From: Shelby Ennis, Elizabethtown, KY USA Date: Sat, 21 Nov 1998 19:56:04 +0000 General Amateur Radio Leonids Summary A number of additional reports have arrived. They do not change the radio peak times as given in the preliminary summary, but they do provide a few more details (and questions). In general, they show a picture that is similar to, but differing just slightly from, the early visual reports. Since radio waves at the frequency usually used (144 MHz) are not hindered by clouds or daylight, this seems to be the primary explanation for several of the differences. Other reasons, some comments, and one interesting new report will be discussed below. TIME of the peak: EUROPE is consistent with a peak on the 17th, about 0600 Z +/- 1 hour. This peak was quite broad, with exceptional conditions between 0300 and 0700 Z. EASTERN NORTH AMERICA is consistent with a peak about the same time, approximately 0700 Z, and covering a broad time period. Both the 16th and 17th were very good for hours at a time! It is the WESTERN U.S. that differs somewhat. Several minor peaks are noted. On the 16th, at about 1400-1700. And on the 17th at about 0800-1000 (two mentioned the 0700 period), and 1300-1700. Actually, as several pointed out, there seemed to be no true peak for those west of approximately 100 degrees W. Long. While the 0800-1000 time on the 17th was mentioned by several as a "good" time, more commented that the Western U.S. may have done better on the 16th. As more operators were available on the 17th, this appears to be a valid conclusion. (The radiant would not be above the horizon for most of them until approximately 07 Z). As in the visual reports, many said that this was one of the most spectacular showers they had ever experienced. The (overdense) burst rate was not quite as high as it was during the peak time of several of the early Perseid peaks earlier this decade. But, instead of lasting for an hour or two, it just kept coming back again and again, with one minor peak after another, apparently as the earth intercepted yet another filament of debris. Short, underdense "pings" were almost non-existent (as has been true during the best peaks of the Perseids). However, several commented that the signals, in general, were not quite as strong as those experienced during the Perseids peaks recently. This is surprising, in light of the number of very bright visual fireballs reported. There were reports of a large number of long distance contacts (beyond 1200 miles/1930 km, with several beyond 1400 miles/2270 km). This would imply strong ionization and a greater than average altitude, as would be expected due to the velocity of this shower and the large number of fireballs reported. The ionization of the E layer should have been increased by this bombardment. However, only 4 reports mentioned a residual background signal. Three of these (from North America) said that this was present in the 50-65 MHz range. One report from Europe said, "we had an Ionization of the e-layer for example SM5BSZ (1200 Km) was readable for almost 2 hour continuous with an 'auroral' sound." Note - it was requested that a list of contacts over about 1950 km be listed. However, the quantity of reports giving this specific information is insufficient to justify it at this time. There were several over 2250 km reported on 144 MHz; and a new North American record may have been set on 432 Mhz (2036 km). Two North American reports brought the first examples ever noted on propagation changes going in "waves" from east to west, over periods of an hour or two. At first this simply appeared to be caused by the radiant's rising in the east and moving west. However, one of the reports indicated two or three of these "waves" each day. Forward scatter of radio waves is most "efficient" or "effective" at different times for different directions, depending upon the geometry of the incoming meteors as determined by the position of the radiant. The Leonid shower is considered to be primarily a north/south-path shower. Several said that they did not find the "effectivity" charts to be valid for this shower (perhaps not surprising, with the large number of fireballs), while several others said that the charts were quite accurate. A quick examination was made of best "effectivity" or "efficiency" in different directions at different times for the part of the midwestern U.S. where this "east to west movement" was reported. MSSoft V. 5 was used for this. There seemed to be a small correlation, especially for the northeast. As the antennas commonly used at this frequency are highly directive (that particular station's antenna should have a forward gain of about 14 dBd, with the expected sharp pattern), signals off the side of the antenna would be greatly attenuated. So when pointing NE, little would be expected from the NW, etc. As the "effectivity" to the NE lessened while increasing slightly to the NW, this would normally cause an operator to rotate the antenna toward the direction of the best signals. Thus, the two effects together - changing effectivity and a highly directional antenna - would possibly explain this observation. This still does not completely cover four separate "waves" noted on the 17th. But the "effectivity" plots are simplified; and there is always the subjective element involved, when routine contacts are being made instead of continuous recording of a signal. As this is the first time this effect has been reported, it is impossible to draw any further conclusions. Thus, it is most likely a real phenomenon, but also caused somewhat by the subjective element introduced by the directional antenna. Reasons for the Differences in Peaks and Times between Radio and Visual Observations. Due to the hopes for a major shower or even a "storm", there were hundreds, probably several thousand, Amateur Radio Operators world-wide operating VHF meteor scatter during the Leonids. Thus, most of Europe and North America should have been well covered (almost no radio reports have been received from Eastern Asia or Australia). Because of the directional nature of the high-gain antennas (and the geometry of the shower) and the fact that communication with a number of stations is the goal, it requires a number of reports to even out these differences. But, as was learned at the time of the early Perseids peaks a few years ago, the observed peak times can be quite accurate. At the time of the reported peak (06-07 on the 17th), it would have been daylight in Europe, while the radiant would still have been rather low in North America until about this time. In summary, the long, broad maximum period of overdense bursts is unlike anything ever experienced. It covered most of the time on the 16th that the radiant was above the horizon (especially for those in Western North America) and then repeated on the 17th, with this being the better day for most European and Eastern North American operators. The greatest peak seemed to be about 0700 Z on the 17th. While there have been better one-hour peak periods in the past, this may have been the best Shower ever experienced.