June Bootids 1998

June Bootids 1998 / Juni Bootiden 1998

Surprising Activity of the June Bootids 1998

Jürgen Rendtel, Rainer Arlt, Valentin Velkov

Introduction

Considerable activity of the June-Bootids was observed at two occasions in 1916 and 1927. Some sources also list the year 1921, but the activity reported from this return is rather low (see Table 1). Additionally, there are some reports of possible activity before and after these returns, but the association to the June-Bootids is not certain. Nevertheless, Hoffmeister (1948) considered the shower (listed as June-Draconids on p. 88) as a ``real shower'' which was excluded from his final catalogue only because of insufficient observation. When the current IMO working list of meteor showers was established, the shower was rejected because its regular activity was practically below the detection limits for many years. However, June is a period of the year which is poorly covered by meteor observations generally, and in particular from the northern hemisphere.

Previous observations of the June Bootids

First, we give a summary of old observations of the June-Bootids regarding their activity. We restrict to those returns where a considerable rate was reported from several locations. Most observers made plots, even when high rates were present. During the 1927 return, plots and counts were made by different observers of the Tashkent group (Sytinsky, 1928). Please note that there are no data for further reduction, such as the limiting magnitude. The data are also diffcult to compare with one another.

Table 1: Activity of the June Bootids during the 1916, 1921 and
1927 returns of the shower which is to be found in the literature.
Rates do not refer to the term ZHR which is in use now, but give
only numbers per hour independent on the observing conditions.
Other papers just give numbers of shower meteors noted by the
observer.

--------------------------------------------------------------------------------------
Date/Time           Activity    Observer                   Source
   (UT)                        and remarks
--------------------------------------------------------------------------------------

1860 and 1861      
Jun 30              ``many''    Lowe                       Denning (1916)
--------------------------------------------------------------------------------------

1916
Jun 28 2225--0010   55 met.     Denning                    Denning (1916)
Jun 29 0045--0115   14 met.     Denning; partly cloudy     Denning (1916)
--------------------------------------------------------------------------------------

1921
Jun 24              2.9/h       summary                    Hoffmeister (1922)
Jun 25              2.5/h       summary                    Hoffmeister (1922)
Jun 26              0.6/h       summary                    Hoffmeister (1922)
Jun 28              7 met.      Denning                    Kronk (1988)
Jun 28              1.7/h       summary                    Hoffmeister (1922)
Jun 28 2145--2250   5 met.      3 obs. Prague              Prey (1921)
                                hazy and Cirrus
Jun 28 2150--2400   5.5/h       Stepanek, Ondrejov         Svoboda (1923)
Jun 28-29 2300-0110 20 met.     Jadot, France              Jadot (1921)
Jun 29.17           7 met.      Dole, USA                  Kronk (1988)
Jun 29              1.1/h       summary                    Hoffmeister (1922)
Jun 29 2135--2310   8 met.      Mrazek, Prague; very hazy  Prey (1921)
Jun 30.10           8 met.      Dole, USA                  Kronk (1988)
Jun 30-31 2110-0050 20met.      Jadot, France; cirrus      Jadot (1921)
Jul 01 2200--2300   6 met.      Heybrock, Frankfurt
                                hazy, clouds               Heybrock (1921)
Jul 03              153 met.    Nakamura                   Yamamoto (1922),
                                                           questioned by Denning (1922)
--------------------------------------------------------------------------------------
          
1927
Jun 24.8            54/h        236 met., 2 obs. Tashkent  Sytinsky (1928)
Jun 25.8            96/h        316 met., 2 obs. Tashkent  Sytinsky (1928) 
Jun 26.8            213/h       1054 met., 2 obs. Tashkent Sytinsky (1928) 
Jun 27.8            357/h       1213 met., 2 obs. Tashkent Sytinsky (1928) 
Jun 26--30          145 met.    Dole, USA                  King (1928)
--------------------------------------------------------------------------------------

The June-Bootid activities of 1916, 1921, and 1927 are quite well documented in the literature. The meteors were often described as faint, but at the same time there were reports of bright meteors and fireballs. A -14 June Bootid fireball was photographed on June 29, 1927 (Yamamoto, 1927).
Denning (1917) immediately associated the shower with comet 7P/Pons-Winnecke, a comet of the Jupiter family. Relatively close encounters with Jupiter caused quite rapid changes of the comet's orbit. These changes shifted the perihelion from inside the Earth's orbit (until 1916) to outside the Earth's orbit (from 1921). The minimum distance between the orbits increased continuously after the 1921 perihelion passage and reached 0.24 AU in 1998 (Fig. 1). So it is quite unlikely that recently released meteoroids approached the Earth in 1998. The event, which is described next, must be linked to meteoroids ejected from the parent comet 7P/Pons-Winnecke earlier in this century.

The 1998 June Bootids: Rates and Radiant

Observers were surprised by a high meteor activity in the night 27/28 June 1998. The display attracted the attention of casual witnesses, because there were numerous bright meteors visible. Due to the short duration of northern summer nights, there were not many reports of regular observers. In total, we received reports or notes from 36 observers:

E. Bojurova (Bulgaria), P. Brown (Canada, radar), G. Carstairs (Australia), S. Crivello (Italy), B. Ewen-Smith (Portugal), M. Dionisi (Italy), D. Girling (Australia), R. Gorelli (Italy), V. Grigore (Romania), R. Haver (Italy), W.K. Hocking (Canada, radar), T. Hashimoto (Japan), D. Ito (Japan), K. Izumi (Japan), P. Jenniskens (USA), J. Kac (Slovenia), K. Kathryn (Australia), A. Knöfel (Germany), H.G. König (Germany), K. Kretsch (Ireland), A. Marsh (Australia), A. Negoescu (Romania), K. Nose (Japan, video), K. Osada (Japan), D. Penn (Portugal), L. Rashkova (Bulgaria), J. Rendtel (Germany), K. Sato (Japan), L.R. Sobkoviak (USA), E. Stomeo (Italy), P. Sütterlin (Germany), K. Suzuki (radio), M. Taylor (USA), J.M. Trigo (Spain), B. Vanderwark (USA), V. Velkov (Bulgaria), R. Vodicka (Australia).

Contrary to most known activity outbursts, this activity lasted for more than 12 hours. This was also reported from the observations in 1916 and 1927. Obviously there was no relation to the actual comet position: the 1916 activity happened almost 300 days after the last perihelion passage (with q=0.970605 AU). The 1927 event occurred just 2 days after the comet passed its perihelion (then q=1.039235 AU). When 7P/Pons-Winnecke passed the perihelion (now q>1.25 AU) last on January 2, 1996, the entire orbit was distant from the Earth's orbit (Fig. 1). Seen the current distance between the orbits of the comet and the Earth, such an enhanced activity was not to be expected. Furthermore, the encounter conditions of the 1998 June-Bootid outburst are of a different type compared to the earlier events of this shower and also compared to the peaks of the Draconids, Leonids, and Perseids, for example.
A considerable amount of 511 magnitude estimates allowed the determination of a population index of the June Bootid meteor shower. We derived r=2.22+-0.07 from observations of the period June 27, 19h30m to June 28, 01h30m UT. This population index as well as the average radiant position of alpha=230^o, delta=+47^o were used to obtain a profile of the ZHR Tab. 2). Highest rates of roughly 100 occurred between June 27, 12^h and 20^h UT. Whereas these rates are based on very few observations, the ZHRs of the period June 27, 20h to June 28, 01^h30^m UT constitute a reliable picture of the activity.

Table 2: Activity of the 1998 June Bootids. The ZHR was calculated
using the radiant position alpha=230 deg, delta=+47 deg and the
population index r=2.22 as determined from the observations.
#Obs gives the number of individual count intervals included
in the average ZHR; nJBO is the number of shower meteors. The
error listed in the last column is the standard deviation of
the average.

-------------------------------------
Date   UT   Sollon #Obs  nJBO ZHR  +-
-------------------------------------
06/26 2310  95.16     2   11  18 +-10
06/27 0730  95.464    1   28  10 +- 4
06/27 1020  95.603    1   21  66 +-29
06/27 1150  95.662    1   25 102 +-41
06/27 1930  95.983    3   40  91 +-29
06/27 2010  95.993    6   68  66 +-16
06/27 2040  96.014    6   69  59 +-14
06/27 2120  96.036    8   97  63 +-13
06/27 2150  96.056   14  150  50 +- 8
06/27 2210  96.072   11  102  48 +-10
06/27 2240  96.093   14  155  60 +-10
06/27 2320  96.122   14  113  53 +-10
06/28 0000  96.144   14  114  50 +- 9
06/28 0040  96.167   10   81  53 +-12
06/28 0100  96.187    6   33  39 +-14
06/28 0120  96.199    2    8  24 +-17
06/28 1220  96.64     1    0   0       (video)
06/29 1100  97.53     6    6   2 +- 2
06/29 2100  97.93     1    0   0
-------------------------------------

Another surprising fact is the large size of the radiant area. This was reported in the early activity events (Fig. 3) as well as during the 1998 display. Other meteor showers producing high rates show a well defined radiant. The analysis of 139 meteor plots by Bojurova, Rashkova and Velkov with the RADIANT software (Arlt, 1992) yields a distinct radiant at (Fig. 4) alpha=230^o+-2^o, delta=+47^o+-2^o (2000.0) which correspods very well to the average radiant position reported by all other observers (Table 3).

Table 3: Radiant positions given for the June Bootids during the returns
in 1916, 1921, 1927, and 1998. Note the large scatter in the radiant
positions observed at the same time. This is mainly due to the fact that
the zenith attraction is not considered. The very low atmospheric entry
velocity of 17.5 km/s causes a zenith attraction which may easily
exceed 10 degrees. If meteors from long obervations are combined, this
will lead to an unsharp radiant. The RADIANT software, however, corrects
each meteor position so that the radiant determined with this tool
show better defined radiants.

----------------------------------------------------------------------
Date, Time           Radiant  Observer and Reference
----------------------------------------------------------------------
1916 Jun 28          203 +53  ? at Birmingham; Olivier (1916)          
1916 Jun 28          221 +56  Denning (1923), no. 183                  
1916 Jun 28          231 +54  Denning (1923), no. 184                 
1916 Jun 28          213 +53  Denning (1923), no. 185a                
1916 Jun 28          223 +41  Denning (1923), no. 185                 
1916 Jun 28          213 +49  Nakamura (in Kronk, 1988)                
----------------------------------------------------------------------
1921 Jun 28          228 +58  Denning (1923), no. 186                 
1921 Jun 28/29       208 +61  Hoffmeister (1922); 12 meteors           
----------------------------------------------------------------------
1927 Jun 26.8        198 +53  2 obs. Tashkent (Sytinsky, 1928)        
1927 Jun 27          213 +55  Dole (King, 1928)                    
1927 Jun 27.8        198 +54  3 obs. Tashkent (Sytinsky, 1928)    
1927 Jun 28.8        198 +54  4 obs. Tashkent (Sytinsky, 1928) 
1927 Jun 29.7        200 +54  2 obs. Tashkent (Sytinsky, 1928) 
1927 Jun 30          218 +60  Dole (King, 1928)                     
1927 Jun 30.7        204 +55  2 obs. Tashkent (Sytinsky, 1928)    
----------------------------------------------------------------------
---- Jun 27--30      212 +58  Bakulin (1973), no. 18 (visual)   
---- Jun 13--Jul 02  229 +48  Bakulin (1973), no. 90 (photographic) 
---- Jul 01          209 +56  Bakulin (1973), no. 52               
----------------------------------------------------------------------
1942 Jul 06          206 +54  Bakulin (1973), no. 29 (telescopic)  
1944 Jun 24          208 +55  Bakulin (1973), no. 30 (telescopic)   
----------------------------------------------------------------------
1998 Jun 27.6        218 +53  report Vodicka and Marsh, 
----------------------------------------------------------------------                              rad. pos. corrected by McNaught (1998, meteorobs)
1998 Jun 27.60       228 +54  Brown and Hocking (1998); radar
1998 Jun 27.60       219 +61  Brown and Hocking (1998); radar, second rad.
1998 Jun 27.9        230 +47  Bojurova, Rashkova, Velkov (pers. comm.)
1998 Jun 27.9        240 +50  Gorelli (1998, imo-news)
1998 Jun 27.9        224 +50  Haver (1998, imo-news)
1998 Jun 27.9        220 +59  Stomeo (1998, imo-news)
----------------------------------------------------------------------

Discussion

When the Earth crossed the meteoroids of comet 7P/Pons-Winnecke in 1916, the particles were far behind the comet -- 298 days. Meteors released from the comet during the perihelion passage in 1915 were substantially disturbed by Jupiter between 1917 and 1919. The closest approach to Jupiter occurred in mid-May 1918 (0.719 AU). The comet and the particles of each ejection phase are disturbed by Jupiter in a different way. This fact should also be the reason for the large scatter of the radiants reported at all occasions. Are the enhanced rates observed at three occasions caused by a superposition of filaments released from 7P/Pons-Winnecke during different perihelion passages, only crossing the Earth's orbit ``by accident'' at (almost) the same time? This could explain both the spread in radiants and the long duration of shower.
In 1916, the enhanced rates were observed when the comet was far away from the perihelion (298 days). 7P/Pons-Winnecke reached its perihelion on 1927 June 21.1, and high rates were observed for more than two nights. The situation was quite similar in 1921, when the perihelion was passed on June 13.4, but the rates remained low.
The available literature and archives do not include hints on significant rates of the June Bootids until 1998. It may well be that a short time activity event of a radiant so far in the northern sky was missed due to the short nights at mid-northern latitudes. However, the analysed returns of the June Bootids show a remarkable long duration, definitely exceeding one night. Difficult to say whether the few meteors reported over the years are real members of a meteoroid stream associated with 7P/Pons-Winnecke or sporadic meteors which are aligned with the large radiant area by chance.

References:

Arlt, R., 1992: The Software Radiant. WGN 20, pp. 62--69.
Bakulin, P.I., 1973: Astronomical Calendar, Constant part. Nauka, Moscow, 1973. (in Russian)
Brown, P., Hocking, W.K., 1998: June Bootid Meteors 1998. IAU Circular no. 6966 (1998 July 4).
Denning, W.F., 1916: Remarkable Meteoric Shower on June 28. MNRAS 76, pp. 740--743.
Denning, W.F., 1917: Meteoric Shower of 1916 June 28, The Observatory 40, p. 95
Denning, W.F., 1922: No Title. The Observatory 45, 83 (comment).
Denning, W.F., 1923: Radiant Points of Shooting Stars observed at Bristol chiefly from 1912 to 1922 inclusive. MNRAS 84, pp. 43--57.
Heybrock, W., 1921: Beobachtung von Meteoren des Winneckeschen Kometen. Astron. Nachr. 214, pp. 215--216.
Hoffmeister, C., 1922: Die Beobachtungen von Meteoren des Winneckeschen Kometen. Astron. Nachr. 215, pp. 455--456.
Hoffmeister, C., 1948: Die Meteorströme. J.A.Barth, Leipzig.
Jadot, A., 1921: Tableau des étoiles filantes enregistrées pendant les 2 nuits des 28-29 juin et 30-1er juillet 1921. Ciel et Terre, 37, pp. 94--95.
King, A., 1928: Meteor Notes -- The Pons-Winnecke shower. The Observatory 51, 25.
Kronk, G., 1988: Meteor Showers. A Descriptive Catalog. Enslow Publ., Hillside.
Olivier, C.P., 1916: The Meteor System of Pons-Winnecke's Comet. MNRAS 77, pp. 71--75.
Prey, A., 1921: Meteoritenbeobachtung in Prag (Sternwartenturm). Astron. Nachr. 214, pp. 39--40.
Sytinsky, N., 1928: Der Strom der Pons-Winneckiden im Jahre 1927. Astron. Nachr. 232, pp. 283--286.
Svoboda, H., 1923: Les météores de la comete de Winnecke. Astron. Nachr. 218, pp. 255--256.
Yamamoto, I., 1922: Observations in Japan of Meteors probably connected with Pons Winnecke's Comet. The Observatory 45, pp. 81--83.
Yamamoto, I., 1923: Photograph of a remarkable meteor. Astrophys. J. 66, pp. 329--332.

Bootid fireball photographed

A June Bootid fireball was photographed by several cameras of the Czech part of the European Network. Trajectory and orbit were determined by Pavel Spurný and Jiri Borovicka of the Ondrejov Observatory.

Figures of the June Bootid text

Fig. 1: Minimum distance between the orbits of comet 7P/Pons-Winnecke and the Earth between the discovery of the comet in 1819 and its last return in 1996. The closest approaches occurred early in this century when the minimum distances were less than 0.03 AU in 1921 and 1927.

Fig. 2: During the 1998 return, the ZHR reached a peak value of 100 and exceeded a figure of 50 for almost one day. The gap between 95.7 deg and 95.95 deg in solar longitude represents the geographical distribution of the observers -- about 4 hours between Japanese and Bulgarian observations.

Fig. 3: Radiant display of the June Bootids 1916. Data of meteor trails from Denning (1916). There is a well-defined radiant at alpha=228 deg, delta=+55 deg which is closest to the no. 184 given by Denning (1923). The display is based on just 17 shower meteors seen on June 28/29.

Fig. 4: Radiant of the June Bootids 1998 derived from plots made by Bojurova, Rashkova and Velkov on June 27/28. The distinct radiant appears at alpha=230deg+-2deg, delta=+47+-2deg.

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