decrease and give a negative peak before settling to normal daytime small values. The negative peak in the morning is particularly very large in case of the filter apparatus at
0.1 m height. Large values of space charge density as measured by the filter apparatus have been observed at nighttime almost throughout the winter season from November to
February, when the stability of the lower atmosphere is good, nocturnal inversions are
Ž .
very low and winds are generally calm. Earlier, Kamra 1982 and Deshpande and Ž
. Kamra 1992 have discussed the space charge distributions in the lowest 2 m of the
atmosphere at this station. Variations in atmospheric electric field and space charge density in Figs. 3–5 do not
show any consistent parallel trend. However, space charge density at these levels undergoes large variations at the time of the morning maximum in electric field. Also,
the magnitude of electric field is comparatively very low when very large negative space Ž
. charge persists close to ground at nighttime e.g. see Fig. 4 . Conductivity at this station
Ž
y1 3
. shows very large values 4 = 10
Srm at nighttime. Unfortunately, these high values saturated the range of our conductivity apparatus throughout the nighttime.
Daytime values of conductivity showed normal values - 2 = 10
y1 4
Srm. The agreement between the three space charge records during daytime exists even for
shorter time scales. For example, Fig. 6 shows a record of the space charge density as measured by the three instruments when observations are taken at the rate of 10 samples
per second and averaged for each second in the afternoon of January 23, 1996 when the atmosphere is well mixed and winds are moderate. The three values are comparable to
each other within the range of experimental error of the instruments.
4. Discussion
The stability conditions of the lower atmosphere at this place are grossly different between the daytime and nighttime periods throughout the winter months. Measurements
Ž .
of Shaha and Ananthakrishnan 1976 , made earlier at Pune, show that the lapse rate reversals occuring almost regularly in the morning and evening hours in this season
clearly show the changeover from the stable to unstable atmosphere in the morning hours and from the unstable to stable atmosphere in the evening hours. So, here we will
limit our discussion only to the differences observed in the vertical space charge distributions between the daytime and nightime observations during winter months. No
attempt will be made to interpret the space charge fluctuations of shorter periods, say on the time-scales of seconds or minutes or to assess the vertical fluxes and the accumula-
tion of space charge in the surface layer. Simultaneous micro-meteorological measure- ments of the atmosphere stability need be used for such studies.
As mentioned earlier, the structures that raise the filter apparatus and Faraday cage above the ground, can distort the local electric field and consequently modify the space
charge distribution around the apparatuses. This can cause an error in the space charge measurements. Unfortunately, no theoretical or experimental estimates of such errors
have been made. However, several observations in our measurements do indicate that the prevailing electric field do not much influence our results on space charge distribu-
tions. For example, the differences in the space charge measurements at no time show
variations parallel to that of the electric field. Secondly, the magnitude of the electric field in calm conditions at nighttime when the error due to this factor should maximize
are relatively low and the differences in the measured space charge values by different techniques are large. On the contrary, there is a very small difference in the space charge
values when the electric field is the maximum at the time of the morning maxima. Lastly, while the polarity of the fair weather field is always negative, the polarity of the
measured space charges with the two techniques can be of either polarity. Therefore, the error caused due to this factor is not likely to significantly affect our results.
The accumulations of radioactive emanations from the ground and aerosol particles below inversions at nighttime cause large gradients of conductivity and space charge
Ž close to the Earth’s surface e.g. Crozier, 1963; Hoppel and Gathman, 1971; Willett,
. Ž
. 1978; Kamra, 1982 . For example, in low wind conditions, Crozier’s 1965 measure-
ments of the electric field and space charge density profiles show the presence of positive space charge, sometimes as high as 750 pCrm
3
close to the Earth’s surface due to electrode effect. Above this layer of positive space charge, Crozier’s observations
show also a layer of negative space charge. Manifestation of this layer of negative space charge is known as the reverse electrode effect. These high values of space charge
density and the reverse electrode effect under low wind conditions have been attributed to the increased ionisation due to the accumulation of radioactive gases and their decay
products in the shallow layer close to the Earth’s surface. The theoretical calculations of
Ž .
Ž .
Hoppel 1969 using the observed ionisation profile of Crozier and Biles 1966 , also show that the reverse electrode effect is observed under nonturbulent conditions if the
layer of high ionization is present near to the Earth’s surface. It has been estimated that the lower positive space charge layer may be confined to only about 1–2 m above the
ground in the nighttime low wind conditions as is clearly shown in the first 4 h of data in Fig. 4. Above this positive space charge layer the negative space charge layer under
the reverse electrode effect may prevail in many tens of meters in height. Turbulent conditions during daytime increase the mixing in the atmosphere and the two layers of
positive and negative space charges mix-up and almost disappear to produce very low values of the net space charge density in that region.
Our measurements confirm the existence of such vastly different values of space charge under turbulent and nonturbulent conditions and also at different vertical levels in
the stable and stratified atmosphere at nighttime. Elsewhere, we plan to discuss the vertical charge distribution and its dependence on the stability of the lower atmosphere.
Here, it will suffice to say that at this location large vertical gradients of space charge density exist near the earth’s surface at nighttime. Under such conditions, therefore, the
selection of the technique adopted to measure the space charge density becomes crucial for studying its vertical distribution near the Earth’s surface.
Both filter apparatus are mounted horizontally with the centers of their intake at 0.1 and 1 m levels. Their intake openings are of 10 cm diameter and the air being drawn
into the filter can enter from a larger vertical depth outside the openings. The measured space charge value at a height should therefore correspond only to a mean value around
the level of the apparatus. So, if the space charge density has a vertical gradient, the measurements made with the filter technique will give a mean value of space charge
averaged for a vertical depth which may be a few tens of centimeter around the
apparatus. On the other hand, the Faraday cage technique gives a measurement of the averaged value of space charge in a volume of the air enclosed inside the cage. In our
measurements, this volume extends to a height of 3.6 m in the vertical. Within this height, our nighttime observations in stable atmosphere show large variations in space
charge density both in its magnitude and polarity. In such a situation the cage is not uniformly filled with charged particles and the assumption to maintain the entire cage in
a region of uniform charge density to obtain a valid measurement of space charge density from the Faraday cage technique, cannot be fulfilled.
There is a third method for measurement of space charge near the ground by Ž
measuring the electric field profile either with a double field mill device Gathman, .
Ž .
1968 or with a tethered balloon mounted field meter Gathman, 1972 . The divergence of electric field between two levels gives a measure of the total space charge that is
present between the two levels. So, similar to the Faraday cage technique, to infer the space charge density at a particular height between the two levels of measurements, one
has to assume that the total space charge is uniformly distributed between the two levels of measurement.
5. Conclusions
