A method is described for determining the return stroke polarity of distant lightning for distances beyond 600 km by detecting the electric field associated with a return stroke of distant lightning, and processing the electric field signal to determine the polarity of the slow tail of the VLF waveform signal associated with the detected electric field. The polarity of the return stroke of distant lightning is determined based upon the polarity of the slow tail portion of the waveform

Blakeslee, Richard J.

Brook, Marx

English

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NASA Case No. MFS-26102-I-CU
PATENT APPLICATION _BSTRACT
Method and Apparatus for Determining
Return Stroke Polarlty of Distant Lightning
The present invention is concerned with
5 determining the return stroke polarity of distant
lightning and is useful in improving the pre-
dictability of thunderstorms and related weather
conditions at distances greater than 600 km. At
these distances, ground wave attenuation has here-
I0 tofore limited the recovery of polarity
information.
Figure 1 shows a flat plate antenna i0 which
detects the electric field associated with the
return stroke of distant lightning, a broad band
15 charge amplifier 14 for amplifying the antenna
output signal; in Fig. I, i0 and 14 constitute an
electrl field-change meter (also known as a slow
antenna). The slow antenna signal goes to a low
pass filter 16 for filtering out radio frequency
20 interference, and the trigger circuit 18 activates
a transient digitizer recorder 20 that outputs a
VLF waveform including a slow tail portion
associated therewith. Figures l(a) and l(b) show
typical VLF waveforms produced by the recorder
25 which respectively exhibit a positive slow tail
(waveform A) and a negative slow tail (waveform B).
The polarity of the slow tail is used to determine
the polarity of the lightning return stroke.
The novelty of the invention appears to be in
30 the use of the slowtail portion of the VLF wave-
form based on the electric field of the return
stroke of distant lightning to determine the po-
larity of that return stroke. The invention is
particularly applicable to lightning beyond 600 km
where severe ground wave attenuation tends to des-
troy useful information in the detected electric
field. The invention can be used to upgrade the
5 predictability of thunderstorms and associated
weather activity by recovering stroke polarity
information associated with lightning at greater
distances than can presently be done, especially
out over the oceans.
Inventors : Richard J. Blakeslee and Marx Brook
Employers:
Date Filed:
Serial Number:
Blakeslee - New Mexico Institute of Mining
Brook - NASAMarshall Space Flight Center
August 28, 1990
07/571,687
NASA MFS26102-I
METHODAND APPARATUS FOR DETERMINING
RETURN STROKE POLARITY OF DISTANT LIGHTNING
Inventor: Brook, M., et al.
NASA Case No. MFS-26102-I-CU PAT_ APPLICATION
METHOD AND APPARATUS FOR DETERMINING
RETURN STROKE POLARITY OF DISTANT LIGHTNING
ORIGIN OF THE INVENTION
The invention described herein was made, in part, by
an employee of the United States Government and may be
manufactured and used by or for the Gove_T_nent for govern-
mental purposes without the payment of any royalties thereon
or therefor.
FIELD OF THE I_ON
I0 The present invention relates to a method and apparatus
for determining return stroke polarity of distant lightning
and, nmre particularly, to a method for determinir_ return
stroke polarity of lightning greater than 600 km away by de-
termining the polarity of the first hump in the sl_ tail
15 portion of the VLF waveform associated with the electric field
produced by the lightning.
20
25
BACKGROUND OF THE INVENTION
Data on lightning and thunderstorm electrical activity,
such as occurrence frequency, spatial distribution and temporal
properties, have become available in significant quantity and
detail in recent years. Reference is made, for example, to
Mach, et al. "Site Errors and Detection Efficiency in a
Magnetic Direction-finder Network for Locating Lightning Strikes
to Ground," J. Atrnos. Oceanic Technol. 3, 67-74 (1986); and
Orville, et al. "Cloud-to-Ground Lightning Flash Characteristics
from June 1984 through May 1985," J. Geophys. Res. 92, 5640-
45644 (1987). More recently, it has been recognized that
polarity patterns of lightning occurrence in mesoscale systems
appear to be related to the direction of the upper level winds.
- 2 -
Reference is made to Orville, et al. "Bipole Pat-
terns Revealed by Lightning Locations in Mesoscale
Storm Systems," Geophys. Res. Let_,, (Feb., 1988)
In view of this recent accumulation of data,
5 there is a growing awareness that return stroke
polarity may be an important indicator of certain
phenomena associated with thunderstorms. For
example, there is evidence that the polarity of
lightning strokes, i.e., the return stroke polarity
i0 of lightning, may be an indicator of the progress
of a thunderstorm and, hence, an understanding of
conditions which lead to the dominance of one
polarity or the other is of importance. One result
of recent studies involving return stroke polari-
15 ties is the finding that positive ground stroke
currents constitute approximately 1 percent of the
total return strokes in summer storms, but dominate
in winter storms, with current rates being as high
as 95 percent.
20 Previous studies to determine polarity of
return strokes of lightning have been limited to
600 km or less because of the attenuation of the
lightning ground wave over distance. Errors in
determining polarity of distant lightning
25 (occurring at greater than 600 km away) can occur
because the ionospheric reflection of the lightning
stroke, unlike the ground wave, is not attenuated
much but is usually reverse_[in polarity.
Studies of distant lightning (occurring at
30 greater than 600 km away) is generally performed by
measurement of the associated electric fields in
the form of radiated waveforms. Return strokes of
both negative and positive polarity exist in such
distant radiated wavef0rms, and an effort has been
- 3 -
made to provide closer examination of the waveforms
of the radiation fields. One observation that has
been made based on this examination is that many
lightning VLF waveforms from a distance beyond 500-
5 600 km exhibit well-defined, positive or negative
slow tails, with a slow tail being defined as a low
frequency dispersion effect occurring in the zeroth
mode of the earth-ionosphere wave guide.
While the observation of the slow tails in a
I0 lightning electric field dates back to 1926, numer-
ous questions regarding some of their properties
have remained to be explained, including the
important question of determining the specific
source of this radiation.
15 SUMMARY OF THE INVENTION
Broadly speaking, the present invention is
based on the discovery that the polarity of the
slow tail portions of VLF waveforms based on light-
ning return strokes can be used in determining the
20 polarity of the lightning strokes from very distant
storms. This is significant for distant lightning
(over 600 km) because, as mentioned above, the
severe attenuation of the lightning ground wave
over long paths has _ereto_ore limited the exami-
25 nation of waveforms produced from lightning return
strokes to those occurring at 600 km or less. At
greater distances, any polarity information con-
tained in the return stroke is generally found to
have been lost in the initial portion of the VLF
30 signal and, as a consequence, a polarity determina-
tion of the return stroke has been difficult to
obtain. Even when such determinations are made,
they have been found to be unreliable for distant
- 4 -
lightning particularly when sought to be obtained
from the initial portion of the VLF signal.
However, applicants have discovered that a determi-
nation of the polarity of the slow tail portion of
5 the VLF waveform, and specifically of the first
hump in the slow tail, provides information as to
whether the electric field has a negative or a
positive return stroke polarity. With the advent
of lightning location networks, i.e., multiple
I0 recording stations separated over a distance where
data from the lightning flashes can be correlated
over the telephone, it has become feasible to
identify such polarity information relating to the
slow tails of lightning waveforms and thus to
15 enable, in accordance with the invention, a
determination of the return stroke polarity of
distant lightning.
It is therefore an object of the present
invention to provide a reliable method and related
20 apparatus for determining lightning stroke polarity
of distant lightning based upon a measurement of
the polarity of the first hump of a slow tail of
the VLF lightning waveform.
With this and other objects in view, there is
25 provided a method for de£e_ining a return stroke
polarity of distant lightning comprising the steps
of receiving an electric field associated with a
return stroke of distant lightning and producing
based thereon a corresponding VLF waveform includ-
30 ing a slow tail, and determining the polarity of
the slow tail to thereby determine the polarity of
the return stroke of distant lightning based upon
the polarity of the slow tail.
- 5 -
In a preferred embodiment, the method involves
receiving the electric field by means of a flat-
plate antenna connected to a wideband charge
amplifier, line driver, and low pass filter, and
5 outputting the processed signal to a transient
digitizer recorder having a 2 MHz sampling rate
with a 12 bit resolution. Further, the recorder is
triggered on by means of a positive or negative
lead signal produced from the detected electric
I0 field. As an alternative method to the triggering
step, a magnetic direction finder unit, such as a
LLP direction finder, can be used to produce a
trigger on signal to the recorder. The polarity of
the return stroke of distant lightning is deter-
15 mined based upon a characteristic of the slow tail
and, specifically, based upon the polarity of the
initial portion, i.e., the first hump, of the slow
tail of the VLF lightning waveform.
In accordance with the foregoing, there is
20 provided apparatus comprising a flat plate antenna,
e.g., for detecting the electric field, a broadband
charge amplifier with a long time constant for
converting the induced charge on the antenna to a
voltage signal, a filtering means for removing
25 undesired signals such as radio frequency inter-
ference, and triggering means associated with a
transient digitizer recorder for producing an
output, such as a waveform, of the electric field,
including the slow tail portion of the VLF waveform
30 associated with the electric field.
Other objects, features and advantages of the
invention will be set forth in, or apparent from,
the detailed description of preferred embodiments
of the invention which follow.
- 6 -
BRIEF DESCRIPTIQN OF THE DRAWTNG$
Figure l(a) is a graph showing a lightning
return stroke VLF waveform exhibiting a positive
slow tail;
5 Figure l(b) is a graph showing a lightning
return stroke VLF waveform exhibiting a negative
slow tail;
Figure 2 is a block diagram of instrumentation
used to measure the positive and negative slow
i0 tails of Figures l(a) and l(b).
DETAILED DESCRIPTION OF THE pREFERRED EMBODIMENTS
Figures l(a) and l(b) show typical return
stroke VLF waveforms of distant lightning which
show a slow tail portion associated with each
15 waveform. Figure l(a) exhibits a positive slow
tail, identified by the letter A, and Figure l(b)
exhibits a negative slow £aYl, identified by the
letter B.
Figure 2 shows, in block diagram form,
20 measurement apparatus of the invention, and as used
in carrying out the method of the invention. A
flate plate antenna i0, connected to a broadband
charge amplifier and line driver 14, with a I-i0
second time constant, is used for receiving a
25 lightning return stroke electric field, indicated
at 12. Together the flate plate antenna i0 and
charge amplifier/line driver 14 comprise a slow
antenna system or electric field change meter (E-
field meter) for receiving electric field 12. A
30 signal from charge amplifier/line driver 14 is then
passed through low pass filter 16 for rejection of
undesired signals, e.g., local radio interference,
while also retaining a wide band response (i micro-
- 7 -
second rise time) down to DC as determined by the
time constant associated with the slow antenna
system 10,14. An output filtered signal from
filter 16 is inputted to a trigger circuit 18.
5 Trigger circuit 18 responds to both positive and
negative polarities, and has an adjustable thres-
hold level for outputting a trigger signal, upon
receipt of either a positive or negative polarity
signal, to start the recording of the return stroke
i0 VLF waveform by a recorder 20.
In noisy environments, the upper frequency of
low pass filter 16 can be reduced to as low a
frequency as I00 khz, causing a reduced bandwidth,
without affecting the slow tail signal. Therefore,
15 in one embodiment, amplification of the signal is
added by optional amplifier 17 in (Figure 2) when
using such a reduced bandwidth. If a reduced
bandwidth is not used in low-pass filter 16,
optional amplifier 17 is not needed, and the output
20 of low pass filter 16 would be connected directly
to trigger circuit 18, as previously discussed.
In an alternative embodiment of this appara-
tus, trigger circuit 18 is replaced with a magnetic
direction finding unit, such as a LLP direction
25 finder, as indicated by the dashed lines of Figure
2, set up near the measurement apparatus. LLP
direction finder 22 receives electronic field 12
concurrent with its recept by slow antenna system
I0, 14. The output from LLP direction finder 22
30 supplies the trigger voltage to recorder 20, and
trigger circuit 18 is deleted from the measurement
apparatus. The output of low pass filter 16 (or of
optional amplifier 17) if fed directly to transient
recorder 20. The LLP electronics normally used in
- 8 -
return stroke waveform observation have a low
frequency cut-off of 1 khz which eliminates the
slow tail part of the signal. This cut-off is
avoided with use of the slow antenna having a
5 virtual DC response.
Considering the operation of the apparatus of
Figure 2, the electric field 12 associated with a
return stroke of distant lightning is detected by
means of slow antenna system I0 and 14. As noted
i0 above, the slow antenna isystem i0 and 14 provides a
near DC frequency response which makes possible the
observation of slow tails on the VLF waveform asso-
ciated with the received or detected electric field
12. An output signal indicative of detected
15 electric field 12 is then converted by broadband
charge amplifier and driver 14 to a voltage signal,
the voltage signal then being filtered by low pass
filter 16 to remove undesired signals, such as
radio frequency interference. As discussed above,
20 the slow antenna filtered signal is then inputted
to trigger circuit 18 which turns on recorder 20 in
response to receipt of the processed signal from
slow antenna system I0 and 14. Alternatively,
where a reduced bandwidth is used in low-pass
25 filter 16, an optional amplifier 17 is used prior
to trigger circuit 18. In the preferred
embodiment, recorder 20 is a transient digitizer
recorder with a 2 MHz sampling rate and a 12 bit
resolution, and outputs a waveform comprising a
30 slow tail portion thereof, from which the polarity
of the first hump, i.e., the initial polarity, of
the slow tail can be determined. As discussed
above, this slow tail polarity is indicative of the
return stroke polarity of the distant lightning.
- 9 -
Also as discussed hereinabove, trigger circuit
18 can be replaced by a LLP direction finder 22
which serves to provide the triggering to turn on
transient digitizer recorder 20, in which case
5 trigger circuit 18 is removed from the apparatus of
Figure 2. In operation, direction finder unit 22
produces an output triggering signal based upon
detection of electric field 12 by an antenna 24
associated with direction finder unit 22. This
I0 contrasts with trigger circuit 18 which produces an
output triggering signal based upon receipt of the
processed signal associated with electric field 12
as initially received by flat-plate antenna i0.
It will be understood by those skilled in the
15 art that although the invention has been described
in relation to exemplary preferred embodiments
thereof, variations and modifications can be
effected in these preferred embodiments without
detracting from the scope and spirit of the
20 invention.
ABSTRACT OF THE DISCLOSURE
A method is described for determining the
return stroke polarity of distant lightning for
distances beyond 600 km by detecting the electric
field associated with a return stroke of distant
lightning, and processing the electric field signal
to determine the polarity of the slow tail of the
VLF waveform signal associated with the detected
electric field. The polarity of the return stroke
of distant lightning is determined based upon the
polarity of the slow tail portion of the waveform.
18
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Method and apparatus for determining return stroke polarity of distant lightning

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