June Bootids 2000

The excellent conditions with a New Moon on July 1 were used by many observers for monitoring the activity of the June Bootids despite activity was not expected to be extra-ordinary. Nevertheless, the detection of a weak, but distinct annual activity was a challenge for visual observing experts, too.

In 1998, the June Bootids produced an outburst of activity with ZHRs near 80, possibly reaching 200, near a solar longitude of 95.7 degrees (J2000.0). The below Table gives an overview of the activity of the June Bootids in 2000. No outburst was observed. The apparent noise of ZHR values suggests a typical visual detection limit of minor showers near ZHR=1. Significant activity was therefore observed between June 27, 0400-2300 UT. This is about 0.3 to 0.8 degrees later (or 7 to 19 hours later) than in 1998. It will be worthwhile to revisit previous moon-free years to confirm an annual activity level of ZHR = 2 to 3.

The radiant position of the June Bootids was found to vary considerably during detections in previous years back to 1916. This could be a physical effect of orbital perturbations, but is strongly enhanced by a purely geometrical effect: As the meteoroids approach the Earth from behind, vectorial addition with the motion of the Earth is very sensitive to slightest orbital variations. (The reader may check this with vector parallelograms.)

Additional uncertainties are thus being introduced if the actual radiant position was not equal to what observers used for shower association. It is strongly suggested to send in the details on plotted meteors for a more thorough analysis. A first radiant analysis of data obtained by ARLRA, BOJEV, RASLI, SAREL, VELVA, MIHMI, YORYO in the observing camp at Avren, Bulgaria, delivers a distinct source at alpha=215, delta=+47. Whereas the radiant is sharply defined at its western edge, a "tail" of convergences extends towards east from this position. The radiant position is corrected for zenithal attraction and diurnal aberration.

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Date    Time (UT), avg.  Sollon nObs nIND  nJBO  ZHR  +/-
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Jun 26  2300-0650  0250  94.81    6    3     2   1.3  0.8
Jun 26  2100-2300  2200  95.57    5    5     2   0.8  0.5
Jun 27  2300-0252  0040  95.68    9    5     4   1.3  0.6
Jun 27  0400-1102  0730  95.95    5    2     8   3.6  1.2
Jun 27  2010-2300  2135  96.51   10    8    14   1.9  0.5
Jun 27  2300-0020  2340  96.59    9    9     2   0.6  0.3
Jun 28  0300-0740  0520  96.82    6    2     4   1.2  0.6
Jun 28  2020-0100  2240  97.51    8    6     1   0.4  0.3
Jun 29  2010-2330  2150  98.43    4    3     0   0.4  0.4
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Solar longitudes refer to equinox J2000.0. nObs is the number of individual observing periods, nIND is the number of individual observers providing them, nJBO is the number of June Bootids seen. The the radiant position was assumed at alpha=224, delta=+48, the population index used wa r=2.2 as derived from the outburst in 1998. The extremely low meteor numbers require a careful statistical treatment. For a given meteor number, various true rates would have been possible. The expectation value of the ZHR is, therefore, the average of all these rates (the integral over the distribution function) which are naturally all greater than or equal to 0. The resuls is most obvious in the last row (June 29): 0 meteors could have been caused by ZHRs between 0.0 and 0.8. The longer the observers keep seeing 0 meteors, the lower the expectation value for ZHR. The actual small-number ZHR formula is

ZHR = (1 + sum nJBO) / sum(Teff/C),

where Teff is the effective observing time and C is the total correction composed of limiting magnitude, clouds, and zenith correction. Zenith attraction was not taken into account here; most of the observing periods had radiant elevations hR>40 degrees.

We are grateful to the following observers for the quick submission of data and careful shower association of their meteors:

ANTKA Karl Anthier (France), KACJA Javor Kac (Slovenia), ARLRA Rainer Arlt (Germany), LINMI Mike Linnolt (USA), ATAJU Jure Atanackov (Slovenia), LUNRO Robert Lunsford (USA), BOJEV Eva Bojurova (Bulgaria), MARPI Pierre Martin (Canada), BUCAN Andreas Buchmann (Switzerland), MIHMI Mihail Mihov (Bulgaria), COOMA Mary Cook (UK), RASLI Lina Rashkova (Bulgaria), DECGO Goedele Deconink (Belgium), SAREL Elena Sarbinska (Bulgaria), GONRU Rui Goncalves (Portugal), STALE Leo Stachowicz (UK), HALCA Cathy Hall (USA), VELVA Valentin Velkov (Bulgaria), HAVRO Roberto Haver (Italy), VERJN Jan Verbert (Belgium), JOHCA Carl Johannink (the Netherlands) and YORYA Yordan Yordanov (Bulgaria).