Orwell Astronomical Society (Ipswich)
Leonid Meteor Shower, 17-19 November 1999
Explanation and overview of the Leonid meteor shower.
The Leonid meteor shower in 1998 produced a spectacular display and observers therefore approached the 1999 event with high hopes. Professional analysis of the 1998 observations suggested that 1999 would produce even higher rates, though fewer fireballs. In fact, work by David Asher of Armagh Observatory and Rob McNaught of the Australian National University accurately predicted the time of the peak in 1999, but underestimated the maximum rate, which was observed at more than 5000 ZHR. (ZHR = Zenithal Hourly Rate is the standard measure for meteor activity. It corresponds to the number of meteors that a hypothetical observer would record in an hour with the radiant at the zenith.)
Three members of OASI reported observations of the 1999 Leonids as follows.
Meteors entering the Earth's atmosphere vapourise and produce ionised trails at altitudes of around 100-110 km. These can be detected because they reflect radio waves. Radio observation of meteors typically uses one of the following techniques:
The ideal frequencies for radio observations of meteors are within the VHF range from around 40 to 70 MHz. Below 30 MHz, the ionosphere reflects all radio waves so meteorite ionisation has no effect. Above 80 MHz, the reflectivity of meteor trails starts to decline, although higher frequencies are possible using specialised equipment. For many years, radio amateurs have taken advantage of ionisation trails, both during and outside recognised meteor showers, to make radio contacts at distances up to around 2,300 km. For this purpose, they typically use frequencies around 50 MHz and around 144 MHz, and employ specialised equipment and operating procedures.
The peak of the 1999 Leonids was predicted for 02:00 UT on 18 November. Unfortunately, just two hours before the peak, the sky above Ipswich clouded over preventing visual observations and photography. (Undeterred, the newspapers the next day passed off pictures of star trails as Leonid meteors!) Prevented from visual observation, I set out to prove that a standard broadcast band FM receiver was capable of detecting meteor pings from commercial broadcast stations located beyond the receiver's horizon. I did not know the frequencies and locations of broadcast stations near the lower end of the band at the time of observation, so a combination of trial and error and good luck was involved. I used two frequencies, 87.5 and 87.6 MHz, and both proved successful. Of the two, 87.6 MHz provided the better results but, by the time I used it, the peak meteor activity had long since passed and pings were much less frequent. Both frequencies are slightly above the frequency where meteor trail reflectivity starts to decline.
I used the FM radio and tape portions of a JVC DR-E2L Midi System and an indoor antenna. There was nothing special about the equipment: it was unmodified and there is no reason why any other hi-fi system or indeed a car radio could not be used to achieve similar results. I used an antenna composed of a few metres of wire connected to the 75 Ohm input. A high gain antenna with better matching, for example a Yagi, would provide better results. I started observations around 02:15 UT, just after the predicted peak of the shower. Initially, I tuned the radio to 87.5 MHz, the lowest frequency that the digital tuner was capable of. I heard lots of pings during the first few minutes, and recorded them on one side of a C90 cassette. The following is typical of what I heard: several pings of a radio beacon, the last being the strongest, followed by another burst which appears to be an FM station off-frequency. I was disappointed not to hear any FM stations clearly although I heard the beacon signal many times, albeit sometimes weakly, for periods of between five and 20 seconds.
Three short pings plus radio off frequency
At 03:00 UT, I changed the tape and also changed the FM radio frequency from 87.5 to 87.6 MHz. This yielded much better results: stronger pings from the beacon (perhaps 87.6 Mhz is closer to the correct frequency) and pings from some FM signals, although none sufficiently long to enable positive identification.
Examples of radio stations:
Short radio bursts
Examples of the beacon signal:
Creation of the .wav files: I marked the points where the best signals were heard using the counter on the tape deck, then transferred the audio extracts to a Windows 95 PC. I used the Windows Recorder application with sampling frequency set to 8 kHz, 8 bit mono mode.
I spent a week at Field View, the astronomy bed and breakfast centre at East Barsham, near Wells-Next-The-Sea. The accommodation was fully booked around the peak night of the Leonids and Nicky and I took the last available room.
The night of 18-19 November was crystal clear early in the evening. Unfortunately, walking back from a meal at The White Horse at 21:00 UT we saw cloud creeping over the Moon, and gradually filling more and more of the sky. The weather forecast predicted clear skies down the middle of the country so we assessed our options for travelling west. The proprietors of Field View (Christine Parker and Simon Batty) had a plan to take a minibus full of guests up to 50 miles to find clear sky but after hearing that it was cloudy in Peterborough they decided to stay put. They have found that the sky often clears at around 02:00 UT and over a year an average of 50% of nights are clear in their area.
At around 23:15 UT I saw a bright meteor (possibly a fireball: I believe magnitude -4 or brighter is defined as a fireball) through cloud which obscured everything except the Moon and Jupiter. Even its train was visible briefly. At that point we decided that things were hotting up and that the cloud appeared to be here to stay, so we decided to head south/south-west. After driving for just over an hour we were about 10 miles from Cambridge and we stopped to check the conditions. It was overcast in all directions so we headed back to East Barsham. For much of the way back we could just see Jupiter but cloud persisted. At Kings Lynn (just west of East Barsham), even Jupiter disappeared. We arrived back at East Barsham at 01:30 UT and stepping out of the car we were amazed to see stars, albeit only 3rd magnitude or brighter. Those who had remained at East Barsham had been observing since 00:30 UT under gradually improving conditions and had seen Leonids with increasing frequency including one bright meteor which fragmented into several pieces. We continued to watch as the cloud occasionally thickened but more often thinned. By 02:00 UT the cloud was fairly thin with occasional gaps and over the next twenty minutes it cleared completely overhead and to the north and east and remained clear for the next 30 minutes. At this time we often saw two or more Leonids together or in rapid succession and, occasionally, sufficiently many appeared within a short period as to give the impression of meteors emanating from the radiant. We also saw two more fireballs. By the end of this period, Leonid rates dropped and the cloud began to roll in again and by 03:00 UT it was almost totally overcast.
One observer at Field View counted 400 Leonids over the two hour period 00:30-02:30 UT despite the poor limiting magnitude for much of this time. The variable conditions meant that none of the observers conducted a formal watch with brightness or timing information but several tried photography. I may have seen (at a guess) 10 to 20 meteors during a five minute interval at the peak. It was the best shower I've seen (I missed last year's Leonid fireball shower).
On the way back home, we stopped off at the Norwich Astronomical Society (NAS) observatory at Seething. Although NAS had clearish patches before and after the peak, they had been under fairly thick cloud during the peak itself.
Nigel travelled to the Sinai Desert with Explorers Tours to observe from clear, dark skies. A selection of his photographs is reproduced below.
Neil Morley, Pete Richards & Nigel Evans