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Radio emission of lightning initiation
Physics Letters A 312 (2003) 228–237 www.elsevier.com/locate/pla
Radio emission of lightning initiation A.V. Gurevich a,∗ , L.M. Duncan b , A.N. Karashtin c , K.P. Zybin a a P.N. Lebedev Institute of Physics, Russian Academy of Sciences, 117924 Moscow, Russia b Thayer School of Engineering, Dartmouth College, NH 03755-8000, USA c Radiophysical Research Institute (NIRFI), 603600 Nizhny Novgorod, Russia
Received 4 March 2003; accepted 14 March 2003 Communicated by V.M. Agranovich
Abstract The wide band (0.1–30 MHz) high time resolution (16 ns) radio interferometer is constructed and used to study the initial radio emission of lightning. It is established that the first lightning radio pulse is bipolar and its form, width and amplitude agree with theoretical predictions [Phys. Lett. A 301 (2002) 320]. From observations follows: (a) The runaway breakdown (RB) conditions are fulfilled in thundercloud during noticeable part of storm time; (b) The lightning is initiated by a combined action of RB and extensive atmospheric shower generated by cosmic ray particle having the energy ∼ 1016 eV. The RB effect in thunderclouds plays a role of lightning “spark chamber” and can be used for radio detection of high energy cosmic ray particles. 2003 Published by Elsevier Science B.V.
1. Introduction The theory of radio emission generated during thunderstorm by cosmic ray particles having high energy was developed by Gurevich et al. [1]. The emission should have a high intensity due to the combined action of runaway breakdown and extensive atmospheric shower. Runaway breakdown is a new physical concept of an avalanche type increase of a number of energetic electrons in gas under the action of the electric field which was proposed by Gurevich et al. [2]. The avalanche can grow in electric field E Ec . The field Ec is almost an order of magnitude less than * Corresponding author.
E-mail address: [email protected] (A.V. Gurevich).
the threshold electric field of conventional breakdown Eth . The growth of number of electrons with energies ε > εc ≈ 0.1–1 MeV is determined by the fact that under the action of electric field E > Ec fast electrons could become runaways, what means that they are accelerated by electric field E as suggested by Wilson [3]. Due to collisions with gas molecules they can generate not only large number of slow thermal electrons, but the new fast electrons having energies ε > εc as well. Directly this process—acceleration and collisions lead to the avalanche type growth of the number of runaway and thermal electrons, which was called in [2] “runaway breakdown” (RB). The detailed kinetic theory of RB was developed in [4–8]. In atmosphere the critical electric field is kV Nm (z) Ec ≈ 200 (1) . m Nm (0)
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Here Nm (z) is the neutral molecules density at the height z and Nm (0) = 2.7 × 1019 cm−3 at see level. Ec falls down with z due to exponential diminishing of Nm . At the thundercloud heights z ≈ 4–6 km, the critical field Ec is 100–150 kV/m and exactly these values of electric field are often observed during thunderstorms [9,10]. When the electric field in thunderstorm cloud reaches the critical value E Ec every cosmic ray secondary electron (its energy ε > 1 MeV) initiates a micro runaway breakdown (MRB). It serves as a source of intensive ionization of air and manifests itself in a strong amplification of X- and γ -rays emission and effective growth of conductivity in thundercloud [2,11]. These effects were observed and compared with RB theory [12–15]. Extensive atmospheric shower (EAS) generated by a high energy cosmic ray particle is accompanied by a strong local growth of cosmic ray secondaries number [16]. A theory of combined effect of RB-EAS was developed in [17]. It was shown that ionization of atmosphere by the shower in RB conditions is growing strong enough to produce a local highly conductive plasma and can serve for lightning leader initiation. On the other hand the same effect can stimulate the excitation by thundercloud electric field a strong local pulse of electric current. This short pulse of electric current can generate intensive radio emission [1]. According to [1] the RB-EAS radio emission should have a form of microsecond bipolar pulse with characteristic frequency 3–10 MHz. Thus we see as follows from the theory [1,17] RBEAS radio emission is connected deeply with lightning initiation. The goal of this Letter is to present the first results of radio observations of lightning initiation process. A special installation for a wide-band radio interferometry (0.1–30 MHz) was constructed. The time resolution of the system (16 ns) is high enough to observe the detailed structure of radio pulses. The main results of our observations could be formulated as follows. Radio emission of all observed lightning begins sharply with a first radio pulse having always a bipolar form and a full width about 0.2– 0.3 µs. The pulse structure is quite analogous to the one predicted by RB-EAS theory [1]. Characteristic amplitude of the first pulse is 50–300 mV/m, characteristic distance to the emission region is 5–20 km. Comparison with the theory [1] allows to state that the observed lightning are initiated by cosmic ray parti-
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cles having the energy ∼ 1016 eV. Thus the lightning initiation due to RB-EAS effect and its radio observations could be used for investigation of the fluxes of cosmic ray particles having high energies.
2. Installation description Measurements were carried out using specially designed installation for short electromagnetic pulse observations in the frequency range from 0.1 to 30 MHz. This installation contains three receiving antennas that can be spaced up to 50 m (determined by connection cable lengths) from the receiving apparatus (central unit). It allows signal arrival angle determination using correlation technique as well as waveform recording with high temporal resolution. Each receiving antenna of the installation (antenna assembly) actually consists of three individual antennas—two mutually perpendicular loop antennas for horizontal magnetic field measurements, and an End-Fed antenna to measure vertical electric field. All three antennas are active and include transistor amplifiers. This allowed to reduce the size of the assembly essentially and to reach uniform gain-frequency characteristics for all antennas in the wide frequency range. Antenna for magnetic field measurements is a screened vertically sited rectangular loop. Loop and prime amplifier screening is needed to provide antenna sensitivity only to magnetic field, in other case its directional pattern becomes non-symmetric and frequency dependent. To equalize frequency-response of the loop it should be loaded by small resistance. Thus it is attached to the amplifier input through a stepup transformer made on long lines working as current transformer. Electric antenna is an electrically short rod attached to the matched transistor amplifier. Its frequencyresponse equalization is achieved using principally capacity character of the electrically short pole impedance as well as of transistor input impedance in the frequency range concerned. So, antenna–amplifier connection is a frequency independent divider with a ratio of antenna capacity to amplifier capacity as a transfer coefficient. Antenna capacity is increased by a plate tip at the end of the rod to increase the transfer coefficient. The increase of antenna capacity extends the antenna frequency range to lower fre-
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quencies also. Low boundary of the antenna frequency range is determined by the response time that is equal to the product of the antenna capacity and an input resistance of the amplifier. High input resistance of the amplifier leads to the growth of low frequency noise and assigns a risk of input transistor damage due to breakdown in strong electric fields. Therefore, the input resistance was chosen in accordance with low boundary of the antenna frequency range (100 kHz approximately) to be about 1 M. Breakdown protection of the amplifier is provided in three stages by high-voltage feed-through capacitor, gas-filled excess voltage preventer, and two parallel-opposition semiconductor diodes successively mounted at its input. All three antenna amplifiers outputs from each antenna assembly are connected to the central unit by coaxial cables of equal length (60 m in the specific implementation). The same cables are used to provide antenna amplifiers power supply through appropriate bypasses. The central unit provides switching and selection up to 4 receiving elements (antennas) most suitable under specific conditions of the experiment. It serves also for preliminary analog data reduction, namely, reduction of the dynamic range of received signals to analog-to-digital converter (ADC) input range to prevent loss of information, and both highpass and low-pass filtering to decrease low-frequency noise and to prevent spectral aliasing during data sampling. After the central unit chosen signals are digitized by 4-channel radio receiver based on the personal computer (PC) using two synchronized two-channel ADC boards with sampling rate up to 60 MHz and 256 MB per board internal memory. High sampling rate leads to high data traffic that exceeds the ability of PC bus. Therefore, pulse operation mode is needed with fast data storage into internal buffer area following by slower data transfer to the PC initiated by trigger pulse. ADC internal buffer memory allows to record up to 1 s sessions with maximum sampling rate of 60 MHz. Radio receiver is operated in the pretrigger mode allowing signal recording before and after the trigger pulse. It can be triggered by external TTL signal as well as internally. In the last case triggering occurred when an input signal of one of the ADC channels exceeds a preset threshold.
The whole installation is supplied by UPS (uninterruptible power supply) with enhanced up to several hours off-line operation capability (using external accumulator of higher capacity).
3. Results of measurements Described electric field measurements were carried out during local isolated thunderstorm on July 13, 2002 between 14:38 and 15:23 Moscow summer time (MST) which is one hour ahead of the local time. Two electric antennas spaced by about 35 m in the east– west direction were used. Record time was chosen to be 1 s with 0.5 s pre-trigger duration. Sampling rate was 60 MHz, and internal triggering was used. During the thunderstorm 50 events correspondent to lightning discharges were recorded. Examples of 1 s and first 250 µs of the discharge are shown at Fig. 1. The initial stage of lightning discharge was observed in 48 events. Each of them began with very short bipolar pulses (front width τ ≈ 100 ns at 0.7 level and full width 0.2–0.3 µs) shown at Fig. 2. Electric field strength can be found by dividing of the signal amplitude by the electric length of the End-Fed antenna (approximately 0.1 m). Both positive and negative polarity of these pulses occurred in approximately equal proportion in different lightning while the polarity of subsequent pulses was always the same in any individual lightning. Number of consecutive short pulses before more complex behavior of electric field varied from few to about ten pulses with gaps from several ten to few hundred microseconds between them. Examples of three first pulses are shown at the Fig. 3. Note that presented experimental data contradict directly to the well-known Kazemirs hypothesis that the lightning is initiated simultaneously in opposite directions—up and down [10,25]. The noise level between the lightning is the same as in quiet (with no thunderstorm) periods at the chosen sensitivity. Very often it is the same between first sub-microsecond pulses observed at the beginning of the lightning. Standard noise deviation σ is approximately 1.1 mV. One can see from the Fig. 2 that the observed amplitudes of the first lightning pulses were mainly 10–20 mV varying in the limits 4.5–50 mV. Consequently electric field strength was around 100– 200 mV/m.
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(a)
(b) Fig. 1. Examples of recorded radio emission. (a) Full lightning discharge (1 s); (b) first 250 µs of the discharge.
Arrival angles are shown in the Table 1 in degrees. Values of cross-correlation function (between signals received by two antennas) at these angles are shown in parentheses. Arrival direction lies at the cone around west–east line, 0◦ corresponds to the west direction, 90◦ is orthogonal to the west–east line (south direction), and 180◦ corresponds to the east direction. Arrival angle accuracy depends on the direction: it is maximal around 90◦ (about 5◦ ) and minimal around 0◦ and 180◦ (about 20◦ ). From the table it follows that the storm occurred southern the
installation, moving in the east–west direction. That agrees with visual observations.
4. Discussion The lightning radio emission was investigated intensively during last decades by Proctor [18], Kreihbel et al. [19], Rhodes et al. [20], Krider et al. [21], Weideman et al. [22], Beasley et al. [23] and others (see monographs [10,24]). New features of light-
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Fig. 2. Initial pulses of lightning discharges.
ning discharge development were observed, different components were singled out and the general
complicated character of the discharge was established.
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Fig. 3. Examples of three pulses of the discharge.
In our study the attention is concentrated at the first pulse of radio emission, which characterizes the lightning initiation process. This concrete goal determined the main peculiarities of our work. The wide band radio interferometry was used—the measurements were done at frequency range 0.1 to 30 MHz. The highest time resolution (in comparison with the previous authors) was reached: the full set of data was obtained every 16 ns. That allowed to distinguish the initial radio pulse for each lightning and to see its detailed structure shown at Fig. 2. The observed radio emission has no features of strongly directed Cherenkov or transmission radiation. It is definitely multidirectional
(see Table 1), generated by electric current pulse in thundercloud. The characteristic pulse frequency spectrum is shown at the Fig. 4. One can see that the main body of the pulse has the frequency f 3–5 MHz. The pulse has no precursors. There is no noise growth before the pulse. It begins always with a sharp front, the characteristic width of the front is about 100 ns. The first radio pulse and consecutive pulses at each individual lightning have the same polarities, what means that the electric current generating radio pulses is unidirectional. From that it follows that the electric current pulse is produced due to the action of
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Table 1 Time
First pulse
Second pulse
Third pulse
14:25:45 14:38:22 14:39:19 14:43:19 14:45:51 14:46:27 14:47:18 14:48:59 14:49:29 14:50:19 14:50:32 14:51:47 14:52:14 14:52:36 14:53:18 14:54:12 14:54:32 14:54:59 14:56:06 14:57:41 14:58:39 14:59:09 14:59:44 15:00:04 15:00:21 15:00:33 15:01:04 15:01:39 15:02:58 15:03:32 15:03:56 15:06:06 15:07:36 15:10:50 15:11:47 15:12:31 15:12:52 15:13:15 15:13:46 15:14:27 15:15:54 15:16:49 15:17:29 15:18:49 15:20:13 15:20:56 15:21:57 15:23:00
34 (0.65) 46 (0.45) 60 (0.85) 27 (0.55) 31 (0.8) 92 (0.65) 34 (0.8) 90 (0.6) 19 (0.7) 115 (0.75) 65 (0.9) 60 (0.7) 46 (0.6) 128 (0.4) 74 (0.75) 156 (0.8) 118 (0.65) 106 (0.55) 122 (0.5) 82 (0.7) 79 (0.75) 56 (0.6) 90 (0.6) 118 (0.8) 82 (0.5) 120 (0.6) 95 (0.8) 115 (0.8) 98 (0.8) 106 (0.65) 79 (0.55) 124 (0.5) 101 (0.85) 106 (0.85) 93 (0.85) 120 (0.8) 95 (0.8) 98 (0.7) 103 (0.6) 113 (0.9) 103 (0.5) 113 (0.7) 103 (0.45) 122 (0.9) 146 (0.5) 124 (0.45) 128 (0.55) 130 (0.8)
46 (0.75) 46 (0.9) 52 (0.6) 56 (0.8) 31 (0.6) 90 (0.8) 50 (0.9) 93 (0.95) 27 (0.55) 134 (0.98) 98 (0.8) 56 (0.65) 34 (0.6) 161 (0.8) 79 (0.8) 166 (0.5) – 101 (0.5) 98 (0.45) 82 (0.6) 93 (0.5) 52 (0.7) 90 (0.4) 104 (0.8) 106 (0.75) 111 (0.75) 90 (0.65) 120 (0.9) 101 (0.8) 104 (0.9) 60 (0.6) 120 (0.85) 106 (0.85) 109 (0.8) 87 (0.8) 82 (0.9) 100 (0.9) 113 (0.9) 100 (0.8) 106 (0.85) 115 (0.9) 118 (0.7) 98 (0.7) 120 (0.85) 156 (0.5) 122 (0.8) 124 (0.7) 118 (0.9)
34 (0.5) 50 (0.75) 52 (0.85) 52 (0.75) 56 (0.8) 90 (0.8) 62 (0.85) 82 (0.65) 24 (0.85) 90 (0.8) 87 (0.75) 65 (0.75) 34 (0.75) – – – 115 (0.7) 109 (0.6) – 82 (0.55) 90 (0.8) 52 (0.55) 90 (0.5) 106 (0.7) 95 (0.7) 126 (0.8) 84 (0.95) – 82 (0.7) 98 (0.75) 74 (0.6) 130 (0.85) 115 (0.97) 113 (0.9) 92 (0.85) 115 (0.95) 90 (0.5) 103 (0.85) 103 (0.55) 115 (0.9) 101 (0.7) 101 (0.5) 115 (0.85) 103 (0.95) 153 (0.8) 106 (0.5) 124 (0.65) 103 (0.95)
the thunderstorm electric field at the thermal electrons. The last statement is confirmed by the coincidence of the lifetime of thermal electrons in air τ ∼ 100 ns with the pulse front characteristic time
scale. Under the action of thundercloud electric field E ∼ 1 kV/cm, thermal electrons in air obtain the average velocity vn eE/mν 2 × 106 cm/s (here ν is electron collision frequency). During its lifetime
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Fig. 4. (a) Pulse frequency spectrum F (f ) and (b)–(d) characteristic integrated spectrum W (f ) = f F (f ).
τ electron can be transported at small distance only lτ ∼ ve τ ∼ 0.2 cm. That means that the ionization source which creates the thermal electrons, to generate the observed half width of the first pulse of radio emission τh ≈ τ , has to move with very high velocity: vs λ/2τ 3 × 1010 cm/s. We took here into account that the electric current pulse is unipolar and has the front scale lc λ/2, where λ = c/fm is the characteristic wavelength of radio emission. The current amplitude as follows from our observations, is I ∼ ER0 c ∼ 100–200 A (here E is the observed radio pulse electric field strength, R0 distance from the receiver to the source). To create this current ionization source has to generate large number of electrons N = I /evn ∼ 5 × 1014 el/cm. Let us discuss the possible sources of this ionization. One natural source is ionization wave in long spark [10,25]. But generation of ionization waves in 5 kV/cm long spark asks average electric field E
and local electric field in leader tip EL ∼ 80– 100 kV/cm. The probability of sudden creation without any precursors of such a high local field inside 1 thundercloud when its average field is only E kV/cm looks negligibly low. Note also that the velocity of ionization wave is always significantly less than observed in our first pulse. Another possible source is ionization by secondary electrons of cosmic ray shower. But too high energy cosmic ray particle is needed to generate 5 × 1014 el/cm. Really, according to [16], the √ number of cosmic ray secondaries is no = 0.3/β ln(/β), where is the energy of cosmic ray particle, β = 7.2 × 107 eV. Taking into account that every secondary generates at 1 cm in air 30–40 thermal electrons we find that to generate 5 × 1014 el/cm the energy should be more than 1021 eV. In runaway breakdown conditions situation looks much more realistic, as due to RB effect cosmic secondaries are multiplied and the number of thermal
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electrons grows several orders of magnitude. Thus we see that RB-EAS interaction is the mostly probable process, which can be used to explain the observational data. Let us compare now the results of observations with the predictions of RB-EAS theory. According to [1] the radio pulse generated due to RB-EAS effect has a bipolar form and characteristic full width 0.2– 0.4 µs. Amplitude of radio pulse E is proportional to the energy of cosmic ray particle and inverse proportional to the distance R0 between the region of RB-EAS interaction and receiver. Both the form and width of the main part of observed radio pulses, presented at Fig. 2, agree well with the theory [1]. The pulse amplitudes as show our new more detailed calculations are in agreement with the theory for R0 = 5–20 km and ≈ 1016 eV too. That gives a possibility to suppose that according to comparison of the theory with observations the first radio pulse which initiates the lightning is generated due to RBEAS effect by high energy cosmic ray particle with ∼ 1016 eV. As one can see from the table during the storm 13.07.2002 from 14.25 to 15.23 local time it was observed 48 lightning. Storm active region was about 100 km2 . That means that the flash density during the storm was PL 48/(48 · 60 · 200) 1.6 × 10−4 km−2 s−1 . The flux of cosmic ray particles having the energy ∼ 1016 eV is [28] Pcr = 2 × 10−2 km−2 s−1 . One can see that though PL is significantly less than Pcr the relation PL /Pcr is noticeable ∼ 10−2 . One can give the following interpretation of this fact. The RB conditions are fulfilled in thundercloud during noticeable part of storm time. At that time the system is in the prepared RB state: electric field E at some height is close to Ec (see [9,10]). The cosmic ray particle with ∼ 1016 eV generate then according to RB-EAS effect a local pulse of electric current I ∼ 100–200 A. It serves for the lightning initiation (LI) and at the same time is the source of the observed first pulse of radio emission. The needed particle energy can of course vary for different thunderstorm conditions. But as the cosmic ray flux depends strongly on the particle energy (Pcr ∼
1/ 2 ), one cannot expect a strong change in particle energy needed for LI. And really according [26] cloud to ground lightning flux lays in the limits 1.5 × 10−4 PL 5 × 10−6 km−2 s−1 . Analogous result follows from [27] (see also [10, Chapter 8, Tables 8.1, 8.2, Fig. 8.46]) PL ∼ 10−5 –10−6 km−2 s−1 . Thus we see that the proposed interpretation of our observations does not contradict to the existing data. On the other hand this interpretation leads to the following fundamental conclusions: (1) The electric field in thunderstorm atmosphere cannot reach the values significantly higher than Ec , but keeps the value E Ec for a noticeable part o thunderstorm time; (2) The lightning is initiated by a high energy cosmic ray particle due to RB-EAS interaction; (3) The measurements of LI first radio pulses could serve as an effective new method for detection of cosmic ray particles with 1016 eV. This statement becomes evident when one compares results of our radio measurements during thunderstorm with usual methods of EAS observations. For example, the TienShan mountain station has a EAS system on the base of scintillation detectors located nearly concentric circumferences up to the radius R = 100 m [14]. To register 50 particles with 1016 eV one needs this installation to work several days. The radio RB-EAS method allowed to see the same 50 particles during 1 thunderstorm hour. Note that for a very high energy cosmic ray particles 1018 –1019 eV the radio pulse is so strong that it could be easily registered at the distance (2–3) × 103 km due to the pulse reflection from ionosphere. Of course proposed interpretation of presented experimental data needs to be verified. To prove it both much more observations and further development of the existing theory is needed.
5. Conclusions In conclusion we formulate briefly the main results of the experiment described in this Letter.
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(1) The wide band high time resolution radio interferometer is constructed and used for studying the initial pulse of lightning radio emission; (2) The observations of 48 lightning are fulfilled. They demonstrate that the first lightning radio pulse is bipolar and its form, width and amplitude are in agreement with the predictions of RB-EAS theory. The following interpretation of observational data is given: (a) The RB conditions are fulfilled in thundercloud during noticeable part of the storm time; (b) The lightning is initiated by combined action of runaway breakdown (RB) and extensive atmospheric shower (EAS); (c) The energy of cosmic ray particles generating observed EAS is ∼ 1016 eV. The further simultaneous observations of cosmic rays together with lightning radio emission, X-rays and γ -rays generated by the RB-EAS process can serve for the development of a new effective method for radio detection of high energy cosmic ray particles with 1017 –1018 eV.
Acknowledgements The authors are grateful to Prof. V.L. Ginzburg, Prof. E.L. Feinberg, Dr. H. Carlson and Dr. I.I. Royzen for useful discussions. The work was supported by Grants EOARD-ISTC N2236p, ISTC-1480.
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Radio emission of lightning initiation A.V. Gurevich a,∗ , L.M. Duncan b , A.N. Karashtin c , K.P. Zybin a a P.N. Lebedev Institute of Physics, Russian Academy of Sciences, 117924 Moscow, Russia b Thayer School of Engineering, Dartmouth College, NH 03755-8000, USA c Radiophysical Research Institute (NIRFI), 603600 Nizhny Novgorod, Russia
Received 4 March 2003; accepted 14 March 2003 Communicated by V.M. Agranovich
Abstract The wide band (0.1–30 MHz) high time resolution (16 ns) radio interferometer is constructed and used to study the initial radio emission of lightning. It is established that the first lightning radio pulse is bipolar and its form, width and amplitude agree with theoretical predictions [Phys. Lett. A 301 (2002) 320]. From observations follows: (a) The runaway breakdown (RB) conditions are fulfilled in thundercloud during noticeable part of storm time; (b) The lightning is initiated by a combined action of RB and extensive atmospheric shower generated by cosmic ray particle having the energy ∼ 1016 eV. The RB effect in thunderclouds plays a role of lightning “spark chamber” and can be used for radio detection of high energy cosmic ray particles. 2003 Published by Elsevier Science B.V.
1. Introduction The theory of radio emission generated during thunderstorm by cosmic ray particles having high energy was developed by Gurevich et al. [1]. The emission should have a high intensity due to the combined action of runaway breakdown and extensive atmospheric shower. Runaway breakdown is a new physical concept of an avalanche type increase of a number of energetic electrons in gas under the action of the electric field which was proposed by Gurevich et al. [2]. The avalanche can grow in electric field E Ec . The field Ec is almost an order of magnitude less than * Corresponding author.
E-mail address: [email protected] (A.V. Gurevich).
the threshold electric field of conventional breakdown Eth . The growth of number of electrons with energies ε > εc ≈ 0.1–1 MeV is determined by the fact that under the action of electric field E > Ec fast electrons could become runaways, what means that they are accelerated by electric field E as suggested by Wilson [3]. Due to collisions with gas molecules they can generate not only large number of slow thermal electrons, but the new fast electrons having energies ε > εc as well. Directly this process—acceleration and collisions lead to the avalanche type growth of the number of runaway and thermal electrons, which was called in [2] “runaway breakdown” (RB). The detailed kinetic theory of RB was developed in [4–8]. In atmosphere the critical electric field is kV Nm (z) Ec ≈ 200 (1) . m Nm (0)
0375-9601/03/$ – see front matter 2003 Published by Elsevier Science B.V. doi:10.1016/S0375-9601(03)00511-5
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Here Nm (z) is the neutral molecules density at the height z and Nm (0) = 2.7 × 1019 cm−3 at see level. Ec falls down with z due to exponential diminishing of Nm . At the thundercloud heights z ≈ 4–6 km, the critical field Ec is 100–150 kV/m and exactly these values of electric field are often observed during thunderstorms [9,10]. When the electric field in thunderstorm cloud reaches the critical value E Ec every cosmic ray secondary electron (its energy ε > 1 MeV) initiates a micro runaway breakdown (MRB). It serves as a source of intensive ionization of air and manifests itself in a strong amplification of X- and γ -rays emission and effective growth of conductivity in thundercloud [2,11]. These effects were observed and compared with RB theory [12–15]. Extensive atmospheric shower (EAS) generated by a high energy cosmic ray particle is accompanied by a strong local growth of cosmic ray secondaries number [16]. A theory of combined effect of RB-EAS was developed in [17]. It was shown that ionization of atmosphere by the shower in RB conditions is growing strong enough to produce a local highly conductive plasma and can serve for lightning leader initiation. On the other hand the same effect can stimulate the excitation by thundercloud electric field a strong local pulse of electric current. This short pulse of electric current can generate intensive radio emission [1]. According to [1] the RB-EAS radio emission should have a form of microsecond bipolar pulse with characteristic frequency 3–10 MHz. Thus we see as follows from the theory [1,17] RBEAS radio emission is connected deeply with lightning initiation. The goal of this Letter is to present the first results of radio observations of lightning initiation process. A special installation for a wide-band radio interferometry (0.1–30 MHz) was constructed. The time resolution of the system (16 ns) is high enough to observe the detailed structure of radio pulses. The main results of our observations could be formulated as follows. Radio emission of all observed lightning begins sharply with a first radio pulse having always a bipolar form and a full width about 0.2– 0.3 µs. The pulse structure is quite analogous to the one predicted by RB-EAS theory [1]. Characteristic amplitude of the first pulse is 50–300 mV/m, characteristic distance to the emission region is 5–20 km. Comparison with the theory [1] allows to state that the observed lightning are initiated by cosmic ray parti-
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cles having the energy ∼ 1016 eV. Thus the lightning initiation due to RB-EAS effect and its radio observations could be used for investigation of the fluxes of cosmic ray particles having high energies.
2. Installation description Measurements were carried out using specially designed installation for short electromagnetic pulse observations in the frequency range from 0.1 to 30 MHz. This installation contains three receiving antennas that can be spaced up to 50 m (determined by connection cable lengths) from the receiving apparatus (central unit). It allows signal arrival angle determination using correlation technique as well as waveform recording with high temporal resolution. Each receiving antenna of the installation (antenna assembly) actually consists of three individual antennas—two mutually perpendicular loop antennas for horizontal magnetic field measurements, and an End-Fed antenna to measure vertical electric field. All three antennas are active and include transistor amplifiers. This allowed to reduce the size of the assembly essentially and to reach uniform gain-frequency characteristics for all antennas in the wide frequency range. Antenna for magnetic field measurements is a screened vertically sited rectangular loop. Loop and prime amplifier screening is needed to provide antenna sensitivity only to magnetic field, in other case its directional pattern becomes non-symmetric and frequency dependent. To equalize frequency-response of the loop it should be loaded by small resistance. Thus it is attached to the amplifier input through a stepup transformer made on long lines working as current transformer. Electric antenna is an electrically short rod attached to the matched transistor amplifier. Its frequencyresponse equalization is achieved using principally capacity character of the electrically short pole impedance as well as of transistor input impedance in the frequency range concerned. So, antenna–amplifier connection is a frequency independent divider with a ratio of antenna capacity to amplifier capacity as a transfer coefficient. Antenna capacity is increased by a plate tip at the end of the rod to increase the transfer coefficient. The increase of antenna capacity extends the antenna frequency range to lower fre-
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quencies also. Low boundary of the antenna frequency range is determined by the response time that is equal to the product of the antenna capacity and an input resistance of the amplifier. High input resistance of the amplifier leads to the growth of low frequency noise and assigns a risk of input transistor damage due to breakdown in strong electric fields. Therefore, the input resistance was chosen in accordance with low boundary of the antenna frequency range (100 kHz approximately) to be about 1 M. Breakdown protection of the amplifier is provided in three stages by high-voltage feed-through capacitor, gas-filled excess voltage preventer, and two parallel-opposition semiconductor diodes successively mounted at its input. All three antenna amplifiers outputs from each antenna assembly are connected to the central unit by coaxial cables of equal length (60 m in the specific implementation). The same cables are used to provide antenna amplifiers power supply through appropriate bypasses. The central unit provides switching and selection up to 4 receiving elements (antennas) most suitable under specific conditions of the experiment. It serves also for preliminary analog data reduction, namely, reduction of the dynamic range of received signals to analog-to-digital converter (ADC) input range to prevent loss of information, and both highpass and low-pass filtering to decrease low-frequency noise and to prevent spectral aliasing during data sampling. After the central unit chosen signals are digitized by 4-channel radio receiver based on the personal computer (PC) using two synchronized two-channel ADC boards with sampling rate up to 60 MHz and 256 MB per board internal memory. High sampling rate leads to high data traffic that exceeds the ability of PC bus. Therefore, pulse operation mode is needed with fast data storage into internal buffer area following by slower data transfer to the PC initiated by trigger pulse. ADC internal buffer memory allows to record up to 1 s sessions with maximum sampling rate of 60 MHz. Radio receiver is operated in the pretrigger mode allowing signal recording before and after the trigger pulse. It can be triggered by external TTL signal as well as internally. In the last case triggering occurred when an input signal of one of the ADC channels exceeds a preset threshold.
The whole installation is supplied by UPS (uninterruptible power supply) with enhanced up to several hours off-line operation capability (using external accumulator of higher capacity).
3. Results of measurements Described electric field measurements were carried out during local isolated thunderstorm on July 13, 2002 between 14:38 and 15:23 Moscow summer time (MST) which is one hour ahead of the local time. Two electric antennas spaced by about 35 m in the east– west direction were used. Record time was chosen to be 1 s with 0.5 s pre-trigger duration. Sampling rate was 60 MHz, and internal triggering was used. During the thunderstorm 50 events correspondent to lightning discharges were recorded. Examples of 1 s and first 250 µs of the discharge are shown at Fig. 1. The initial stage of lightning discharge was observed in 48 events. Each of them began with very short bipolar pulses (front width τ ≈ 100 ns at 0.7 level and full width 0.2–0.3 µs) shown at Fig. 2. Electric field strength can be found by dividing of the signal amplitude by the electric length of the End-Fed antenna (approximately 0.1 m). Both positive and negative polarity of these pulses occurred in approximately equal proportion in different lightning while the polarity of subsequent pulses was always the same in any individual lightning. Number of consecutive short pulses before more complex behavior of electric field varied from few to about ten pulses with gaps from several ten to few hundred microseconds between them. Examples of three first pulses are shown at the Fig. 3. Note that presented experimental data contradict directly to the well-known Kazemirs hypothesis that the lightning is initiated simultaneously in opposite directions—up and down [10,25]. The noise level between the lightning is the same as in quiet (with no thunderstorm) periods at the chosen sensitivity. Very often it is the same between first sub-microsecond pulses observed at the beginning of the lightning. Standard noise deviation σ is approximately 1.1 mV. One can see from the Fig. 2 that the observed amplitudes of the first lightning pulses were mainly 10–20 mV varying in the limits 4.5–50 mV. Consequently electric field strength was around 100– 200 mV/m.
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(a)
(b) Fig. 1. Examples of recorded radio emission. (a) Full lightning discharge (1 s); (b) first 250 µs of the discharge.
Arrival angles are shown in the Table 1 in degrees. Values of cross-correlation function (between signals received by two antennas) at these angles are shown in parentheses. Arrival direction lies at the cone around west–east line, 0◦ corresponds to the west direction, 90◦ is orthogonal to the west–east line (south direction), and 180◦ corresponds to the east direction. Arrival angle accuracy depends on the direction: it is maximal around 90◦ (about 5◦ ) and minimal around 0◦ and 180◦ (about 20◦ ). From the table it follows that the storm occurred southern the
installation, moving in the east–west direction. That agrees with visual observations.
4. Discussion The lightning radio emission was investigated intensively during last decades by Proctor [18], Kreihbel et al. [19], Rhodes et al. [20], Krider et al. [21], Weideman et al. [22], Beasley et al. [23] and others (see monographs [10,24]). New features of light-
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Fig. 2. Initial pulses of lightning discharges.
ning discharge development were observed, different components were singled out and the general
complicated character of the discharge was established.
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Fig. 3. Examples of three pulses of the discharge.
In our study the attention is concentrated at the first pulse of radio emission, which characterizes the lightning initiation process. This concrete goal determined the main peculiarities of our work. The wide band radio interferometry was used—the measurements were done at frequency range 0.1 to 30 MHz. The highest time resolution (in comparison with the previous authors) was reached: the full set of data was obtained every 16 ns. That allowed to distinguish the initial radio pulse for each lightning and to see its detailed structure shown at Fig. 2. The observed radio emission has no features of strongly directed Cherenkov or transmission radiation. It is definitely multidirectional
(see Table 1), generated by electric current pulse in thundercloud. The characteristic pulse frequency spectrum is shown at the Fig. 4. One can see that the main body of the pulse has the frequency f 3–5 MHz. The pulse has no precursors. There is no noise growth before the pulse. It begins always with a sharp front, the characteristic width of the front is about 100 ns. The first radio pulse and consecutive pulses at each individual lightning have the same polarities, what means that the electric current generating radio pulses is unidirectional. From that it follows that the electric current pulse is produced due to the action of
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Table 1 Time
First pulse
Second pulse
Third pulse
14:25:45 14:38:22 14:39:19 14:43:19 14:45:51 14:46:27 14:47:18 14:48:59 14:49:29 14:50:19 14:50:32 14:51:47 14:52:14 14:52:36 14:53:18 14:54:12 14:54:32 14:54:59 14:56:06 14:57:41 14:58:39 14:59:09 14:59:44 15:00:04 15:00:21 15:00:33 15:01:04 15:01:39 15:02:58 15:03:32 15:03:56 15:06:06 15:07:36 15:10:50 15:11:47 15:12:31 15:12:52 15:13:15 15:13:46 15:14:27 15:15:54 15:16:49 15:17:29 15:18:49 15:20:13 15:20:56 15:21:57 15:23:00
34 (0.65) 46 (0.45) 60 (0.85) 27 (0.55) 31 (0.8) 92 (0.65) 34 (0.8) 90 (0.6) 19 (0.7) 115 (0.75) 65 (0.9) 60 (0.7) 46 (0.6) 128 (0.4) 74 (0.75) 156 (0.8) 118 (0.65) 106 (0.55) 122 (0.5) 82 (0.7) 79 (0.75) 56 (0.6) 90 (0.6) 118 (0.8) 82 (0.5) 120 (0.6) 95 (0.8) 115 (0.8) 98 (0.8) 106 (0.65) 79 (0.55) 124 (0.5) 101 (0.85) 106 (0.85) 93 (0.85) 120 (0.8) 95 (0.8) 98 (0.7) 103 (0.6) 113 (0.9) 103 (0.5) 113 (0.7) 103 (0.45) 122 (0.9) 146 (0.5) 124 (0.45) 128 (0.55) 130 (0.8)
46 (0.75) 46 (0.9) 52 (0.6) 56 (0.8) 31 (0.6) 90 (0.8) 50 (0.9) 93 (0.95) 27 (0.55) 134 (0.98) 98 (0.8) 56 (0.65) 34 (0.6) 161 (0.8) 79 (0.8) 166 (0.5) – 101 (0.5) 98 (0.45) 82 (0.6) 93 (0.5) 52 (0.7) 90 (0.4) 104 (0.8) 106 (0.75) 111 (0.75) 90 (0.65) 120 (0.9) 101 (0.8) 104 (0.9) 60 (0.6) 120 (0.85) 106 (0.85) 109 (0.8) 87 (0.8) 82 (0.9) 100 (0.9) 113 (0.9) 100 (0.8) 106 (0.85) 115 (0.9) 118 (0.7) 98 (0.7) 120 (0.85) 156 (0.5) 122 (0.8) 124 (0.7) 118 (0.9)
34 (0.5) 50 (0.75) 52 (0.85) 52 (0.75) 56 (0.8) 90 (0.8) 62 (0.85) 82 (0.65) 24 (0.85) 90 (0.8) 87 (0.75) 65 (0.75) 34 (0.75) – – – 115 (0.7) 109 (0.6) – 82 (0.55) 90 (0.8) 52 (0.55) 90 (0.5) 106 (0.7) 95 (0.7) 126 (0.8) 84 (0.95) – 82 (0.7) 98 (0.75) 74 (0.6) 130 (0.85) 115 (0.97) 113 (0.9) 92 (0.85) 115 (0.95) 90 (0.5) 103 (0.85) 103 (0.55) 115 (0.9) 101 (0.7) 101 (0.5) 115 (0.85) 103 (0.95) 153 (0.8) 106 (0.5) 124 (0.65) 103 (0.95)
the thunderstorm electric field at the thermal electrons. The last statement is confirmed by the coincidence of the lifetime of thermal electrons in air τ ∼ 100 ns with the pulse front characteristic time
scale. Under the action of thundercloud electric field E ∼ 1 kV/cm, thermal electrons in air obtain the average velocity vn eE/mν 2 × 106 cm/s (here ν is electron collision frequency). During its lifetime
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Fig. 4. (a) Pulse frequency spectrum F (f ) and (b)–(d) characteristic integrated spectrum W (f ) = f F (f ).
τ electron can be transported at small distance only lτ ∼ ve τ ∼ 0.2 cm. That means that the ionization source which creates the thermal electrons, to generate the observed half width of the first pulse of radio emission τh ≈ τ , has to move with very high velocity: vs λ/2τ 3 × 1010 cm/s. We took here into account that the electric current pulse is unipolar and has the front scale lc λ/2, where λ = c/fm is the characteristic wavelength of radio emission. The current amplitude as follows from our observations, is I ∼ ER0 c ∼ 100–200 A (here E is the observed radio pulse electric field strength, R0 distance from the receiver to the source). To create this current ionization source has to generate large number of electrons N = I /evn ∼ 5 × 1014 el/cm. Let us discuss the possible sources of this ionization. One natural source is ionization wave in long spark [10,25]. But generation of ionization waves in 5 kV/cm long spark asks average electric field E
and local electric field in leader tip EL ∼ 80– 100 kV/cm. The probability of sudden creation without any precursors of such a high local field inside 1 thundercloud when its average field is only E kV/cm looks negligibly low. Note also that the velocity of ionization wave is always significantly less than observed in our first pulse. Another possible source is ionization by secondary electrons of cosmic ray shower. But too high energy cosmic ray particle is needed to generate 5 × 1014 el/cm. Really, according to [16], the √ number of cosmic ray secondaries is no = 0.3/β ln(/β), where is the energy of cosmic ray particle, β = 7.2 × 107 eV. Taking into account that every secondary generates at 1 cm in air 30–40 thermal electrons we find that to generate 5 × 1014 el/cm the energy should be more than 1021 eV. In runaway breakdown conditions situation looks much more realistic, as due to RB effect cosmic secondaries are multiplied and the number of thermal
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electrons grows several orders of magnitude. Thus we see that RB-EAS interaction is the mostly probable process, which can be used to explain the observational data. Let us compare now the results of observations with the predictions of RB-EAS theory. According to [1] the radio pulse generated due to RB-EAS effect has a bipolar form and characteristic full width 0.2– 0.4 µs. Amplitude of radio pulse E is proportional to the energy of cosmic ray particle and inverse proportional to the distance R0 between the region of RB-EAS interaction and receiver. Both the form and width of the main part of observed radio pulses, presented at Fig. 2, agree well with the theory [1]. The pulse amplitudes as show our new more detailed calculations are in agreement with the theory for R0 = 5–20 km and ≈ 1016 eV too. That gives a possibility to suppose that according to comparison of the theory with observations the first radio pulse which initiates the lightning is generated due to RBEAS effect by high energy cosmic ray particle with ∼ 1016 eV. As one can see from the table during the storm 13.07.2002 from 14.25 to 15.23 local time it was observed 48 lightning. Storm active region was about 100 km2 . That means that the flash density during the storm was PL 48/(48 · 60 · 200) 1.6 × 10−4 km−2 s−1 . The flux of cosmic ray particles having the energy ∼ 1016 eV is [28] Pcr = 2 × 10−2 km−2 s−1 . One can see that though PL is significantly less than Pcr the relation PL /Pcr is noticeable ∼ 10−2 . One can give the following interpretation of this fact. The RB conditions are fulfilled in thundercloud during noticeable part of storm time. At that time the system is in the prepared RB state: electric field E at some height is close to Ec (see [9,10]). The cosmic ray particle with ∼ 1016 eV generate then according to RB-EAS effect a local pulse of electric current I ∼ 100–200 A. It serves for the lightning initiation (LI) and at the same time is the source of the observed first pulse of radio emission. The needed particle energy can of course vary for different thunderstorm conditions. But as the cosmic ray flux depends strongly on the particle energy (Pcr ∼
1/ 2 ), one cannot expect a strong change in particle energy needed for LI. And really according [26] cloud to ground lightning flux lays in the limits 1.5 × 10−4 PL 5 × 10−6 km−2 s−1 . Analogous result follows from [27] (see also [10, Chapter 8, Tables 8.1, 8.2, Fig. 8.46]) PL ∼ 10−5 –10−6 km−2 s−1 . Thus we see that the proposed interpretation of our observations does not contradict to the existing data. On the other hand this interpretation leads to the following fundamental conclusions: (1) The electric field in thunderstorm atmosphere cannot reach the values significantly higher than Ec , but keeps the value E Ec for a noticeable part o thunderstorm time; (2) The lightning is initiated by a high energy cosmic ray particle due to RB-EAS interaction; (3) The measurements of LI first radio pulses could serve as an effective new method for detection of cosmic ray particles with 1016 eV. This statement becomes evident when one compares results of our radio measurements during thunderstorm with usual methods of EAS observations. For example, the TienShan mountain station has a EAS system on the base of scintillation detectors located nearly concentric circumferences up to the radius R = 100 m [14]. To register 50 particles with 1016 eV one needs this installation to work several days. The radio RB-EAS method allowed to see the same 50 particles during 1 thunderstorm hour. Note that for a very high energy cosmic ray particles 1018 –1019 eV the radio pulse is so strong that it could be easily registered at the distance (2–3) × 103 km due to the pulse reflection from ionosphere. Of course proposed interpretation of presented experimental data needs to be verified. To prove it both much more observations and further development of the existing theory is needed.
5. Conclusions In conclusion we formulate briefly the main results of the experiment described in this Letter.
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(1) The wide band high time resolution radio interferometer is constructed and used for studying the initial pulse of lightning radio emission; (2) The observations of 48 lightning are fulfilled. They demonstrate that the first lightning radio pulse is bipolar and its form, width and amplitude are in agreement with the predictions of RB-EAS theory. The following interpretation of observational data is given: (a) The RB conditions are fulfilled in thundercloud during noticeable part of the storm time; (b) The lightning is initiated by combined action of runaway breakdown (RB) and extensive atmospheric shower (EAS); (c) The energy of cosmic ray particles generating observed EAS is ∼ 1016 eV. The further simultaneous observations of cosmic rays together with lightning radio emission, X-rays and γ -rays generated by the RB-EAS process can serve for the development of a new effective method for radio detection of high energy cosmic ray particles with 1017 –1018 eV.
Acknowledgements The authors are grateful to Prof. V.L. Ginzburg, Prof. E.L. Feinberg, Dr. H. Carlson and Dr. I.I. Royzen for useful discussions. The work was supported by Grants EOARD-ISTC N2236p, ISTC-1480.
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