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Radio pulses from extensive air showers during thunderstorms—the atmospheric electric field as a possible cause
Journal of Atnmpheric and Terrestrial Physics,1874,Vol.56, PP. 1431-1456. PergamonPress. Printedin NorthernIreland
SHORT PAPER
Radio pulses from extensive air showers during
thunderstorms-the atmospheric electric field as a possible cause N. ~NDOLESI, G. MORI~I and G. G. C. PALUMBO Consiglio N&on& delle Ricerche, TE.S.R.E. Leborstory, Via de’ Castagnoli, 40126 Bologna, Italy
1
(Received 25 October 1973; in revised form 2 January 1974) &&&-Radio pulses from Extensive Air Showers have been observed during thunderstorms. The measured RF electric fields in the pulses observed at 46-65 and 110 MHz cannot be explained by the geomagnetic charge separation theory. Alternative causes are discussed; it is proposed that the thundercloud electric field is responsible for the observed pulses.
pulses associated with Extensive Air Showers (EAS) have been studied, since the time of their discovery (JELLEY et al., 1965), with the aim to investigate in more details the nature of the EAS cascade and of the generating primary Cosmic Ray particle. There is substantial evidence (ALLAN, 1971) that the electromagnetic emission at radio frequencies from the shower disc is mainly due to the effect of the geomagnetic field on the charged particles. It has been suggested however (CHARMAN, 1967) that during thunderstorms the high static electric field produced, acting on the shower particles, could cause pulses as large as those produced by the geomagnetic mechanism. Recently the University of Michigan group operating at Mt. Chacaltaya (HAZEN and HENDEL, 1971) has found three showers during thunderstorms which showed pulses whose origin was attributed to the atmospheric electric field mechanism. In this letter we wish to present experimental evidence that high electric fields during thunderstorms may be indeed the cause for the production of radio pulses from EAS. At the EAS array of Medicina (Bologna) our group has been studying radio emission from showers for quite sometime (ALLAN, 1971; MANDOLESI et al., 1973). During the period February-June 1973 we have been subjected to several thunderstorms, some of long duration and with copious lightning and electric activity. During the same period the array haa been running continuously, detecting EAS of size N N lo5 -i- 10’ particles at a rate of ~3 h-l and radio pulses from them at a rate of ~1 week-l. What made us suspect a mechanism different from the geomagnetic one at work, was that we recorded two consecutive showers (which occurred RADIO
We would like to mention that other contemporary work suggesting an electrostatic contribution to the origin of radio pulses associated with cosmic-ray showers has recently appeared (GREOORY et al., 1973). 1431
N. MANDOLESI,
1432
G. MORIUI and G. G. C. PALUMBO
~8 min one after the other) both showing radio pulses. These two showers occurred during a violent thunderstorm. In fact we estimated that the probability of observing and that a normal radio shower could two radio showers in ~10 min was -lo-’ occur during a thunderstorm was ~10-~. At Medicina we can measure, for each detected shower: 6 zenith angle, p azimuth angle, core position, size N (total number of particles at sea level) by means of five scintillators, and we detected radio emission at three frequencies, namely 46, 65 and 110 MHz (bandwidth by ~2 MHz), all NS and EW polarized (MANDOLESI et al., l(373), with two (1/Z) folded dipoles, (n/2) apart, (n/4) above ground, for each polarization. After the first two consecutive showers, a record of thunderstorms was kept and the corresponding showers carefully analyzed. In about 150 days, seven thunderstorms had occurred at the array site. During two of those the array could not operate since the main power had been out off. Of all the showers recorded during the stormy activity five of them had associated radio emission (Table 1). Table 1. Master register
Event
NO.
No.
Date 6 6 6 8 14
34171 34172 41392 41641 51482
46 MHz NS EW
March March April April May
In Table 2 are summarized showers’ recorded.
no no Yes yes yes
Operating frequencies 65 MHz NS EW
yes yes Yes yes yes
the parameters
no no yes yes yes
no no yes Yes Yes
110 MHz NS EW Yes yes yes yes yes
of these five ‘thunderstorm
yes yes Yes Yes yes
radio-
Table 2. Event hT0.
1 2 3 4 5
1.3 2.6 4.9 1.7 1.3
x x x x x
107 10’ 105 lo6 106
50 126 21 36 37
45 124 30 32 50
40 122 37 30 43
20 35 1 46 20
214 134 126 206
17 25 30 36 15
>3260 3350 881
>1200 >660 695
110 320 4490 5530 4930
R, = perpendicular distance from the antenna site of frequency v to the shower axis. a = angle between shower direction and earth magnetic field B (at Medicina B forms an angle of ~30” with the vertical towards the south and has a strength of ~0.46 Gauss). &N,, = total RF electric field recorded at the antenna in p P m--l MHz-l normalized to N = 107 and obtained as a vectorial sum of the two polarization NS and EW.
1433
Radio pulses from extensive air showers during thunderstorms
v is taken from the magnetic north (~1’ from the geographic north at the array) counterclockwise. > indicates minimum values since the top of the pulses were not readable because off scale. From Table 1 one can see that all showers come from around the south and are in a very unfavourable position for the Geomagnetic charge separation mechanism (small a). The electric fields detected at all frequencies are enormous compared to the fields generated by Geomagnetic separation ( cTNvN a few ,u‘CT m-1 MHz-l), because are given as if the Geomagnetic mechanism was responsible for them. According to CHARMAN (1967, 1968), the deflection Ax due to the atmospheric electric field can be of the order of AM, deflection due to the Earth’s magnetic field on each charged particle if H,c - ET, where H, is the component of the Geomagnetic field perpendicular to the shower axis, ET is the component of the atmospheric electric field perpendicular to the shower motion and c the velocity of the light. The measured fields are in fact the sum of both effects but one can separate them if the expected field for each shower for pure Geomagnetic effect is known. Rather than using theoretical expressions for computing the contribution to the pulses due to pure Geomagnetic separation, we have used the average values of gNy obtained in the lateral distribution curves of radio pulses measured at Medicina (~HANDOLESI et al., 1973). Although the lateral distributions are not smooth curves and there is scatter among the points, the scatter is only ~10 $7 m-l MHz-l at the most, certainly not severe if compared with the values obtained in Table 2. Taking the values obtained for each value of R, shower size N, angle ccand bandwidth & into account, one can compute the average values of ET assuming that: AE+M AM
cc gE+M 6,
and therefore using Charman’s relationship for the whole shower:
E,-
(2 -
l)Hlc.
Substituting in this equation the values &‘E+Mmeasured, L?~ taken from the experimental lateral distribution curves and H,, the perpendicular component of the local magnetic field, one obtains Table 3: Table 3. Event No. 1 2 3 4 5
-%,4e(Vm-1) >3.7 x 104 1.4 x 105 2.0 x 104
-%,65(Vm-1) >104 >2.0 x 104 104
ET,llO(Vm-l) 2.3 4.2 1.9 5.3 3.0
x x x x x
lo3 IO6 104 104 104
where ET is the atmospheric electric field component perpendicular to the shower direction computed for each frequency.
1434
N. MAXXWLESI,G. MORI~I 8nd G. G. C.
PAL-O
From Table 3 one observes that: (a) The values of Err for each shower do not dXer substantially at the three frequencies. (b) E, for the last three showers for which all three frequencies were operating is of the same order of magnitude.
One should keep in mind however that the values obtained come from the oversimplifying hypothesis that the shower particles behave like single electrons that move along the shower track. Moreover, one needs to know the volume of the thunderstorm cloud across which the showers have gone. Clouds may extend over many (~10~) cubic kilometers with 1953) (COMERS, electrical gradients that may reach ~5 x lo* V m--l ($OH~~L~D, 1967). The estimated values of Er in Table 3 do not seem unreasonable although the precise values are of course unknown. The above seems to us the more likely hypothesis to explain the observed pulses. We have also investigated other possible causes rather than Charman’s mechanism for the production of the observed pulses i.e. : (i) noise background pulses; (ii) spurious random large pulses ; (iii) charge excess mechanism. We have ruled out the first two possib~ties with the following arguments: (i) although noise back~ound increases on onr records during thunde~~rms we never observed bandwidth limited pulses; and if there were any there is a negligible probability of observing them at three different frequencies occurring at the right time. (ii) No spurious random pulses have been observed on our oscilloscopic traces having the right shape and width (b-l) throughout the whole experiment. Furthermore the probability argument about time of occurrence on all channels still applies. (iii) The third possibility, i.e. charge excess mechanism, is a more subtle one. Strong electric fields could cause partial vertical separation of the positrons and electrons in the shower front, leading to a charge excess emission, analogous to the Askaryan mechanism (ASHARYAN,1962). However, such a mechanism in quiet atmosphere conditions, if at all present, does not contribute more than a few per cent to the total pulse height (PRESCOTTet aE., 1971). During thunderstorms a cont~bution to the pulses may well come form charge excess, but it appears that the bulk of the pulses is generated by a Charman like mechanism. It should be mentioned at this point that the shower array itself, and the trigger electronics, was found to be insensitive to the lightning. In conclusion we believe that the observed pulses were mainly caused by separation of charges in the enhanced electric fields of the thunderstorm clouds. It seems imperative to clarify how much the atmospheric eleotric field in normal fine-weather conditions also contributes to the radio emission from EAS, before any conclusion about the EAS parameters or the nature of the Cosmic Ray primary particles can be drawn. Ack~wle~e~~~8-~he authors wish to thank Prof. D, BEXNIend Dr. S. CECCHINI for helpful criticism and stimulating discussions.
Radio pulses from extensive air showers during thunderstorms
1435
REFERENCES ALLAN H. R.
1971
ASKARYAN G. A. CHALMERSJ. A.
1962 1967
CHAFLMAN W. N.
1967 1968 1973
GREGORYA. G., CLAY R. W. and PRESCOTTJ. R. HAZEN W. E. and HENDEL A. Z. JELLEY J. V., FRUIN J. H., PORTERN. A., WEEKS T. C., S~~ITHF. C. and PORTERR. A. MANDOLESIN., MORI~I G. end PALUMBO G. G. C. PRESCOTTJ. R., HOU~H J. H. and PIDCOCKJ. K.
1971 1965
1973
Progress in Elenzentary Particle and Cosmic Ray Physica, Vol. X (Edited by J. G. WILSON and S. A. WOUTWYSEN), p. 169.North-Holland, Amsterdam. Soviet Phys (JETP), 14, 441. Atmospheric Electricity (2nd edn.). Pergamon Press, Oxford. Nature, Lo&. 215, 497. J. atmos. terr. Phys. 30, 196. Nature, Phys. Sci. 245, 86. Pro& 12th Int. Conf. Cosmic Rays, 3, 1127 Hobart. Nature, Lond. 205, 327.
Proo. 13th Ist. Conf. Cosmic Rays 4, 2414 Denver. Nature Phys. Sci. 233, 109.
SHORT PAPER
Radio pulses from extensive air showers during
thunderstorms-the atmospheric electric field as a possible cause N. ~NDOLESI, G. MORI~I and G. G. C. PALUMBO Consiglio N&on& delle Ricerche, TE.S.R.E. Leborstory, Via de’ Castagnoli, 40126 Bologna, Italy
1
(Received 25 October 1973; in revised form 2 January 1974) &&&-Radio pulses from Extensive Air Showers have been observed during thunderstorms. The measured RF electric fields in the pulses observed at 46-65 and 110 MHz cannot be explained by the geomagnetic charge separation theory. Alternative causes are discussed; it is proposed that the thundercloud electric field is responsible for the observed pulses.
pulses associated with Extensive Air Showers (EAS) have been studied, since the time of their discovery (JELLEY et al., 1965), with the aim to investigate in more details the nature of the EAS cascade and of the generating primary Cosmic Ray particle. There is substantial evidence (ALLAN, 1971) that the electromagnetic emission at radio frequencies from the shower disc is mainly due to the effect of the geomagnetic field on the charged particles. It has been suggested however (CHARMAN, 1967) that during thunderstorms the high static electric field produced, acting on the shower particles, could cause pulses as large as those produced by the geomagnetic mechanism. Recently the University of Michigan group operating at Mt. Chacaltaya (HAZEN and HENDEL, 1971) has found three showers during thunderstorms which showed pulses whose origin was attributed to the atmospheric electric field mechanism. In this letter we wish to present experimental evidence that high electric fields during thunderstorms may be indeed the cause for the production of radio pulses from EAS. At the EAS array of Medicina (Bologna) our group has been studying radio emission from showers for quite sometime (ALLAN, 1971; MANDOLESI et al., 1973). During the period February-June 1973 we have been subjected to several thunderstorms, some of long duration and with copious lightning and electric activity. During the same period the array haa been running continuously, detecting EAS of size N N lo5 -i- 10’ particles at a rate of ~3 h-l and radio pulses from them at a rate of ~1 week-l. What made us suspect a mechanism different from the geomagnetic one at work, was that we recorded two consecutive showers (which occurred RADIO
We would like to mention that other contemporary work suggesting an electrostatic contribution to the origin of radio pulses associated with cosmic-ray showers has recently appeared (GREOORY et al., 1973). 1431
N. MANDOLESI,
1432
G. MORIUI and G. G. C. PALUMBO
~8 min one after the other) both showing radio pulses. These two showers occurred during a violent thunderstorm. In fact we estimated that the probability of observing and that a normal radio shower could two radio showers in ~10 min was -lo-’ occur during a thunderstorm was ~10-~. At Medicina we can measure, for each detected shower: 6 zenith angle, p azimuth angle, core position, size N (total number of particles at sea level) by means of five scintillators, and we detected radio emission at three frequencies, namely 46, 65 and 110 MHz (bandwidth by ~2 MHz), all NS and EW polarized (MANDOLESI et al., l(373), with two (1/Z) folded dipoles, (n/2) apart, (n/4) above ground, for each polarization. After the first two consecutive showers, a record of thunderstorms was kept and the corresponding showers carefully analyzed. In about 150 days, seven thunderstorms had occurred at the array site. During two of those the array could not operate since the main power had been out off. Of all the showers recorded during the stormy activity five of them had associated radio emission (Table 1). Table 1. Master register
Event
NO.
No.
Date 6 6 6 8 14
34171 34172 41392 41641 51482
46 MHz NS EW
March March April April May
In Table 2 are summarized showers’ recorded.
no no Yes yes yes
Operating frequencies 65 MHz NS EW
yes yes Yes yes yes
the parameters
no no yes yes yes
no no yes Yes Yes
110 MHz NS EW Yes yes yes yes yes
of these five ‘thunderstorm
yes yes Yes Yes yes
radio-
Table 2. Event hT0.
1 2 3 4 5
1.3 2.6 4.9 1.7 1.3
x x x x x
107 10’ 105 lo6 106
50 126 21 36 37
45 124 30 32 50
40 122 37 30 43
20 35 1 46 20
214 134 126 206
17 25 30 36 15
>3260 3350 881
>1200 >660 695
110 320 4490 5530 4930
R, = perpendicular distance from the antenna site of frequency v to the shower axis. a = angle between shower direction and earth magnetic field B (at Medicina B forms an angle of ~30” with the vertical towards the south and has a strength of ~0.46 Gauss). &N,, = total RF electric field recorded at the antenna in p P m--l MHz-l normalized to N = 107 and obtained as a vectorial sum of the two polarization NS and EW.
1433
Radio pulses from extensive air showers during thunderstorms
v is taken from the magnetic north (~1’ from the geographic north at the array) counterclockwise. > indicates minimum values since the top of the pulses were not readable because off scale. From Table 1 one can see that all showers come from around the south and are in a very unfavourable position for the Geomagnetic charge separation mechanism (small a). The electric fields detected at all frequencies are enormous compared to the fields generated by Geomagnetic separation ( cTNvN a few ,u‘CT m-1 MHz-l), because are given as if the Geomagnetic mechanism was responsible for them. According to CHARMAN (1967, 1968), the deflection Ax due to the atmospheric electric field can be of the order of AM, deflection due to the Earth’s magnetic field on each charged particle if H,c - ET, where H, is the component of the Geomagnetic field perpendicular to the shower axis, ET is the component of the atmospheric electric field perpendicular to the shower motion and c the velocity of the light. The measured fields are in fact the sum of both effects but one can separate them if the expected field for each shower for pure Geomagnetic effect is known. Rather than using theoretical expressions for computing the contribution to the pulses due to pure Geomagnetic separation, we have used the average values of gNy obtained in the lateral distribution curves of radio pulses measured at Medicina (~HANDOLESI et al., 1973). Although the lateral distributions are not smooth curves and there is scatter among the points, the scatter is only ~10 $7 m-l MHz-l at the most, certainly not severe if compared with the values obtained in Table 2. Taking the values obtained for each value of R, shower size N, angle ccand bandwidth & into account, one can compute the average values of ET assuming that: AE+M AM
cc gE+M 6,
and therefore using Charman’s relationship for the whole shower:
E,-
(2 -
l)Hlc.
Substituting in this equation the values &‘E+Mmeasured, L?~ taken from the experimental lateral distribution curves and H,, the perpendicular component of the local magnetic field, one obtains Table 3: Table 3. Event No. 1 2 3 4 5
-%,4e(Vm-1) >3.7 x 104 1.4 x 105 2.0 x 104
-%,65(Vm-1) >104 >2.0 x 104 104
ET,llO(Vm-l) 2.3 4.2 1.9 5.3 3.0
x x x x x
lo3 IO6 104 104 104
where ET is the atmospheric electric field component perpendicular to the shower direction computed for each frequency.
1434
N. MAXXWLESI,G. MORI~I 8nd G. G. C.
PAL-O
From Table 3 one observes that: (a) The values of Err for each shower do not dXer substantially at the three frequencies. (b) E, for the last three showers for which all three frequencies were operating is of the same order of magnitude.
One should keep in mind however that the values obtained come from the oversimplifying hypothesis that the shower particles behave like single electrons that move along the shower track. Moreover, one needs to know the volume of the thunderstorm cloud across which the showers have gone. Clouds may extend over many (~10~) cubic kilometers with 1953) (COMERS, electrical gradients that may reach ~5 x lo* V m--l ($OH~~L~D, 1967). The estimated values of Er in Table 3 do not seem unreasonable although the precise values are of course unknown. The above seems to us the more likely hypothesis to explain the observed pulses. We have also investigated other possible causes rather than Charman’s mechanism for the production of the observed pulses i.e. : (i) noise background pulses; (ii) spurious random large pulses ; (iii) charge excess mechanism. We have ruled out the first two possib~ties with the following arguments: (i) although noise back~ound increases on onr records during thunde~~rms we never observed bandwidth limited pulses; and if there were any there is a negligible probability of observing them at three different frequencies occurring at the right time. (ii) No spurious random pulses have been observed on our oscilloscopic traces having the right shape and width (b-l) throughout the whole experiment. Furthermore the probability argument about time of occurrence on all channels still applies. (iii) The third possibility, i.e. charge excess mechanism, is a more subtle one. Strong electric fields could cause partial vertical separation of the positrons and electrons in the shower front, leading to a charge excess emission, analogous to the Askaryan mechanism (ASHARYAN,1962). However, such a mechanism in quiet atmosphere conditions, if at all present, does not contribute more than a few per cent to the total pulse height (PRESCOTTet aE., 1971). During thunderstorms a cont~bution to the pulses may well come form charge excess, but it appears that the bulk of the pulses is generated by a Charman like mechanism. It should be mentioned at this point that the shower array itself, and the trigger electronics, was found to be insensitive to the lightning. In conclusion we believe that the observed pulses were mainly caused by separation of charges in the enhanced electric fields of the thunderstorm clouds. It seems imperative to clarify how much the atmospheric eleotric field in normal fine-weather conditions also contributes to the radio emission from EAS, before any conclusion about the EAS parameters or the nature of the Cosmic Ray primary particles can be drawn. Ack~wle~e~~~8-~he authors wish to thank Prof. D, BEXNIend Dr. S. CECCHINI for helpful criticism and stimulating discussions.
Radio pulses from extensive air showers during thunderstorms
1435
REFERENCES ALLAN H. R.
1971
ASKARYAN G. A. CHALMERSJ. A.
1962 1967
CHAFLMAN W. N.
1967 1968 1973
GREGORYA. G., CLAY R. W. and PRESCOTTJ. R. HAZEN W. E. and HENDEL A. Z. JELLEY J. V., FRUIN J. H., PORTERN. A., WEEKS T. C., S~~ITHF. C. and PORTERR. A. MANDOLESIN., MORI~I G. end PALUMBO G. G. C. PRESCOTTJ. R., HOU~H J. H. and PIDCOCKJ. K.
1971 1965
1973
Progress in Elenzentary Particle and Cosmic Ray Physica, Vol. X (Edited by J. G. WILSON and S. A. WOUTWYSEN), p. 169.North-Holland, Amsterdam. Soviet Phys (JETP), 14, 441. Atmospheric Electricity (2nd edn.). Pergamon Press, Oxford. Nature, Lo&. 215, 497. J. atmos. terr. Phys. 30, 196. Nature, Phys. Sci. 245, 86. Pro& 12th Int. Conf. Cosmic Rays, 3, 1127 Hobart. Nature, Lond. 205, 327.
Proo. 13th Ist. Conf. Cosmic Rays 4, 2414 Denver. Nature Phys. Sci. 233, 109.