From: James Richardson, AMS, Graceville, Florida
Date: Nov. 22, 1997


Based upon my known system response, I am quite sure, that the
enhancement I monitored was well within the visible range, with very little
enhancement indicated on the faint end of the specrum.  Because Leonids are
so incredibly fast, their faint meteors will ionize at higher altitudes
than other showers.  These higher altitudes will result in higher electron
diffusion rates after trail formation, which, in turn, results in shorter
radio echo durations.  The fainest meteors which are normally detected by a
radio system may be reduced only to meteor head-echoes when Leonids are
considered - if at all.  Counteracting this effect somewhat, the higher
speed meteors will have a higher ionization efficiency,; therefore, more
faint fast meteors will actually be detected than if only the diffusion
rate effect is considered.

Most radiometeor systems, including the professionally run back-scatter
systems, do not detect fastj, higher altitude, underdense trails very
effectively.  This is called the height-ceiling efect, and was first
documented in the early 1960's (see McKinley, 1961).  The Adelade,
Australia, meteor radar scientist reported this year that their modern
system wil only detect the overdense trails for meteors of greater than 50
km/sec, or meteors with altitudes of greater than 100 km.  Their operating
frequency is about 54 MHz, which is quite near my own frequency of 55.26
MHz.

Due to this effect, the Poplar Springs system is most sensitive to faint
meteors with medium to low velocities.  My best overall shower detections
occurs with the Quadrantids and Delta Aquarids.  When meteor speeds reach
those of the Eta Aquarids and Perseids, my detection is generally reduced
to overdense trails only:  meteors within the visible range -- which can
penetrate deep enough into the atmosphere to cause longer duration echoes.
The Leonids certainly fall into this catagory as well, with the higher
ionization eficiency allowing a few faint ones to be seen -- but at very
short durations (usually les than 1 second).

Thus, my system is primarily sensitive to a bright Leonid enhancement much
more than a faint one -- something which i was trying to explain to
Werfried during the chat a couple of weeks ago.

I think the real culprit in why the east coast visual observers did not see
the best enhanced rates was in the time that they occured.

I reported that my rates began to noticeably ramp up around 0330 CST, (0930
UT).  Bob L also reported a noticed increase in rates about 0100 PST (0900
UT).  This would have been around 4 to 4:30 am in the east, with twilight
not far ahead.

My radio rates really began to pick up around 0430-0530 CST (1030-1130 UT),
and Bob also reported a marked increase around 0330 PST (1130 UT).  This
would have been too late for visual observersin the eastern time zone (5:30
am) due to morning twilight.

I had expected the Leonid rate to drop off some as the radiant reached
angular altitudes which are not as effecive for radio scatter, a "lull"
which would be centered on the culmination time for Leo.  However, no such
lull was observed, indicating that the Leonid rate was strong enough to
counteract any attenuation due to geometry.  My best guess is that we
experienced a broad, bright Leonid peak, probably centered around 1200 to
1400 UT.  However, radiant set finally added its own attenuation factor,
and did not allow my system to see the rate ramp down in the same manner
that it had built up.  The drop in rate was much steeper as the Leonid
approached and dropped below the horizon.

I hope that the folks on the main island of Hawaii got a nice break through
the weather, allowing them to potentially see the trailing edge of the
activity.