394:
standing resonant waves modes can exit. The fundamental mode which transports electric charge to the ground most effectively, has thus a wavelength λ four times the channel length L. In the case of the K stroke, the lower boundary is the same as the upper boundary. Of course, this picture is valid only for wave mode 1 (λ/4 antenna) and perhaps for mode 2 (λ/2 antenna), because these modes do not yet "feel" the contorted configuration of the real lightning channel. The higher order modes contribute to the incoherent noisy signals in the higher frequency range (> 100 kHz).
140:
sources of direct information about thunderstorm activity on the ground. Transients electric currents during return strokes (R strokes) or intracloud strokes (K strokes) are the main sources for the generation of impulse-type electromagnetic radiation known as sferics (sometimes called atmospherics). While this impulsive radiation dominates at frequencies less than about 100 kHz, (loosely called long waves), a continuous noise component becomes increasingly important at higher frequencies. The longwave electromagnetic propagation of sferics takes place within the
554:
magnetosphere, etc. dominates. In the VLF-range, there are the coherent impulses from R- and K-strokes, appearing out of the background noise. Beyond about 100 kHz, the noise amplitude becomes more and more incoherent. In addition, technical noise from electric motors, ignition systems of motor cars, etc., are superimposed. Finally, beyond the high frequency band (3–30 MHz) extraterrestrial noise (noise of galactic origin, solar noise) dominates.
31:
424:= very low frequencies, 3–30 kHz). These waves are reflected and attenuated on the ground as well as within the ionospheric D layer, near 70 km altitude during day time conditions, and near 90 km height during the night. Reflection and attenuation on the ground depends on frequency, distance, and
452:
out of a broadband signal. The 15 kHz signal dominates at distances greater than about 5000 km. For ELF waves (< 3 kHz), ray theory becomes invalid, and only mode theory is appropriate. Here, the zeroth mode begins to dominate and is responsible for the second window at greater distances.
139:
channel with all its branches and its electric currents behaves like a huge antenna system from which electromagnetic waves of all frequencies are radiated. Beyond a distance where luminosity is visible and thunder can be heard (typically about 10 km), these electromagnetic impulses are the only
521:
of a sferic signal at different frequencies together with its direction of arrival. The group time delay difference of neighbouring frequencies in the lower VLF band is directly proportional to the distance of the source. Since the attenuation of VLF waves is smaller for west to east propagation and
455:
Resonant waves of this zeroth mode can be excited in the Earth–ionosphere waveguide cavity, mainly by the continuing current components of lightning flowing between two return strokes. Their wavelengths are integral fractions of the Earth's circumference, and their resonance frequencies can thus be
451:
T(ρ, f) depending mainly on distance ρ and frequency f. In the VLF range, only mode one is important at distances larger than about 1000 km. Least attenuation of this mode occurs at about 15 kHz. Therefore, the Earth–ionosphere waveguide behaves like a bandpass filter, selecting this band
443:
At distances greater than about 500 km, sky waves reflected several times at the ionosphere must be added. Therefore, mode theory is here more appropriate. The first mode is least attenuated within the Earth–ionosphere waveguide, and thus dominates at distances greater than about 1000 km.
409:. The maximum spectral amplitude of the sferic typically is near 5 kHz. Beyond this maximum, the spectral amplitude decreases as 1/f if the Earth's surface were perfectly conducting. The effect of the real ground is to attenuate the higher frequencies more strongly than the lower frequencies (
364:
The visible part of a lightning channel has a typical length of about 5 km. Another part of comparable length may be hidden in the cloud and may have a significant horizontal branch. Evidently, the dominant wavelength of the electromagnetic waves of R- and K-strokes is much larger than their
393:
simulating the electric properties of the channel. At the moment of contact with the perfectly conducting Earth surface, the charge is lowered to the ground. In order to fulfill the boundary conditions at the top of the wire (zero electric current) and at the ground (zero electric voltage), only
360:
Both R-strokes as well as K-strokes produce sferics seen as a coherent impulse waveform within a broadband receiver tuned between 1–100 kHz. The electric field strength of the impulse increases to a maximum value within a few microseconds and then declines like a damped oscillator. The
525:
For the regional range (< 1,000 km), the usual way is magnetic direction finding as well as time of arrival measurements of a sferic signal observed simultaneously at several stations. Presumption of such measurements is the concentration on one individual impulse. If one measures
553:
The steady electric discharging currents in a lightning channel cause a series of incoherent impulses in the whole frequency range, the amplitudes of which decreases approximately with the inverse frequency. In the ELF-range, technical noise from 50 to 60 Hz, natural noise from the
522:
during the night, thunderstorm activity up to distances of about 10,000 km can be observed for signals arriving from the west during night time conditions. Otherwise, the transmission range is of the order of 5,000 km.
439:
When distances are less than about 500 km (depending on frequency), then ray theory is appropriate. The ground wave and the first hop (or sky) wave reflected at the ionospheric D layer interfere with each other.
517:
at only a few stations around the world can monitor the global lightning activity fairly well. One can apply the dispersive property of the Earth–ionosphere waveguide by measuring the
81:, and can be received thousands of kilometres from their source. On a time-domain plot, a sferic may appear as a single high-amplitude spike in the time-domain data. On a
365:
channel lengths. The physics of electromagnetic wave propagation within the channel must thus be derived from full wave theory, because the ray concept breaks down.
373:
The channel of a R stroke can be considered as a thin isolated wire of length L and diameter d in which negative electric charge has been stored. In terms of
510:
located mainly in the continental areas at low and middle latitudes. In order to monitor the thunderstorm activity, sferics are the appropriate means.
791:
Lin, Y.T.; et al. (1979). "Characterization of lightning return stroke electric and magnetic fields from simultaneous two-station measurements".
557:
The atmospheric noise depends on frequency, location and time of day and year. Worldwide measurements of that noise are documented in CCIR-reports.
164:, that occur at mesospheric altitudes, are short-lived electric breakdown phenomena, probably generated by giant lightning events on the ground.
1182:
386:
253:
in the upper part of the channel and an equivalent amount of negative charge in its lower part neutralize within a typical time interval of
526:
simultaneously several pulses, interference takes place with a beat frequency equal to the inversal average sequence time of the pulses.
329:
for K-strokes. Often, a continuing current component flows between successive R-strokes. Its "pulse" time typically varies between about
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1111:
1087:
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1005:
981:
957:
930:
689:
662:
1157:
433:
78:
906:
728:
1102:
Spaulding, A. D. (1995). "Atmospheric noise and its effects on telecommunication system performance". In
Volland, H. (ed.).
826:
Weidman, C.D.; Krider, E. P. (1979). "The radiation field wave forms produced by intracloud lightning discharge processes".
1167:
579:
157:
141:
85:, a sferic appears as a vertical stripe (reflecting its broadband and impulsive nature) that may extend from a few
123:
signal. Because the source of the whistler is an impulse (i.e., the sferic), a whistler may be interpreted as the
1172:
1134:
429:
153:
66:
428:. In the case of the ionospheric D-layer, it depends, in addition, on time of day, season, latitude, and the
417:
776:
Volland, H. (1995), "Longwave sferics propagation within the atmospheric waveguide", in
Volland, H. (ed.),
743:
Serhan, G. L.; et al. (1980), "The RF spectra of first and subsequent lightning return strokes in the
1177:
535:
361:
orientation of the field strength increase depends on whether it is a negative or a positive discharge
184:
stored within the lightning channel is lowered to the ground within a typical impulse time interval of
1129:
1029:
835:
800:
680:
Proctor, D. E. (1995), "Radio noise above 300 kHz due to
Natural Causes", in Volland, H. (ed.),
177:
In a typical cloud-to-ground stroke (R stroke), negative electric charge (electrons) of the order of
514:
495:
280:
The energy of K-strokes is in general two orders of magnitude weaker than the energy of R-strokes.
112:
246:
is the speed of light). In typical intracloud K-strokes, positive electric charge of the order of
1162:
1020:
Grandt, C. (1992), "Thunderstorm monitoring in South Africa and Europe by means of VLF sferics",
447:
The Earth–ionosphere waveguide is dispersive. Its propagation characteristics are described by a
374:
161:
1107:
1083:
1059:
1001:
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953:
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902:
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Sferics can be simulated approximately by the electromagnetic radiation field of a vertical
124:
120:
116:
39:
550:
is one of the most important sources for the limitation of the detection of radio signals.
1054:
Orville, R. E. (1995), "Lightning detection from ground and space", in
Volland, H. (ed.),
996:
Williams, E. R. (1995), "Meteorological aspects of thunderstorms", in
Volland, H. (ed.),
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839:
804:
884:
633:
518:
406:
43:
1151:
543:
149:
108:
897:
Harth, W. (1982), "Theory of low frequency wave propagation", in
Volland, H. (ed.),
260:
The corresponding values for average electric current, frequency and wavelength are
507:
506:
About 100 lightning strokes per second are generated all over the world excited by
191:
This corresponds to an average current flowing within the channel of the order of
148:
D- and E- layers. Whistlers generated by lightning strokes can propagate into the
17:
571:
283:
The typical length of lightning channels can be estimated to be of the order of
82:
74:
35:
1078:
Fraser-Smith, A. C. (1995), "Low-frequency radio noise", in
Volland, H. (ed.),
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the speed of light). These resonant modes with their fundamental frequency of
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145:
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distance or greater have their frequencies slightly offset in time, producing
47:
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654:
425:
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136:
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discharges. Sferics may propagate from their lightning source without major
70:
63:
972:
Vonnegut, B. (1982), "The physics of thundercloudes", in
Volland, H (ed.),
103:
When the electromagnetic energy from a sferic escapes the Earth-ionosphere
30:
649:
Lewis, E. A. (1982), "High frequency radio noise", in
Volland, H. (ed.),
546:
must clearly exceed the noise amplitude in order to become detectable.
1041:
127:
of the magnetosphere (for the conditions at that particular instant).
948:
Sentman, D. D. (1995), "Schumann resonances", in
Volland, H. (ed.),
1130:
http://www.srh.noaa.gov/oun/wxevents/19550525/stormelectricity.php
901:, vol. II, Boca Raton, Florida: CRC Press, pp. 133–202,
403:
86:
29:
1106:. Vol. I. Boca Raton, Florida: CRC Press. pp. 359–395.
1082:, vol. I, Boca Raton, Florida: CRC Press, pp. 297–310,
1058:, vol. I, Boca Raton, Florida: CRC Press, pp. 137–149,
952:, vol. I, Boca Raton, Florida: CRC Press, pp. 267–295,
925:, vol. I, Boca Raton, Florida: CRC Press, pp. 111–178,
684:, vol. I, Boca Raton, Florida: CRC Press, pp. 311–358,
723:, vol. II, Boca Raton, Florida: CRC Press, pp. 21–77,
708:, vol. II, Boca Raton, Florida: CRC Press, pp. 155–193
1000:, vol. I, Boca Raton, Florida: CRC Press, pp. 27–60,
606:(International Consultation Committee on Radio Communications).
976:, vol. I, Boca Raton, Florida: CRC Press, pp. 1–22,
921:
Polk, C. (1982), "Schumann resonances", in Volland, H. (ed.),
780:, vol. II, Boca Raton, Florida: CRC Press, pp. 65–93
421:
416:
R strokes emit most of their energy within the ELF/VLF range (
89:
to several tens of kHz, depending on atmospheric conditions.
208:
Maximum spectral energy is generated near frequencies of
704:
Hayakawa, M. (1995), "Whistlers", in Volland, H. (ed.),
604:
Comité Consultatif International des Radiocommunications
69:
impulse that occurs as a result of natural atmospheric
719:
Park, C. G. (1982), "Whistlers", in Volland, H (ed.),
42:
signals amidst a background of sferics as received at
432:
in a complicated manner. VLF propagation within the
436:can be described by ray theory and by wave theory.
398:Transfer function of Earth–ionosphere waveguide
538:determines the sensibility and sensitivity of
420:= extremely low frequencies, < 3 kHz;
502:Monitoring thunderstorm activity with sferics
8:
859:
857:
771:
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943:
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333:its electric current is of the order of
62:(sometimes also spelled "spheric") is a
1104:Handbook of Atmospheric Electrodynamics
1080:Handbook of Atmospheric Electrodynamics
1056:Handbook of Atmospheric Electrodynamics
998:Handbook of Atmospheric Electrodynamics
950:Handbook of Atmospheric Electrodynamics
778:Handbook of Atmospheric Electrodynamics
706:Handbook of Atmospheric Electrodynamics
682:Handbook of Atmospheric Electrodynamics
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617:
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494: ≃ 7.5 Hz are known as
7:
1143:, G.Wiessala in RadioUser 1/2013, UK
653:, vol. I, Boca Raton, Florida:
542:systems (e.g., radio receivers). An
144:between the Earth's surface and the
567:1955 Great Plains tornado outbreak
25:
27:Broadband electromagnetic impulse
385:, where the charge is stored, a
340:corresponding to the numbers of
377:theory, one can adopt a simple
1:
1183:Severe weather and convection
974:CRC Handbook of Atmospherics
923:CRC Handbook of Atmospherics
899:CRC Handbook of Atmospherics
721:CRC Handbook of Atmospherics
651:CRC Handbook of Atmospherics
456:approximately determined by
92:Sferics received from about
866:Atmospheric Electrodynamics
158:upper-atmospheric lightning
34:A frequency vs. time plot (
1199:
1135:Radio in Space and Time -
434:Earth–ionosphere waveguide
142:Earth-ionosphere waveguide
79:Earth–ionosphere waveguide
156:lines of force. Finally,
578:track using sferics and
369:Electric channel current
56:radio atmospheric signal
1158:Atmospheric electricity
881:Wave Propagation Theory
848:10.1029/JC084iC06p03159
813:10.1029/JC084iC10p06307
761:10.1029/RS015i006p01089
630:The Lightning Discharge
483:the Earth's radius and
471:) ≃ 7.5
389:of the channel, and an
173:Basic stroke parameters
51:
536:signal-to-noise ratio
225:or at wavelengths of
33:
1168:Electrical phenomena
879:Wait, J. R. (1982),
864:Volland, H. (1984),
657:, pp. 251–288,
628:Uman, M. A. (1980),
1034:1992JGR....9718215G
840:1979JGR....84.3159W
805:1979JGR....84.6307L
515:Schumann resonances
496:Schumann resonances
50:on August 24, 2005.
868:, Berlin: Springer
306:for R-strokes and
115:by the near-Earth
52:
38:) showing several
1137:Whistler, Sferics
1042:10.1029/92JD01623
548:Atmospheric noise
540:telecommunication
530:Atmospheric noise
449:transfer function
430:geomagnetic field
413:'s ground wave).
379:transmission line
168:Source properties
94:2,000 kilometres'
18:Radio atmospheric
16:(Redirected from
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131:Introduction
119:, forming a
102:
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59:
55:
53:
602:stands for
580:dawn chorus
572:Cluster One
331:10–150 ms ,
154:geomagnetic
146:ionospheric
83:spectrogram
75:attenuation
36:spectrogram
1152:Categories
908:0849332273
745:ℓ ≈ 100 km
730:0849332273
613:References
576:Pink Floyd
411:Sommerfeld
391:inductance
387:resistance
152:along the
48:Antarctica
1163:Lightning
655:CRC Press
587:Footnotes
426:orography
383:capacitor
358:3–40 Mm .
137:lightning
113:dispersed
105:waveguide
71:lightning
64:broadband
747:range",
561:See also
404:Hertzian
351:7–100 Hz
345:1–20 C ,
278:7.5 km .
271:40 kHz ,
223:10 kHz ,
189:100 μs .
121:whistler
40:whistler
1030:Bibcode
836:Bibcode
801:Bibcode
755:(108),
338:100 A ,
323:
311:
300:
288:
265:400 A ,
258:25 μs .
242:(where
233:⁄
216:⁄
206:10 kA .
199:⁄
162:sprites
77:in the
1141:Tweeks
1110:
1086:
1062:
1004:
980:
956:
929:
905:
727:
688:
661:
117:plasma
98:tweeks
60:sferic
251:10 mC
240:30 km
1139:and
1108:ISBN
1084:ISBN
1060:ISBN
1002:ISBN
978:ISBN
954:ISBN
927:ISBN
903:ISBN
725:ISBN
686:ISBN
659:ISBN
600:CCIR
574:, a
534:The
467:/(2π
353:and
327:4 km
309:ℓ ≈
304:8 km
286:ℓ ≈
273:and
228:λ =
211:f ≈
194:J ≈
1038:doi
844:doi
809:doi
757:doi
422:VLF
418:ELF
356:λ ≈
349:f ≈
343:Q ≈
336:J ≈
325:λ =
302:λ =
276:λ ≈
269:f ≈
263:J ≈
256:τ ≈
249:Q ≈
187:τ =
182:1 C
180:Q ≈
160:or
87:kHz
58:or
1154::
1036:,
1026:97
1024:,
940:^
856:^
842:.
832:84
830:.
807:.
797:84
795:.
768:^
753:15
751:,
672:^
641:^
620:^
498:.
465:mc
135:A
100:.
54:A
46:,
1116:.
1040::
1032::
850:.
846::
838::
815:.
811::
803::
759::
492:1
489:f
485:c
481:a
477:m
473:m
469:a
460:m
458:f
320:2
317:/
314:1
297:4
294:/
291:1
244:c
238:≈
235:f
231:c
221:=
218:τ
214:1
204:=
201:τ
197:Q
20:)
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