. Zigo(1), G. Pupillo(3)(4), G. Cevolani(3),G. Bortolotti(3), C. Franceschi(3) and G. Trivellone(3)(1) Department of Astronomy, Physics of the Earth and MeteorologyFMPI Comenius University - 842 48 Bratislava, Slovakia(2) Astronomical Institute of the Slovak Academy of Sciences - 845 04 Bratislava, Slovakia(3) CNR-ISAC - via Gobetti 101, 40129 Bologna, Italy(4) Osservatorio di Campi Salentina, 73012 Campi Salentina (Lecce), Italy(ricevuto il 20 Dicembre 2005; revisionato il 16 Giugno 2006; approvato il 10 Luglio 2006)Summary. — Observations of the Lyrid meteor shower in 1997-2005 by a forward-scatter radio system for meteor observation operating simultaneously along twobaselines, Bologna-Lecce (Italy) and Bologna-Modra (Slovakia), are analysed anddiscussed. The activity curves of long-duration echoes (≥ 8 s) and their variationsindicate a complex structure of the stream. The Earth crosses the stream at a halfmaximum rate level in about two days. The overall activity peak derived from allobserved returns is at solar longitude 32.30o (eq. 2000.0). The mass distribution ex-ponent s and its variations in the period of the shower maximum indicate a relativelystable population of meteoroids in the stream exhibiting filamentary structure.PACS 96.30.Za – Meteors, meteorites and textites.PACS 95.85.Bh – Radio, microwave (> 1 mm).1. – IntroductionThe Lyrid meteor shower ranks among regular annual meteor showers active for aboutone week of the second half of April in most years with a fairly low visual rate about10-15 meteors per hour at maximum on April 21-22. The shower radiant derived fromphotographic observations has the right ascension α = 272.0◦ and the declination δ =33.3◦ at solar longitude 31.8◦ (equinox: 1950.0) [1]. The parent of the stream is C/1861G1 (Thatcher), which is the comet with the longest period of revolution (P = 415 years)having a known meteoroid stream. The Lyrids occasionally exhibit enhanced activitymaxima, the last one in 1982 with the visual Zenithal Hourly Rate (ZHR) of about 250meteors [2].References to the shower activity are infrequent, most are concerned with individualyears. The longest series of Lyrid observations analyzed so far are eighteen returns (overa period of thirty years) of the stream observed by meteor radars in Springhill, Canadac© Societa` Italiana di Fisica 601602 V. PORUBCˇAN, P. ZIGO, G. PUPILLO, ETC.(1958-1967) and Ondrˇejov, Czechoslovakia (1980-1987) giving the width of the rate profileat half-amplitude 1.5 days and maximum for solar longitude 31.5◦±0.05 (eq. 1950.0) [3].The first test observations of the Lyrids by the BLM (Bologna-Lecce-Modra) forward-scatter system were acquired in 1992, however, along the Budrio-Lecce baseline only.Regular simultaneous observations of the shower along both the baselines started in1996. The shower was almost regularly monitored in the period April 18-25.The present paper presents observations of the Lyrid shower observed in nine returnsof the stream (1997-2005) obtained by a forward-scatter system operating along twobaselines Budrio (44.6oN; 11.5◦E, Italy)-Lecce (40.3◦N; 18.2◦E, Italy) and Budrio-Modra(48.4◦N; 17.3◦E, Slovakia). In the paper an analysis of activity and mass distribution ofmeteoroids in the stream are presented and discussed.2. – Lyrids in 1997-20052.1. Equipment . – Since September 1996, there has been operating over Italy andSlovakia a forward-scatter system for meteor observations, with a radio signal transmittedalong two mutually almost rectangular baselines. The transmitter is located at Budrio(44.6◦N; 11.5◦E, Italy) near Bologna and receivers are at Lecce (40.3◦N; 18.2◦E, Italy)and Modra (48.4◦N; 17.3◦E, Slovakia). The BLM system was built up for a systematicmonitoring of meteor activity in order to study meteor flux from different baselinesdirections and study the structure and potential sources of the population in a closesurrounding 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 directionof 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. [4].2.2. Activity . – Earlier analyses of activity of meteor showers observed by the BLMexhibit that shower echoes can be clearly recognized from sporadic background echoesby the system only for overdense echoes [5]. In all echo counts shower echoes are veryfrequently almost entirely overlapped by sporadic echoes. Therefore, for the presentanalysis the shower activity was derived for echoes of duration ≥ 8 s and was calculatedby subtracting sporadic echoes counts from all echoes counts in corresponding hours. Forsporadic the days were taken well off the shower maximum and whenever it was possiblethe sporadic background is the mean value relative to two days (one before and one afterthe peak of the shower activity).2.3. Mass distribution exponent . – The mass distribution exponent s was derived fromcumulative echo counts of partitioned in different echo duration groups, considering theambipolar diffusion for dominant process of an echo decay, using the known formuladerived by Kaiser [6] in the formlogNc =(−34)(s− 1) log TD + const,(1)where Nc is the cumulative number of echoes with the duration equal to or greater thanTD.THE LYRID METEOR STREAM: ACTIVITY AND MASS DISTRIBUTION 603020401998Ns020401999Ns020402000Ns020402001Ns020402003Ns020402004Ns31 31.5 32 32.5 33 33.5 34020402005NsSolar longitude (J2000.0)Fig. 1. – Counts of the Lyrid meteor shower echoes of duration ≥ 8 s in one-hour intervalsobserved by the BLM forward-scatter radio system in Lecce in 1998-2005.The mass exponent was found by a solution of a least-squares fit of 13 echo durationsubsets equidistantly distributed in a logarithmic scale in the 1–20 s duration interval.The exponents for shower echoes (ssh) and for sporadic echoes (ssp) have been calculatedon 6 h time intervals centered on the peak of the shower activity and on the sporadicbackground activity, respectively.2.4. Results. – The activity curves of the Lyrids corrected for the zenith distance ofthe shower radiant, derived for three days around the maximum of activity separatelyfor both receiving stations (Lecce and Modra) and depicted in hourly shower echo countsare shown in figs. 1 and 2. The Lyrid radiant (α = 272◦, δ = 33◦) culminates at 04:05LT and is above horizon for the Northern mid-latitude sites for about 18 hours. Theobserved solar longitudes of the shower maxima (eq. 2000.0) are listed in table I. Onlythe echo counts for which the shower radiant elevation was h ≥ 20◦ are in the depictedplots. Large correction for the radiant elevation below 20◦ can result in unreal distortedhourly echo counts. As shown in figs. 1 and 2 the echo counts observed in Modra until2000 are systematically lower than those observed in Lecce. This was due to a lowersensitivity of the Modra receiver with respect to the Lecce one.Table I lists also the values of s derived separately for the shower and sporadic meteorsat both receiving stations.In 1997 Lyrids were observed in direction to Modra and the observed maximum israther flat with two peaks at 32.17◦ and 32.30◦ (higher one). The mass exponent derived604 V. PORUBCˇAN, P. ZIGO, G. PUPILLO, ETC.01020301997Ns01020301998Ns01020301999Ns01020302000Ns01020302001Ns31 31.5 32 32.5 33 33.5 3401020302004NsSolar longitude (J2000.0)Fig. 2. – Counts of the Lyrid meteor shower echoes of duration ≥ 8 s in one-hour intervalsobserved by the BLM forward-scatter radio system in Modra in 1997-2004.for the period of 6 hours around the peak s = 1.96 and the value for sporadic backgrounddue to low number of echoes is uncertain (in brackets).In the next four years 1998-2001 the observations were carried out along both baselinesand positions of the Lyrid maxima found separately for Lecce and Modra are consistent(table I). The only deviation is observed for the year 2000 when the difference in theTable I. – The maxima of activity and mass distribution exponent s of the Lyrid meteor shower(sh) observed by the BLM forward-scatter radio system in 1997-2005 and sporadic (sp) meteors(L—Lecce, M—Modra). Values in brackets are due to low numbers of uncertain echoes.Year Lecce Modra ssh ssh ssp ssp(eq. 2000.0) (eq. 2000.0) L M L M1997 - 32.30◦ - 1.96 - (2.71)1998 32.01◦ 32.01◦ 1.87 1.71 2.59 2.511999 32.65◦ 32.62◦ 2.16 2.18 2.47 2.642000 32.36◦ 32.48◦ 2.05 1.65 2.65 2.742001 32.23◦ 32.23◦ 2.28 - 2.42 -2003 32.30◦ - 2.15 - (2.18) -2004 (32.46◦) 32.50◦ - 2.01 - 2.772005 32.13◦ - 1.80 - 2.14 -THE LYRID METEOR STREAM: ACTIVITY AND MASS DISTRIBUTION 605maxima observed at the two stations is 0.12◦ (32.36◦ for Lecce and 32.48◦ for Modra)corresponding to about 2.9 h. This shift in maxima between relatively close stationsmay indicate existence of very narrow filaments in the stream consisting of populationsof particles of different sizes [7]. This can be inferred also from the different values of themass exponent s between both receivers for the peaks of activity (2.0 and 1.65 for Lecceand Modra, respectively). The peak observed at Modra was composed predominately oflarger particles. The corresponding sporadic background values of s (2.65 and 2.74) areat the same time in comparison with the differences in the shower values consistent.On the other hand, a large-scale filamentary structure of the Lyrids confirms alsothe different values of s in 1998 and 1999. For each of these two years solar longitudesof the peak and s are consistent for both stations. But the s value in 1998 and 1999 issignificantly different, while s for the sporadic background in both years is also consistent.The 1998 return of the stream was relatively reacher in larger particles than in 1999.The observations of the Lyrids in 2002 were interrupted just at the expected showerpeak and therefore were not analyzed. In 2003 and 2005 due to malfunction of transmis-sion towards Modra the observations were made only in direction to Lecce and in 2004observations were performed only in direction to Modra. Even so the observed activitypeaks and s are in agreement with the results of observations in the preceding years.Though, the observed Lyrid maxima in individual returns show a variance, the overallactivity peak derived from all eight returns at solar longitude 32.30o (eq. 2000.0) iscompletely consistent with the results of the previous radio studies of the Lyrid long-term activity profile setting the maximum at solar longitude 32.2o [3].The close agreement of these results over a time span of some decades with thepresence of a sharp peak in the Lyrid maximum activity documents that the Lyrids area permanent meteor stream with relatively stable populations of particles not seriouslyaffected by planetary perturbations (as for Orionids and Eta Aquarids, for example).This is well supported also by the evidence of no motion in the streams orbital nodes forat least 2600 years [8].3. – ConclusionsObservations of the Lyrid meteor shower by the Bologna-Lecce-Modra forward-scattersystem in 1997-2005 were analysed and the activity curves for long-duration echoes (≥8 s)and mass distribution exponents were derived.The activity curves confirm that the Lyrids are a regular and low activity stream,which crosses the Earth’s orbit by its central denser part in less than two days. Theshape of the activity curve and position of maximum may change from year to year.For the period of observations (1997-2005) the position of the shower peak varied within32.0◦–32.6◦.The shower exhibits a multiple peak structure in almost all returns and shows afilamentary structure consisting of different populations of meteoroids as confirmed bythe mass distribution exponent. Besides a large-scale filamentary structure evident fromthe variable s in different returns, the observations indicate the existence of very narrowfilaments in the stream consisting of population of particles of different sizes.The mass exponent derived for the central part of the stream reveals that the mete-oroids within the stream are not distributed homogeneously and their distribution maychange from year to year.The overall activity profile exhibits a peak at solar longitude 32.30◦ which is consis-tent with the previous global analysis of radio observations of the Lyrids over a time606 V. PORUBCˇAN, P. ZIGO, G. PUPILLO, ETC.span of thirty years giving the overall activity peak at 32.20◦ [3]. These very similarresults suggest that Lyrids have the characteristics of a meteor stream with relativelystable populations of particles not seriously affected by planetary perturbations and non-gravitational forces.∗ ∗ ∗The authors acknowledge support of the research to the Institute ISAC-CNR, Bolognaand to VEGA, the Slovak Grant Agency for Science, grant Nos. 1/0204/03 and 2/3024/23.REFERENCES[1] Lindblad B. A. and Porubcˇan V., Bull. Astron. Inst. Czechosl., 42 (1991) 354.[2] Adams M. T., Meteor News, 58 (1982) 1.[3] Porubcˇan V., Sˇimek M. and McIntosh B. A., Bull. Astron. Inst. Czechosl., 40 (1989)298.[4] Cevolani G., Bortolotti G., Franceschi C., Grassi G., Trivellone G., Hajduk A.and Kingsley S. P., Planet. Space Sci., 43 (1995) 765.[5] Porubcˇan V., Hajduk A., Cevolani G. and Pupillo G., Nuovo Cimento C, 26 (2003)679.[6] Kaiser T. R., Meteors (Pergamon Press, London) 1955.[7] Cevolani G., Gabucci M. F., Grassi G. and Trivellone G., Planet. Space Sci., 46(1998) 869.[8] Kronk G. W., Meteor Showers (Enslow Publishers, Inc.) 1988.

The Lyrid meteor stream: Activity and mass distribution

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The Lyrid meteor stream: Activity and mass distribution

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