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Atmospheric electric field measurements in Northern Nigeria during the dry season
Journal of Atmonpheric and Terrestrial Physics, 1971, Vol. 58. pp. 581-588. Pergamon Press. Printed inNorthern Ireland
Atmospheric electric field measurements in Norfhern Nigeria during the dry season D. J. HARRIS* Department of Electrical Engineering, Ahmadu Be110University, Zaria, Nigeria (Received 17 September
1970)
Ah&&--The atmosphericelectricfield and the air-to-earth currenthave been measuredduring the dry season (November-March) at Zaria in Northern Nigeria. The results show a polarity reversal and magnitude increase by a factor of up to 60 during early morning in both field and current, whilst night time results are those normally obtained under fair weather conditions. Possible causes and implication8of these results &rediscussed. Detail8 of a fully tran&torked field mill amplifier are given. 1. INTRODTJCTI~N
Northern Nigeria is characterised by two distinct seasons, one exceedingly dry and the other with heavy rain and thunderstorm activity. The period of extreme dryness extends from November to March, with maximum snd minimum temperatures at about 95’ and 60°F respectively, and relative humidity figures often becoming less than 20 per cent at 7 a.m. and less than 10 per cent at, 4 p.m. at Zaria where the measurements reported were taken. By contrast the wet period, from May to September, has temperatures up to 100°F and a relative humidity that is above 60 per cent most of the time, and often over 90 per cent. The measurement of atmospheric electrical effects has been carried out during both these seasons for the past three years. During the dry season measurements have been concentrated on ‘fair weather’ electrical effects, whilst during the wet se&son measurements hsve concentrated on the lightning discharge and its effects. This paper describes some of the measurements taken during the dry season. An extensive srea of West Africa, ranging along the southern limits of the Sahara Desert, is subjected to exceedingly dry conditions for the period indicated, and this dryness is accompanied for much of the time by a dust haze. Northern winds, emenating from the Sahara desert, carry with them fine dust particles in suspension. This dust-laden wind is known as the Harmattan, and can reduce visibility to considerably less than a mile, often extending for many days at a time. The extreme dryness leads to excellent electricsl insulation properties for many materials, with a subsequent prevalence of static electricity effects. Bodies that become electrically charged may retain their charge for a very long period and static electric shocks are a commonplace experience. It was thought that the presence of so much dust in the atmosphere accompanied as it usually is by brisk winds, might well lead to a substantial effect on the electric field in the atmosphere, and equipment was therefore set up to measure any effects that might occur. THE
ULIMATE
of
* Present address: Department of Electronic and Electrical Engineering,The Polytechnic, Portsmouth, England. 681
582
U.
J . HARRIS
The nature of the Harmattan and the dust particles carried by it has been the subject of investigations by HAWLTON and ARCHBOLD (1945) ; GROVE (1962) ; and MCKEOWN (1958). The influence of the Harmattan is experienced by virtually the whole of West Africa, being most severe close to the desert area. Reports from aircraft flying in to Kano show that it frequently extends up to a height of 8000 ft,, and layers in excess of 12,000 ft have been measured. Measurements on composition and size of dust particles have been made at Ibadan, about 500 miles south of Zaria. These showed the particles to be small, mostly in the 0*1--O-5 ,u range, and the constituents to be mainly silica (560/), a1umina (1Oo/o), lime (50/6), ferric oxide (4%) and magnesia (2%) with smaller additions of potash, soda and titanium oxide. In view of the much larger distance that these particles have travelled, it can be assumed that many of the heavier constituent particles will have been precipitated by the time that the dust has travelled down to Tbadan, and that the size distribution will be shifted to larger dimensions at Zaria where these measurements of the electrical effects have been made. The co-ordinate position of Zaria is approx. latitude N ll”, longitude E 8”. There is some evidence for anticipating substantial electric: effects of the dust in motion. Very high voltages can of course be generated by blowing dry dust from one place to another and electric generators can be made this way. LANE (1965) has published a remarkable photo~aph of ‘lightni~’ occurring in volcanic dust above Surtsey, the Icelandic volcano.DEMON (1953) and UCH~KAW,~ (l95I) have reported enhanced electric fields in severe dust storms in the Sahara and in The measurements by Demon were made at Beni-Abbes Japan respectively. in the Northern Sahara, and gave fields up to 16 kV/m in the dust cloud, the polarity corresponding to a predominance of positively charged dust particles in the air. The particles were of sand and a few microns in diameter. Uchikawa found fields corresponding to negatively charged dust particles with values up to 600 V/m, i.e. a field reversal took place as well as a field enhancement. Investigation of charged partiole size indicated that the larger particles had a positive charge and the lighter particles a negative charge. Measurements at Zaria were initially concentrated on the electric field magnitude and were carried out by two independent methods. -4 measurement of the air-to earth current was also made by an approximate method. Some results of these measurements have been given; HARRIS (1966), (1969), and more details of the measurements are presented in this paper. Me~~rements during tfia thunderstorm period will be given in a later companion paper. 2. EXPERIMENTAL METHODS AND EQUIPMENT The majority of the electric field measurements were made using a field mill. Such mills have been widely used for electric field measurements, and are described by CHALMERS (1957). The field mill used consisted of six highly insulated horizontal conducting segments, of total area 135 cm2, alternately shielded and exposed to the electric field to be measured by a rotating segmented earthed rotor. This gave an alternating output current of periodi~ity 200 G/S, and an r.m.s. current magnitude of approximately 1O-8 amp for an electric field of 200 V/m. The output from the field mill is proportional to the magnitude of the electric field, and is calibrated
!J!he atmospheric electric field in Nigeri&
583
by applying a known electric field. The output from the field mill for the earlier measurements was applied to a cathode-follower stage placed close to the field mill so as to reduce the input capacit&nce and therefore keep the time const&nt of the input circuit well below the period of the output wave. This stage acted as an impedance convertor, having approx, unity gain, a high input impedance of several megohms, and a low o&put impedance of a few kilohms. The valve used was an EF37A, and the stage w&s connected via a long coaxial cable to a high-gain stable amplifier followed by a deteotor and chart recorder. The field mill was set out in a space clear of buildings, trees etc. In the later work, the EF37A valve stage was replaced by a field-effect transistor ~PFlO5, whioh proved to be very satisfactory. It had a low current drain from a 18-V dry battery, and was operated for s, long period from such & cell. The input impedance was about 2 MS& and it had good long term stability. A transistorised amplifier to follow the FET stage was constructed, giving a fully trans~~&ble instrument ~&libr&~d in volts per metre and with a drive output for a chart recorder9 and is shown in the oircuit diagram of Fig. 1. Maximum
BV
Fig. 1. Field mill amplifk
circuit.
sensitivity for full scale deflection of the meter was approx. 250 V/m, and five rsnges enabled fields up to 10,000 V/m to be readily measured. The rectified output could be fed to a chart recorder for continuous recording. Frequent calibration checks were made by imposing known eleotrio fields, snd the overall accuracy is estimated to lie within 410% & 10 V/m. The latter contribution is included because of & tendency of the zero to drift, although this problem was much less acute with the FET stage than with the initial cathode follower circuit, Measurements were made continuously and consistently over periods of many months. The method does not differentiate between positive and negative fields, although spot checks for polarity could be made by introducing a chatrged conductor of known polarity near the field mill and noting whether the output from the mill increased or decreased. Simultaneous measurements were made by setting up a radio-active probe, using 25 PC of Radium 226 foil as the radio-active source, about half a metre above the surface of the ground. The voltage of the probe relative to earth was measured by a high impedance resistive potential divider (200 MQ) and a d.c. electrometer. The output was recorded along with the field mill output. Whilst a high accuracy cannot be claimed for these measurements, it has the advantage of showing the
polarity of the field and also acts as a confirmation check on the general pattern of the field changes. The air-to-earth current was investiga~d by setting up a i m square horizontal aluminium plate close to the surface of the earth but well insutated from it, and measuring the potential of the plate relative to earth by an electrometer of 1O1*112 input resistance. The time constant of the system was increased to a few minutes by the addition of a high quality capacitor between plate and earth. This was essential to reduce the effect of a fluctuating displacement current, probably caused by charged dust clouds moving in the vicinity of the current collecting plate. The current m~g~tude and polarity were recorded &longside the field measurements on the chart reeorder. However, an order of magnitude result only is claimed for this current measurement. 3. MEASUREMENTS
IXJRSNOTHE I~ARMATTAK SEA~CW
Almost continuo~ls measurements of the electric field were taken during the period October 1966-April 1967, and ~on~rrnati~n readings were taken at intervals through the 1967-68 and 1968-69 dry seasons. The form and ~harac~risti~s of the field variation obtained were consistent during the periods, showing that the phenomena observed were not freak results due to an unusual eombination of circumstances, but were the normal repeatable characteristics to be expected each year. Measurements of electric field at the surface of the Earth made at Zaria f&l into three categories. During the summer months there is thunderstorm activity during some part of each day, with wide ~uctuations of electric field. With no nearby clouds the field has its normal fair-weather value in the range 50-100 V/m. When thunderstorms form in the neighborhood there is an aceomFanying dramatic increase of field, mostly with a polarity corresponding to a negative lower charge on the cloud and with a magnitude which may be many kV/m. A maximum value of 40 kV/m was recorded, and there were oocasions when a high positive field of several kVfm were recorded. The thunderstorm season is usually over by mid-October, after which the field exhibits normal fair weather variations. From mi~ight to noon the field is between 40 and 80 V/m with no distinctive variations, and from noon onwards there is an increase to about 160 V/m around 2000 hr, the value subsequently decreasing to the fewer vahxe by midnight. The peak value is thus reached about 1900 hr GMT (Zaria time is GMT -l-1), which correlates with experience in other parts of the world. The third category of measurement is that associated with the Harmattan season and is ch~a~~ri~d by a remarkable reversal of polarity and increase of ma~itude in the early morning. The effect is shown in Figs, 2 and 3. The onset of the effect is gradual in that during the initial part of the Harmattan season the only indication is a reduction in the positive electric field during the early hours of the morning. This decrease in magnitude becomes enhanced as the dry season progresses until eventually the field suf.Fersa reversal in polarity and thereafter the reversed polarity field becomes much greater in magnitude than the normal fair weather field, values up to 5 kV/m being recorded. The d%erenee in scale factor for positive and negative fields should be noted. Rgure 2 shows the actual
-
t3 Dec.
----
22 Jon.
--
4 Feb.
E
3 --I--
--I f
3-8
Dec.
MT
12-26 Dec. f 3ODac.-4 Jcm. N-23 Jun.
---
27Jon.-7Feb.
-.-
3-10
April
v~~~tio~ of field with time of d&y for three ~~rtie~&r day rep& ~bt~~ by average v8Jue for severa interv&, during th H~~tt~ season. The and the ex~n~~~ of the petiod during the day when it 0~~~s cm. be No ~~~~~t variation of the field pattern Tom normal during the 0~0~ hr w&s noted over the H&rrn&t~~ period during the rest of the day. This c~~tin~~t~on of the regular fair weather paths is p~~~o~~ly ma&ed in the figure bang value averaged owr sevmd days. The ch&~ge in geld in the e&y rno~~g e&n be a rapid one, the t~~~~r from the
normal fair weather positive field to the reverse polarity high magnitude value often occurring in a period of an hour, but the recovery during the afternoon and early evening usually occupied several hours. It is noteworthy also that the rapid changeover did not usually occur directly at sunrise, but an hour or two later. Another interesting feature is that the magnitude of the anomalous effect was not always related to the visible range in the dust haze, some of the most marked negative field recordings occurring when the visibility was not particularly bad.
Fig. 4. Air-earth
current and electric field.
The form of the results was confirmed by the radioactive probe measurements, although there were differences in detail. Since, however, the probe method is open to more difficulty of interpretation whilst the electric field mill readings depend directly on the electric flux at the measuring segments, and therefore directly on the vertical electric field, the probe technique has been used solely to give field polarity and to confirm independently the main features of the field mill result. Simultaneous measurements of electric field and air-to-earth currents are shown in Fig. 4. The current showed some fluctuations in spite of the time constant of the equipment, and a smoothed out curve is shown. It can be seen that the form of the current variation corresponds very closely to that of the electric field variation, reversing polarity at about the same time and being approximately proportional to the field throughout the daily variation. Whilst high accuracy is not claimed for the current measurements because of theirfluctuations, it does show that the conduotivity of the air does not change significantly during the daily period, and it shows also that the sign of the nett charge reaching the surface of the earth does change with the reversal of the field polarity. A conductivity of approx. 2 . lo-l5 mho/m is indicated. 4.
DISCUSSION
OF
RESULTS
The ‘fair weather’ field, as recorded before the Harmattan effect sets in, has a variation during the day that is in accord with experience elsewhere. The form
The atmospheric electric field in Nigeria
687
reproducible from day to da.y, although there were modifications in overall level of field and in minor detail. The increase to a maximum value st epprox. 1900 hr GMT corresponds to the time of maximum global thunderstorm activity. The transition from a normal fair weather variation to the anomalous condition is a gradual one, a period of about a month elapsing between the onset of the effect and its maximum. It would seem that both a Harmattan dust haze and a very low humidity are required since a dust haze early or late in the dry season, when the relative humidity is not so low, is not accompanied by the characteristic effect on the electric field. Measurements at Zaria show that the relative humidity is at its lowest in February when the effect on the electric field is greatest. It is significant that there is little effect on the electric field during the night period even though the dust haze persists, the moon often being obscured at that time of the year. Three possible causes sre that there is a marked reduction in air movement as the surface of the earth is cooled (in contrast to the brisk breezes during the day), that there is an absence of photoionization when the radiation from the sun is removed, snd that there is condensation of water vapour on the dust particles when the temperature is reduced during the night. The effect during the day is a dramatic one however. It should be emphasized that these are not occasional occurrences but the regular patterns of daily variation in the Harm&tan season that have been confirmed over a three year period. The effect is undoubtedly due to the charging of the dust particles in the air, and rapid fluctustions of field occur as the dust laden air moves in the vicinity of the field mill. It is clear that during the day the atmosphere close to the surface of the earth acquires a negative charge, approx. 5 . lo--* C/m2 of surface being required for a field of 5000 V/m. This could be due to the whole of the dust layer being negatively charged, or result from a dipole layer with the upper region positive and the lower negative in polarity. The latter is thought to be the most likely explanation. Ionization or charge transfer could result from photoionization by solar radiation or by ‘frictional’ effects under the stirring action of the atmosphere by surface heating of the earth. Production of ions would hsve to be accompanied by charge separation in which positive charge is carried to the upper layers of the atmosphere. This is in contrast to the work of UCHIRAWA (1951) who measured the charge of particles in a dust storm and reports that larger particles have positive charge and smaller particles have negative by the friction between themselves. The particle sizes in the Harmattan are likely to be very much smaller, however, than those in the Uchikawa case. This is consistent with the presence of strong reverse fields in the absence of a marked visible Harmattan haze at times, suggesting that it is the smaller particles that are important, and these may behave differently. A mono-polarity dust particle layer would require that the positive charge associated with the negative dust charge be left elsewhere, e.g. at the point of origin of the dust, and that the dust laden atmosphere retain its charge as it moves across the country. The possibility is unlikely in view of the apparent loss of charge during the night and replenishment in the early morning, and with the distance of the presumed point of origin and the wind velocity taken into account. The measurement of current to the Earth shows that this also reverses polarity was
and increases by about the same factor as the electric field. The effective conductivity of the air remains approx. constant throughout the day, and is of the same order of magnitude as that normally assumed. It is seen that a current of about 2 . lo-” Alma is rna~ta~~ for a period of several hours, and sinncethis current will exhaust a charge of 5 . 10-* C/ma in less than an hour, it wouId seem that oharge replenishment must continue during the day. If a bipolar layer is assumed, positive charge must also be transferred to the upper atmosphere and thereby help to maintain the positive potential there. It is estimated that an area of West Africa of about a million square miles is affected by the Harmattan, and a similar effect may be found in other areas around the Sahara, and indeed be associated with other desert areas of the world. If this is so, the effect could well be of significance in the current balance of the earth and also make a significant contribution to the maintenance of the positive potential of the upper atmosphere. Similar me~~emants in or near other desert areas of the world should be made to see if such an effect occurs elsewhere. An attempt to verify the bi-polar nature of the atmosphere has been made. A pair of lightweight field mills together with amplifier and tape recorder, with a combined weight Iess than 4 lb, has been mounted for ascent using a tethered balloon teohnique. It was found, however, that with the small excess lift of the balloons available and the brisk breezes associated with the Harmattan period, the measurements could not be carried out and must await a greater lifting capability. Aci&wu&x$gems&--Theauthor gladly aoknowledge~ the assistance of Mr. J. BUCZKOWSKI who designd the tran&&or amplifier and who, together with other colleagues at Abmadu Be110 University, gave valuable support during this work.
REFEILBXWES 1987
J. A.
CEA-
1963 1962
DE&CON&f.
GROVE A. T. -rLTONR.A.o;nd HARRIS HARRIS
D.J. D. J.
LANE F.W. McK&owz+zJ. D. J. UC=WA K.
ARU~OLD
J.W.
1945 1966 1969 1965 1968 1961
dt?aoq&f-ic ELeotrictig.Pergamon Pret38, Oxford. J. beck.Cepa,N&S. Bee. Sci. p. 12% Hamttcsra .Dwt in Nigtwiu, Cambridge
Atmospheric electric field measurements in Norfhern Nigeria during the dry season D. J. HARRIS* Department of Electrical Engineering, Ahmadu Be110University, Zaria, Nigeria (Received 17 September
1970)
Ah&&--The atmosphericelectricfield and the air-to-earth currenthave been measuredduring the dry season (November-March) at Zaria in Northern Nigeria. The results show a polarity reversal and magnitude increase by a factor of up to 60 during early morning in both field and current, whilst night time results are those normally obtained under fair weather conditions. Possible causes and implication8of these results &rediscussed. Detail8 of a fully tran&torked field mill amplifier are given. 1. INTRODTJCTI~N
Northern Nigeria is characterised by two distinct seasons, one exceedingly dry and the other with heavy rain and thunderstorm activity. The period of extreme dryness extends from November to March, with maximum snd minimum temperatures at about 95’ and 60°F respectively, and relative humidity figures often becoming less than 20 per cent at 7 a.m. and less than 10 per cent at, 4 p.m. at Zaria where the measurements reported were taken. By contrast the wet period, from May to September, has temperatures up to 100°F and a relative humidity that is above 60 per cent most of the time, and often over 90 per cent. The measurement of atmospheric electrical effects has been carried out during both these seasons for the past three years. During the dry season measurements have been concentrated on ‘fair weather’ electrical effects, whilst during the wet se&son measurements hsve concentrated on the lightning discharge and its effects. This paper describes some of the measurements taken during the dry season. An extensive srea of West Africa, ranging along the southern limits of the Sahara Desert, is subjected to exceedingly dry conditions for the period indicated, and this dryness is accompanied for much of the time by a dust haze. Northern winds, emenating from the Sahara desert, carry with them fine dust particles in suspension. This dust-laden wind is known as the Harmattan, and can reduce visibility to considerably less than a mile, often extending for many days at a time. The extreme dryness leads to excellent electricsl insulation properties for many materials, with a subsequent prevalence of static electricity effects. Bodies that become electrically charged may retain their charge for a very long period and static electric shocks are a commonplace experience. It was thought that the presence of so much dust in the atmosphere accompanied as it usually is by brisk winds, might well lead to a substantial effect on the electric field in the atmosphere, and equipment was therefore set up to measure any effects that might occur. THE
ULIMATE
of
* Present address: Department of Electronic and Electrical Engineering,The Polytechnic, Portsmouth, England. 681
582
U.
J . HARRIS
The nature of the Harmattan and the dust particles carried by it has been the subject of investigations by HAWLTON and ARCHBOLD (1945) ; GROVE (1962) ; and MCKEOWN (1958). The influence of the Harmattan is experienced by virtually the whole of West Africa, being most severe close to the desert area. Reports from aircraft flying in to Kano show that it frequently extends up to a height of 8000 ft,, and layers in excess of 12,000 ft have been measured. Measurements on composition and size of dust particles have been made at Ibadan, about 500 miles south of Zaria. These showed the particles to be small, mostly in the 0*1--O-5 ,u range, and the constituents to be mainly silica (560/), a1umina (1Oo/o), lime (50/6), ferric oxide (4%) and magnesia (2%) with smaller additions of potash, soda and titanium oxide. In view of the much larger distance that these particles have travelled, it can be assumed that many of the heavier constituent particles will have been precipitated by the time that the dust has travelled down to Tbadan, and that the size distribution will be shifted to larger dimensions at Zaria where these measurements of the electrical effects have been made. The co-ordinate position of Zaria is approx. latitude N ll”, longitude E 8”. There is some evidence for anticipating substantial electric: effects of the dust in motion. Very high voltages can of course be generated by blowing dry dust from one place to another and electric generators can be made this way. LANE (1965) has published a remarkable photo~aph of ‘lightni~’ occurring in volcanic dust above Surtsey, the Icelandic volcano.DEMON (1953) and UCH~KAW,~ (l95I) have reported enhanced electric fields in severe dust storms in the Sahara and in The measurements by Demon were made at Beni-Abbes Japan respectively. in the Northern Sahara, and gave fields up to 16 kV/m in the dust cloud, the polarity corresponding to a predominance of positively charged dust particles in the air. The particles were of sand and a few microns in diameter. Uchikawa found fields corresponding to negatively charged dust particles with values up to 600 V/m, i.e. a field reversal took place as well as a field enhancement. Investigation of charged partiole size indicated that the larger particles had a positive charge and the lighter particles a negative charge. Measurements at Zaria were initially concentrated on the electric field magnitude and were carried out by two independent methods. -4 measurement of the air-to earth current was also made by an approximate method. Some results of these measurements have been given; HARRIS (1966), (1969), and more details of the measurements are presented in this paper. Me~~rements during tfia thunderstorm period will be given in a later companion paper. 2. EXPERIMENTAL METHODS AND EQUIPMENT The majority of the electric field measurements were made using a field mill. Such mills have been widely used for electric field measurements, and are described by CHALMERS (1957). The field mill used consisted of six highly insulated horizontal conducting segments, of total area 135 cm2, alternately shielded and exposed to the electric field to be measured by a rotating segmented earthed rotor. This gave an alternating output current of periodi~ity 200 G/S, and an r.m.s. current magnitude of approximately 1O-8 amp for an electric field of 200 V/m. The output from the field mill is proportional to the magnitude of the electric field, and is calibrated
!J!he atmospheric electric field in Nigeri&
583
by applying a known electric field. The output from the field mill for the earlier measurements was applied to a cathode-follower stage placed close to the field mill so as to reduce the input capacit&nce and therefore keep the time const&nt of the input circuit well below the period of the output wave. This stage acted as an impedance convertor, having approx, unity gain, a high input impedance of several megohms, and a low o&put impedance of a few kilohms. The valve used was an EF37A, and the stage w&s connected via a long coaxial cable to a high-gain stable amplifier followed by a deteotor and chart recorder. The field mill was set out in a space clear of buildings, trees etc. In the later work, the EF37A valve stage was replaced by a field-effect transistor ~PFlO5, whioh proved to be very satisfactory. It had a low current drain from a 18-V dry battery, and was operated for s, long period from such & cell. The input impedance was about 2 MS& and it had good long term stability. A transistorised amplifier to follow the FET stage was constructed, giving a fully trans~~&ble instrument ~&libr&~d in volts per metre and with a drive output for a chart recorder9 and is shown in the oircuit diagram of Fig. 1. Maximum
BV
Fig. 1. Field mill amplifk
circuit.
sensitivity for full scale deflection of the meter was approx. 250 V/m, and five rsnges enabled fields up to 10,000 V/m to be readily measured. The rectified output could be fed to a chart recorder for continuous recording. Frequent calibration checks were made by imposing known eleotrio fields, snd the overall accuracy is estimated to lie within 410% & 10 V/m. The latter contribution is included because of & tendency of the zero to drift, although this problem was much less acute with the FET stage than with the initial cathode follower circuit, Measurements were made continuously and consistently over periods of many months. The method does not differentiate between positive and negative fields, although spot checks for polarity could be made by introducing a chatrged conductor of known polarity near the field mill and noting whether the output from the mill increased or decreased. Simultaneous measurements were made by setting up a radio-active probe, using 25 PC of Radium 226 foil as the radio-active source, about half a metre above the surface of the ground. The voltage of the probe relative to earth was measured by a high impedance resistive potential divider (200 MQ) and a d.c. electrometer. The output was recorded along with the field mill output. Whilst a high accuracy cannot be claimed for these measurements, it has the advantage of showing the
polarity of the field and also acts as a confirmation check on the general pattern of the field changes. The air-to-earth current was investiga~d by setting up a i m square horizontal aluminium plate close to the surface of the earth but well insutated from it, and measuring the potential of the plate relative to earth by an electrometer of 1O1*112 input resistance. The time constant of the system was increased to a few minutes by the addition of a high quality capacitor between plate and earth. This was essential to reduce the effect of a fluctuating displacement current, probably caused by charged dust clouds moving in the vicinity of the current collecting plate. The current m~g~tude and polarity were recorded &longside the field measurements on the chart reeorder. However, an order of magnitude result only is claimed for this current measurement. 3. MEASUREMENTS
IXJRSNOTHE I~ARMATTAK SEA~CW
Almost continuo~ls measurements of the electric field were taken during the period October 1966-April 1967, and ~on~rrnati~n readings were taken at intervals through the 1967-68 and 1968-69 dry seasons. The form and ~harac~risti~s of the field variation obtained were consistent during the periods, showing that the phenomena observed were not freak results due to an unusual eombination of circumstances, but were the normal repeatable characteristics to be expected each year. Measurements of electric field at the surface of the Earth made at Zaria f&l into three categories. During the summer months there is thunderstorm activity during some part of each day, with wide ~uctuations of electric field. With no nearby clouds the field has its normal fair-weather value in the range 50-100 V/m. When thunderstorms form in the neighborhood there is an aceomFanying dramatic increase of field, mostly with a polarity corresponding to a negative lower charge on the cloud and with a magnitude which may be many kV/m. A maximum value of 40 kV/m was recorded, and there were oocasions when a high positive field of several kVfm were recorded. The thunderstorm season is usually over by mid-October, after which the field exhibits normal fair weather variations. From mi~ight to noon the field is between 40 and 80 V/m with no distinctive variations, and from noon onwards there is an increase to about 160 V/m around 2000 hr, the value subsequently decreasing to the fewer vahxe by midnight. The peak value is thus reached about 1900 hr GMT (Zaria time is GMT -l-1), which correlates with experience in other parts of the world. The third category of measurement is that associated with the Harmattan season and is ch~a~~ri~d by a remarkable reversal of polarity and increase of ma~itude in the early morning. The effect is shown in Figs, 2 and 3. The onset of the effect is gradual in that during the initial part of the Harmattan season the only indication is a reduction in the positive electric field during the early hours of the morning. This decrease in magnitude becomes enhanced as the dry season progresses until eventually the field suf.Fersa reversal in polarity and thereafter the reversed polarity field becomes much greater in magnitude than the normal fair weather field, values up to 5 kV/m being recorded. The d%erenee in scale factor for positive and negative fields should be noted. Rgure 2 shows the actual
-
t3 Dec.
----
22 Jon.
--
4 Feb.
E
3 --I--
--I f
3-8
Dec.
MT
12-26 Dec. f 3ODac.-4 Jcm. N-23 Jun.
---
27Jon.-7Feb.
-.-
3-10
April
v~~~tio~ of field with time of d&y for three ~~rtie~&r day rep& ~bt~~ by average v8Jue for severa interv&, during th H~~tt~ season. The and the ex~n~~~ of the petiod during the day when it 0~~~s cm. be No ~~~~~t variation of the field pattern Tom normal during the 0~0~ hr w&s noted over the H&rrn&t~~ period during the rest of the day. This c~~tin~~t~on of the regular fair weather paths is p~~~o~~ly ma&ed in the figure bang value averaged owr sevmd days. The ch&~ge in geld in the e&y rno~~g e&n be a rapid one, the t~~~~r from the
normal fair weather positive field to the reverse polarity high magnitude value often occurring in a period of an hour, but the recovery during the afternoon and early evening usually occupied several hours. It is noteworthy also that the rapid changeover did not usually occur directly at sunrise, but an hour or two later. Another interesting feature is that the magnitude of the anomalous effect was not always related to the visible range in the dust haze, some of the most marked negative field recordings occurring when the visibility was not particularly bad.
Fig. 4. Air-earth
current and electric field.
The form of the results was confirmed by the radioactive probe measurements, although there were differences in detail. Since, however, the probe method is open to more difficulty of interpretation whilst the electric field mill readings depend directly on the electric flux at the measuring segments, and therefore directly on the vertical electric field, the probe technique has been used solely to give field polarity and to confirm independently the main features of the field mill result. Simultaneous measurements of electric field and air-to-earth currents are shown in Fig. 4. The current showed some fluctuations in spite of the time constant of the equipment, and a smoothed out curve is shown. It can be seen that the form of the current variation corresponds very closely to that of the electric field variation, reversing polarity at about the same time and being approximately proportional to the field throughout the daily variation. Whilst high accuracy is not claimed for the current measurements because of theirfluctuations, it does show that the conduotivity of the air does not change significantly during the daily period, and it shows also that the sign of the nett charge reaching the surface of the earth does change with the reversal of the field polarity. A conductivity of approx. 2 . lo-l5 mho/m is indicated. 4.
DISCUSSION
OF
RESULTS
The ‘fair weather’ field, as recorded before the Harmattan effect sets in, has a variation during the day that is in accord with experience elsewhere. The form
The atmospheric electric field in Nigeria
687
reproducible from day to da.y, although there were modifications in overall level of field and in minor detail. The increase to a maximum value st epprox. 1900 hr GMT corresponds to the time of maximum global thunderstorm activity. The transition from a normal fair weather variation to the anomalous condition is a gradual one, a period of about a month elapsing between the onset of the effect and its maximum. It would seem that both a Harmattan dust haze and a very low humidity are required since a dust haze early or late in the dry season, when the relative humidity is not so low, is not accompanied by the characteristic effect on the electric field. Measurements at Zaria show that the relative humidity is at its lowest in February when the effect on the electric field is greatest. It is significant that there is little effect on the electric field during the night period even though the dust haze persists, the moon often being obscured at that time of the year. Three possible causes sre that there is a marked reduction in air movement as the surface of the earth is cooled (in contrast to the brisk breezes during the day), that there is an absence of photoionization when the radiation from the sun is removed, snd that there is condensation of water vapour on the dust particles when the temperature is reduced during the night. The effect during the day is a dramatic one however. It should be emphasized that these are not occasional occurrences but the regular patterns of daily variation in the Harm&tan season that have been confirmed over a three year period. The effect is undoubtedly due to the charging of the dust particles in the air, and rapid fluctustions of field occur as the dust laden air moves in the vicinity of the field mill. It is clear that during the day the atmosphere close to the surface of the earth acquires a negative charge, approx. 5 . lo--* C/m2 of surface being required for a field of 5000 V/m. This could be due to the whole of the dust layer being negatively charged, or result from a dipole layer with the upper region positive and the lower negative in polarity. The latter is thought to be the most likely explanation. Ionization or charge transfer could result from photoionization by solar radiation or by ‘frictional’ effects under the stirring action of the atmosphere by surface heating of the earth. Production of ions would hsve to be accompanied by charge separation in which positive charge is carried to the upper layers of the atmosphere. This is in contrast to the work of UCHIRAWA (1951) who measured the charge of particles in a dust storm and reports that larger particles have positive charge and smaller particles have negative by the friction between themselves. The particle sizes in the Harmattan are likely to be very much smaller, however, than those in the Uchikawa case. This is consistent with the presence of strong reverse fields in the absence of a marked visible Harmattan haze at times, suggesting that it is the smaller particles that are important, and these may behave differently. A mono-polarity dust particle layer would require that the positive charge associated with the negative dust charge be left elsewhere, e.g. at the point of origin of the dust, and that the dust laden atmosphere retain its charge as it moves across the country. The possibility is unlikely in view of the apparent loss of charge during the night and replenishment in the early morning, and with the distance of the presumed point of origin and the wind velocity taken into account. The measurement of current to the Earth shows that this also reverses polarity was
and increases by about the same factor as the electric field. The effective conductivity of the air remains approx. constant throughout the day, and is of the same order of magnitude as that normally assumed. It is seen that a current of about 2 . lo-” Alma is rna~ta~~ for a period of several hours, and sinncethis current will exhaust a charge of 5 . 10-* C/ma in less than an hour, it wouId seem that oharge replenishment must continue during the day. If a bipolar layer is assumed, positive charge must also be transferred to the upper atmosphere and thereby help to maintain the positive potential there. It is estimated that an area of West Africa of about a million square miles is affected by the Harmattan, and a similar effect may be found in other areas around the Sahara, and indeed be associated with other desert areas of the world. If this is so, the effect could well be of significance in the current balance of the earth and also make a significant contribution to the maintenance of the positive potential of the upper atmosphere. Similar me~~emants in or near other desert areas of the world should be made to see if such an effect occurs elsewhere. An attempt to verify the bi-polar nature of the atmosphere has been made. A pair of lightweight field mills together with amplifier and tape recorder, with a combined weight Iess than 4 lb, has been mounted for ascent using a tethered balloon teohnique. It was found, however, that with the small excess lift of the balloons available and the brisk breezes associated with the Harmattan period, the measurements could not be carried out and must await a greater lifting capability. Aci&wu&x$gems&--Theauthor gladly aoknowledge~ the assistance of Mr. J. BUCZKOWSKI who designd the tran&&or amplifier and who, together with other colleagues at Abmadu Be110 University, gave valuable support during this work.
REFEILBXWES 1987
J. A.
CEA-
1963 1962
DE&CON&f.
GROVE A. T. -rLTONR.A.o;nd HARRIS HARRIS
D.J. D. J.
LANE F.W. McK&owz+zJ. D. J. UC=WA K.
ARU~OLD
J.W.
1945 1966 1969 1965 1968 1961
dt?aoq&f-ic ELeotrictig.Pergamon Pret38, Oxford. J. beck.Cepa,N&S. Bee. Sci. p. 12% Hamttcsra .Dwt in Nigtwiu, Cambridge