Meteor shower radiant positions and structures as determined from single station video observations

Plwwr. .spK” SC;.. Vol. 4s. No. 7. pp. 857-864. 1997 !; 1997 Elsevier Science Ltd All rights reserved. Printed in Great Britain 0032N633’97 $17.00+0.00

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PIl: SOO32-0633(97)00021~

Meteor shower radiant positions and structures as determined from single station video observations S. Molau’,’ and R. Ark2 ‘DLR, Institute of Planetary Exploration, Rudower Chaussee 5. D-12489 Berlin, Germany ‘International Meteor Organization, PF 600118. D-14401 Potsdam. Germany Received 1 September

1996: revised 9 December

1996: accepted

of meteors can be used for the precise determination of radiant posifions bf major and minor meteor showers. Video systems combine large quantities of recorded events with high accuracies, and are therefore applied in the investigation of fine structures within the radiant. In the past four years, amateurs from the ArchenholdObservatory Berlin have been operating the wide angle video system A&OVIE. The maxima of several major meteor showers could be recorded. The video tapes were digitized and analysed on PCs. Using the Radiant software developed within the International Meteor Organization, the position was determined of the Quadrantid (a = 229.4” If: l.Y, 6 = +49.7”+ 1.5”), Lyrid (a = 271.6”+_ 1.5”, 6 = -t-32.9” + l”), Per&d (a = 46.0”+2”, 6 = + 57.7” +2”), Orionid (a = 93.6 f l”, 6 = +14.9”&1”) and Leonid (a = 154.5”+_2”, 6 = +21.4” & 1”) meteor shower radiants and possible radiant fme structures searched for. The obtained radiant positions are in good agreement with the common values in the literature ; no significant fine structures could be found. From the video records of the 1995 a-Monocerotid outburst, a radiant at a= 117”&3” and 6 = + lo _+2” was derived, which proves older results to be in error by several degrees. 0 1997 Elsevier Science Ltd Ab&act.

Single station video observations

Introduction

I3 January

I997

of known meteor showers. such as radiant positions, population indices, activity profiles and other characteristics. The observations are also used to analyse the reliability of visual meteor observations. and to search for new meteor showers. This paper focuses on the determination of radiant positions and structures. It describes the video system, the analysis procedures and results of recent observations.

The video system The main part of the meteor camera MOb.fE is a first generation image intensifier with three amplification stages. The intensifier is equipped with a photocathode and an output screen of 25 mm diameter and achieves a gain > 60,000. At the front end. a 3.820mm wide-angle lens projects the sky onto the photocathode. The intensifier’s output screen is recorded by a camcorder (Fig. 1). Finally, a VHS video recorder stores the frame sequence. In this configuration, the camera records a circular field ofview of 60 diameter. The limiting magnitude is 6.5 mag for stars and - 5 mag for meteors. Unlike most other video systems currently in operation (e.g. Hawkes. I990 ; Ueda and Fujiwara. 1993 : de Lignie and Jobse, 1996; Shigeno and Hiroyuki, 1993), MO V/E is a wide-angle meteor camera. Advantages of that configuration are the large field of view resulting in large numbers of bright meteors and a high percentage of complete meteor trails. In addition. the percentage of shower

Single station video meteor observations have been carried out occasionally for several years (e.g. Jones and Sarma, 1979; Duffy et ul., 1987). Amateurs of the ArchenholdObservatory Berlin have been applying this technique since 1992 to observe the maxima of selected meteor showers (Molau. 1994). The main aim is to determine properties Wide angle

Image

photo lens Correspon&xcx,

fo : S. Molau (e-mail : Sirko.Molau(tr

dlr.de)

Fig. 1. Schematic

intensifier

1

structure

of ,110 I’IE

Camcorder

858

S. Molau and R. Arlt: Meteor shower radiant positions and structures

meteors is higher as for other systems which record on average fainter meteors. Disadvantages as compared to the usual configuration are the high noise level of the first generation image intensifier, a strong geometric image distortion and a lower limiting magnitude compared to other video systems. Furthermore, less accurate meteor positions result from the lower spatial resolution.

Calibration

Images recorded by MOVIE are severely distorted. For the measurement of object positions, the radial symmetric distortion function of the system was determined once from test records of a chess-board type pattern (Fig. 2). The exponential function is comparable to distortion functions of fish-eye lenses (Molau, 1995). The sensitivity of the image intensifier changes significantly in the field of view. However, once the background illumination is subtracted from an image, the meteor brightness can be computed from the sum of all pixels belonging to the meteor, calibrated with the known brightness of reference stars.

Video tape analysis

To extract the few meteors from the many hours of night sky recording, the video tapes need to be manually inspected first. Recently, attempts to automate the meteor search have succeeded (Molau and Nitschke, 1996), but the procedure works with recordings from essentially noise-free second generation intensifiers (MCPs) only. All video frames belonging to a meteor are digitized with a 512 x 512 pixel 8 bit frame grabber. They are processed independently resulting in one brightness and position measurement each l/25 s (PAL video standard). First, the images are geometrically corrected with the calibration data obtained earlier. Then, the positions are transformed from line/sample to equatorial coordinates using a linear plate constant fit. The reference stars are taken from the PPM star catalogue (epoch 52000). Finally, we compute the mean path, duration, angular velocity, light curve and possible shower membership of the meteor. All values are stored in a database using IMO’s PosDat format. Currently, the database contains more than 800 entries of video meteors with brightnesses between - 4 and 4 mag. A more detailed description of the analysis process is given in Molau (1995).

Radiant analysis

In contrast to multiple station video or photographic records, where radiants for individual meteors can be derived, single station observations provide a statistical estimate obtained from a sample of many shower meteors. The basic idea is the backward prolongation of the meteors moving on great circles over the celestial sphere. The spatial distribution of these backward prolongations is

A gnomonically projected map with two meteors and their backward prolongations. The equatorial grid is marked by dotted lines. Elements of the distribution matrix are only increased over a certain section of the backward prolongation according to the meteor’s angular velocity. These array elements are indicated by grey squares. The black squares were touched by both meteors Fig. 3.

calculated in a two-dimensional grid revealing the most probable radiant position. A mean requirement for this method is a uniform distribution of meteors around the radiant. The gnomonic projection is used to form a two-dimensional grid on the celestial sphere. All great circles are straight lines on a gnomonic map making it easy to prolong the meteors over the sky. The area of the gnomonic map should be small, as strong scale changes will occur otherwise. Not every point on the backward prolongation is a reasonable radiant of the meteor. In this study we investigate meteor showers with known geocentric velocities v,. An expected radiant position is calculated from the angular velocity of the meteor. Thus, according to the error limits of the speed determination, only a section of the backward prolongation is used (Fig. 3). These sections have constant weights in the accumulation of radiant points. Each prolongation is equally weighted in this method. The elevation of the expected radiant point was used to correct the meteor’s path for the zenithal attraction ; the aberration is not considered here. The angular velocity can be computed with an accuracy of about 1” SK’ from video recordings. We determine the position of the meteor in each video frame (typically five or more frames), calculate a mean path assuming a constant meteor speed, and derive the speed from the corrected beginning and end position. The software Radiant (Arlt, 1992) considers the angular velocity of the meteor, its elevation above the horizon and the geocentric velocity of the investigated stream. It adds a certain section of the backward prolongation to the gnomonic grid, which can be understood as a matrix con-

S. Molau and R. Arlt: Meteor shower radiant positions and structures

Fig. 2. Image of a chess-board type test pattern before and after the correction with the inverse distortion function

Fig. 4. Shower image of the Lyrids. All bright meteors recorded on April 21/22, 1995, are combined in one image to show the distribution of meteors around the radiant

859

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S. Molau and R. Arlt: Meteor shower radiant

positions

Fig. 5. Radiant plot of the Lyrids. The camera’s field of view was changed two times in the night, producing an optimal meteor distribution (Fig. 4). The resulting radiant is well defined; no subradiant structures were found

Fig. 6. Radiant plot of the Orionids. The uniform distribution results in a narrow peak of the backtraced meteors at the radiant

of the meteor position

around

the radiant

and structures

S. Molau and R. Arlt: Meteor shower radiant positions and structures

Fig. 7. Radiant plot of the a-Monocerotids. The short outburst of this minor shower was entirely recorded by MO VIE. The field of view was not changed during the 30 min of high activity. However, the radiant is still much more accurate than the position obtained from earlier visual observations (marked by a dark circle)

861

863

S. Molau and R. Arlt: Meteor shower radiant positions and structures taining the radiant probability distribution over a certain region in the sky. Every matrix element within the backward prolongation section is increased by 1 plus a slight scale correction. Another method of determining radiant distributions is the accumulation of intersection points between two backward prolongations. Again, intersections are only valid if they lie within the allowed sections of the prolongations according to the angular speeds of the meteors.

Results Examples for meteor shower images and resulting radiant plots are given in Figs 4-7. The radiant position of the Quadrantids, Lyrids, Perseids, Orionids and Leonids are confirmed by this study (Table 1). We achieved a typical positional accuracy of l-2”. This is slightly less accurate than positions obtained from photographic or other video observations, which is caused by the lower spatial resolution in the large field of view. Even though the radiant is not always a single Gaussian-shaped peak, there are no indications for significant sub-radiant structures of the analysed meteor showers. Most important for single station observations is an appropriate field of view. Radiant plots become artificially elongated and less accurate, if the meteors are not equally

distributed around the radiant. For our system we found an optimum by aiming the camera in different directions at about 20-30” radiant distance. The short, but intense outburst of the a-Monocerotids in 1995 could be observed under favourable circumstances in southern Germany. The radiant obtained from backward prolongations of 28 recorded shower members (Fig. 7) is elongated, which is caused by the unequal meteor distribution. A similar effect occurred in the 1995 Leonid observation analysis, where the radiant was elongated towards the centre of the camera’s field of view too. The plot proves the radiant position determined earlier to be in error by several degrees (Rendtel et al., 1995). So far, only three outbursts of that shower had been witnessed by occasional visual observers (Jenniskens, 1995). Thus, there was no reliable data available before the 1995 event, which was well observed by many observers in Europe thanks to the accurate predictions.

Conclusions and future prospects Single station video observations can be used for the detailed investigation of meteor showers. Using this technique, we have determined the radiant positions of several well-known meteor showers. The values derived are consistent with results of other investigations, no sub-radiant

Table 1.

Radiant position Meteor

shower

-

Year of observation

Shower meteors

Ref. solar longitude

This study

Other investigationsa

Quadrantids

1995

39

283.1”

GI= 229.4” ) 1.5” 6 = +49.7”* 1.5”

(a) (b) (c) (d)

Lyrids

1995

31

32.1”

c( = 271.6”+1.5” 6 = f32.9”f I”

(a) CL= 271.9”~0.3” (d) c( = 271.9”+0.9” (e) CI= 271.4”If:O.5”

6 = +33.3”f0.3” 6 = +33.4”f0.4” 6 = +33.4”+0.5”

Perseids

1993194

228

139.5”

CI= 46.0” f 2” 6 = +57.7”+ 1”

(a) (f) (g) (h)

6 6 6 6

Orionids

1995

27

208.4”

CI= 93.6”* 1” s = +14.9”*1”

(a) c1= 94.7” f0.8” (d) c( = 94.2”+3.6” (i) c( = 94.9”

6 = + 15.9”f0.8” 6 = + 15.8”~0.9” 6 = +15.4”

Leonids

1995

35

235.2”

CL= 154.5”*2” 6 = +21.4”+1”

(a) tl = 152.7”+0.3” (d) tl = 153.2”* 1.0”

6 = +22.5”+0.3” 6 = +22.0”+0.8”

cr-Monocerotids

1995

28

239.3”

c! = 117.1”&3” 6 = +1.0”+2”

(j) a = 109”*5’

6 = -6”f5”

“These values may refer to a different epoch and solar longitude. (a) KresAk and Porubcan (1970). (b) de Lignie and Jobse (1996). (c) Trigo (1996). (d) Neslusan et al. (1995). (e) Steyaert (1986). (f) Lindblad and Porubcan (1995). (g) Jones and Sarma (1979). (h) Suzuki et al. (1994). (i) Duffy et al. (1988). (j) Kronk (1988).

c( = c( = c( = c( =

CI= c( = @I= OL=

229.8”f 1.1” 231.0”f2.1” 229”+ 1” 230.6”_$2.2”

46.8” + 1.3” 46.6” f2.3” 47.9”k3” 47.8Ok2.3”

6 6 s 6

= = = =

= = = =

+49.4”* 1.1” +49.1”&-0.8” +49”*1” +49.1”_+1.0”

+57.7”f 1.3” +57.8”+ 1.1” +58.4”+3” +57.5”$1.0”

864

S. Molau and R. Arlt: Meteor shower radiant positions and structures

structures were found. From observations of the 1995 outburst of the cr-Monocerotids we were able to obtain a reliable radiant for the first time. In the future we want to extend our studies to smaller meteor showers. New camera systems and faster computers will significantly improve the efficiency of analysis. Further, multi-station video observations are planned to obtain also heliocentric orbits of meteoroid streams. Acknowledgements. We wish to express our thanks to the Arch-

enhold-Observatory Berlin for the ongoing support of our work, without which video observations could not have been carried out.

in meteor streams. I. The size of the radiant areas. Bull. Astron. Inst. Czech. 21, 153-170. Kronk, G. W. (1988) Meteor Showers: a Descriptive Catalog. Enslow, Hillside, New Jersey. Lindblad, B. A. and Porubcan, V. (1995) Radiant ephemeris and radiant area of the Perseid meteoroid stream. In Meteoroids, eds V. Porubcan, I. P. Williams and V. Vanysek. Earth, Moon, and Planets 68,409-418.

Molau, S. (1994) MOVIE-meteor observation with video equipment. In Proc. Znt. Meteor Conf., ed. P. Roggemans, pp. 71-75. IMO, Mechelen. Molau, S. (1995) MOVIE-analysis of video meteors. In Proc. Znt. Meteor Conf., eds A. Knofel and P. Roggemans, pp. 1 l24. IMO, Mechelen. Molau, S. and Nitschke, M. (1996) Computer based meteor search-a new dimension in video meteor observation. WGN, ZMO J. 24,119-123.

Arlt, R. (1992) The software Radiant. WGN, ZMO J. 20,62-70. de Lignie, M. and Jobse, K. (1996) Double-station video observations of the 1995 Quadrantids. WGN, ZMO J. 24,20-25. Duffy, A. G., Hawkes, R. L. and Jones, J. (1987) The determination of shower meteor parameters from single station observations. Mon. Not. R. Astron. Sot. 228, 55-75. Duffy, A. G., Hawkes, R. L. and Jones, J. (1988) Television observations of the 1984 Orionid shower. Mon. Not. R. Astron. Sot. 234,643-654.

Hawkes, R. L. (1990) Double station TV meteor observations. WGN, IMO J. 18, 14>151.

Jenniskens, P. (1995) Good prospects for alpha-Monocerotid outburst 1995. WGN, ZMO J. 23,84-86. Jones, J. and Sarma, T. (1979) Television observations of the Perseid meteors. Mon. Not. R. Astron. Sot. 185,225-232. Kresak, L. and Porubcan, V. (1970) The dispersion of meteors

Neslusan, L., Svoren, J. and Porubcan, V. (1995) A procedure of selection of meteors from major streams for determination of mean orbits. In Meteoroids, eds V. Porubcan, I. P. Williams and V. Vanysek. Earth, Moon, and Planets 68, 427433.

Rendtel, J., Brown, P. and Molau, S. (1996) The 1995 outburst and possible origin of the cr-Monocerotid meteoroid shower. Mon. Not. R. Astron. Sot. 279, L31-L36.

Shigeno, Y. and Hiroyuki, S. (1993) Double station TV meteor observations. WGN, ZMO J. 21,37-42. Steyaert, C. (1986) The photographic Lyrid radiant. WGN, ZMO J. 14,46-47.

Suzuki, S., Yoshida, T., Suzuki, K. and Akedo, T. (1994) Multistation TV observations of the 1993 Perseids. WGN, ZMO J. 22,137-139.

Trigo, J. (1996) Photographic analysis of the 1992 Quadrantids. WGN, ZMO J. 24,27-29.

Ueda, M. and Fujiwara, Y. (1993) TV observations of the 1991 Geminid meteor stream. WGN, ZMO J. 21, 215-217.