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2011 Geminids

Here is Alastair McBeath's analysis of the 2011 Geminids:

Today it's the Geminids' turn for an update on how they performed last December. Although the shower's maximum has been reliably strong for many years, with a peak likely to last for almost a day at fairly similar ZHRs, the bright Moon and some typically dismal northern winter weather globally meant observers almost everywhere struggled to see much of the shower's best in 2011. However, it has been possible to compile the radio meteor analysis now, which when combined with data from other sources and observing techniques, gives a modest, if incomplete, overview of the near-peak activity.

More visual results were received by the SPA for the Geminids than for most of the other Moon-affected showers during late 2011, albeit many were quite casually made and reported, owing to poor skies across Britain and elsewhere. Even the IMO's "live" online preliminary visual results were quite patchy, indicating a probable main maximum time near 15h UT on December 14. It has remained unclear how accurate the estimated strongest ZHR of 198 ± 13 then really was, since it is possible the value was inflated thanks to the bright sky. Usually, the ZHR would be around 120-130 or so. The timing fell well within the predicted maximum range based on long-term IMO visual studies, expected to persist from roughly 01h-22h UT on December 14.

Intriguingly, the IMO data also hinted at a possible secondary maximum outside this predicted peak period, around 02h-03h UT on December 15, with ZHRs estimated at ~148 ± 13. Casual reports, and comments based on fireball camera observations in America from Bill Cooke (NASA's Meteoroids Environment Officer; helpfully forwarded by Rich Taibi), indicated that the Geminids overall appeared to have been significantly brighter on December 14-15 than 13-14. This was not unexpected, as previous studies going back to the 1960s have found mass-sorting of particles to be present within the Geminid meteoroid stream, meaning brighter shower meteors tend to happen predominantly somewhat after the visual peak.

The IMO's video results for the Geminids (published in "WGN", 40:2 for 2012 April, pp. 69-75, especially pp. 69-70) found only a single, noticeably sharp, maximum at 03:15 ± 15 minutes UT on December 14. As most of the video results were obtained from Europe, there were large gaps in these as well as the visual data, so uncertainties have remained regarding these seemingly discrepant peak timings. It is worth remembering that many video meteor cameras are quite infra-red sensitive, so can detect a range of meteors from the visual down into the sub-visual. Possibly, the video results indicated something of the "fainter meteors" peak earlier in the likely maximum interval.

Using the available radio meteor data from North America and Europe, the SPA's radio analysis found most systems sufficiently active and recording accurately favoured the better Geminid rates as having happened on December 13-14, between approximately 23h-09h UT. Although it is difficult to be certain, it seems plausible there were two stronger phases within this time, from about 23h-01h and 03h-05h UT, the latter perhaps very marginally the better-detected. European radio data collected during the period the Geminid radiant was regrettably undetectable for most of the operational North American observers, also indicated a distinct secondary peak during the interval from ~23h-01h UT on December 14-15 (remembering that the radio meteor results are typically given only in one-hour long data-bins). This included peaks in the few longer-duration echo-count results presented, which would tally with the generally brighter Geminids reported visually on the latter night, assuming as we usually do that longer-duration radio echoes equate with brighter meteors. These factors may explain why the IMO visual results favoured a Geminid maximum on December 14-15, while the video data preferred the previous night. As always when we have too little data to work with, we end up with more questions than proper answers!

A list of observers whose data contributed to this report can be found at : forum.popastro.com/viewtopic.php

Many and grateful thanks go to everyone for your results and comments on the Geminids.
Alastair McBeath,
Assistant Meteor Director, Society for Popular Astronomy

Added by:  Tracie Heywood