Our identification of the location of at least one event provides strong support for the ground reflection model. The fact that it is a cloud pulse and not a cloud-to-ground stroke provides equally strong indication that the middle-atmosphere double-discharge model is not appropriate since upward-going discharges are associated with positive cloud-to-ground strokes [Boccippio et al., 1995]. Furthermore, the calculations upon which this initial hypothesis was based has recently been redone with significantly different results [Symbalisty et al., 1997]. For the tropospheric-cloud double- discharge mechanisms to be correct one would have to assume that the pulse separation was coincidentally the same as the echo delay expected for this event. It seems clear from this triangulated example and the untriangulated results of Zuelsdorf et al. [1998b] that TIPPs are generated in a single event near the tops of clouds followed by the reflection of that energy by the surface of the Earth. The inference by Zuelsdorf et al. [1998b] that the energy of the pulse does not travel much beyond the line sight from the cloud seems to be confirmed by our case study in which only two of the five neighboring NLDN stations detected the pulse that caused the TIPP under discussion.
If we examine the various electric phenomena that are associated with
thunderstorms we can identify one that is a good candidate for being associated
with TIPP production.
Le Vine [1980] found that the strongest
producer
of radio frequency noise had a bipolar structure in which a strong pulse was
followed by an overshoot in the other direction over a period of 10-20 
s.
These pulses occurred in isolation. Later
Willett et al. [1989] studied
these same pulses, naming them Narrow Positive Bipolar Pulses. Both conclude
that these pulses are the strongest source of cloud generated HF radiation.
However,
Willett et al. [1989] and
Smith et al., [Direct isolated
thunderstorm radio emissions, unpublished manuscript, 1998; hereafter referred
to as SSHR98] agree that NPBPs have a weaker amplitude than cloud-to-ground
flashes as measured when sampling the fast electric field. This is consistent
with our limited detection by NLDN stations of TIPP correlated pulses, whereas
cloud-to-ground pulses are detected at many stations. Furthermore, our
correlated pulses do not exhibit a signal strength any stronger than the
uncorrelated cloud pulses detected by NLDN stations
[Zuelsdorf et al.,
1998b]. That we are able to locate the TIPP source at an altitude of 8.0 
0.7 km on an isochron in the vicinity of an active storm lends further support
to the theory that TIPPs are associated with NPBPs
[Smith et al., 1997].
In addition, the summation of our rise and peak-to-zero times is not
inconsistent with the
Willett et al. [1989] measurement of the total
duration including the undershoot of the pulse being 30 s or SSHR98 who
measure 25.8 4.9 s. These observations appear to be entirely
consistent with the narrow bipolar pulse source for TIPPs but, as in our study,
they do not identify what process in clouds leads to the production of these
very special, radio-frequency producing events.
In closing we note that we used the NLDN raw data not because it was
ideally suited for the detection of the cloud pulses responsible for TIPPs.
These data were chosen because of their retrospective availability and because
we did not know with what cloud discharges TIPPs would correlate. The high
bandwidth recordings of waveforms such as those performed most recently by
Smith
et al. [1998] are much more suited to the study of these pulses. The short
duration of these bipolar pulses suggest that the NLDN stations that do not
record pulses of duration less than 4 s could miss many pulses especially
close to their source before they dispersed. Moreover, on strong short pulses
it is possible that NLDN stations could trigger on the long negative
"undershoot" portion of the bipolar pulse. We also note that the signal may
propagate over the horizon from the cloud pulse for stronger events. The radio
horizon is further than the line of sight optical horizon and a "ground wave"
may also extend the range of the signal
[Proctor, 1995]. Nevertheless,
the coincidence of cloud to ground activity with our isochron deduced solely
from the two cloud-pulse delays gives us much confidence that we have indeed
found the geographic location of the source of at least one TIPP detected by
Blackbeard.