INTERESTING ASPECTS OF THE AIR-EARTH CURRENT DENSITY OVER THE OCEANS AS 

 DERIVED FROM ATMOSPHERIC-ELECTRIC DATA OF CRUISE VH OF THE CARNEGIE 



In the tables in sections V and VI of the present 

 volume there have been tabulated computed values of air - 

 earth current density expressed in 10-7 esu. In section 

 V there are 263 values of the current density, each rep- 

 resenting a period of approximately one to two hours in 

 the early afternoon, local time, for 263 days at sea. In 

 section VI there are twenty-four complete, or essential- 

 ly complete, diurn^-variation series and seven partial 

 series of values of current density, containing 645 indi- 

 vidual values. On the average, one diurnal-variation 

 series was obtained every two weeks during the cruise. 

 It is of interest to examine these air-earth current den- 

 sity data as an aid to interpretation of the changes con- 

 stantly taking place in the electrical conditions in the 

 lower atmosphere. 



For the present discussion the earth and the high 

 atmosphere may be regarded as two conductors, each of 

 negligible resistance, which constitute a spherical con- 

 denser, the outer element having a potential E relative 

 to the inner element. Then if R is the resistance of a 

 vertical column of unit cross section of the atmosphere 

 between the elements of this condenser, at a given geo- 

 graphical location, the air-earth current density, i, 

 through that column, assuming Ohm's law to apply, is 

 E/R. From measurements of conductivity in the region 

 between the earth's surface and an altitude of 18 kilome- 

 ters, Gish (1) has made an estimate of the columnar re- 

 sistance up to the latter altitude, and gives the result as 

 1021 ohms or 1.11 x 10^ esu. In a preceding paper, the 

 quite small variation of the conductivity through the day 

 over the oceans, under least-disturbed conditions during 

 fair weather, has been discussed and diurnal-variation 

 curves for various periods of cruise VH have been pre- 

 sented (figs. 3, 4, and 5). Those results indicate that the 

 variation is from 5 to 10 per cent of the mean. When the 

 conductivity is so constant one may fairly conclude that 

 disturbing factors such as smoke, fog, mist or haze, 

 which act to reduce the conductivity, are largely absent, 

 not only in the lowest part of the atmosphere where the 

 conductivity is measured, but also throughout the regions 

 above which are even less likely to come under the in- 

 fluence of any disturbing factor, so that the columnar re- 

 sistance R, like the conductivity, may be regarded as 

 essentially constant through the day. For the present 

 paper, this constant value of R will be taken as 1021 

 ohms or 1.11 x 109 esu, as given by Gish, and will rep- 

 resent the total columnar resistance rather than the re- 

 sistance to a height of 18 kilometers. Such a procedure 

 introduces no important error in the conclusions to be 

 drawn here because, as Gish has pointed out, the upper 

 8 of the 18 kilometers contribute only 5 per cent of the 

 total resistance, and from the region beyond the 18 kil- 

 ometers on up to the highly conducting upper region one 

 might expect an even smaller contribution to the total. 



When disturbed conditions arise, and conductivity 

 values are lower than usual, disturbing factors must be 

 present in a region of unknown height near the earth's 

 surface. The magnitude of the drop in conductivity (or 

 increase in resistivity) and the height to which the dis- 

 turbing factor penetrates are important in determining 



the degree to which the columnar resistance R may be 

 affected. A very thick layer containing the disturbing 

 element, with the conductivity greatly reduced through- 

 out that layer, would produce a considerable increase in 

 R. On cruise VTI there were occasions when conductivity 

 was notably reduced below the usually prevailing value, 

 and on some of these occasions potential- gradient was 

 much increased, whereas on others it was not greatly 

 different from the normal value. Values of air-earth 

 current density sometimes were much lower, therefore, 

 than ordinarily encountered while at other times they 

 were only slightly low. Some of the disturbed periods 

 will be discussed in detail later, and the attempt will be 

 made to determine the height to which the disturbing fac- 

 tor existed in the atmosphere during each period. First, 

 however, an examination will be made of the values of 

 air -earth current density generally found for "normal" 

 or "least-disturbed" conditions, in order that a basis 

 may be established for recognizing disturbed values. 



Since the air-earth current density 1 equals E/R, 

 and R is essentially constant under least-disturbed con- 

 ditions over the oceans, variations in i under those con- 

 ditions are the result chiefly of variations in E. Meas- 

 urements of potential-gradient in those circumstances 

 reveal the extent and character of the variations in E , 

 and cruise VII data discussed earlier in this volume have 

 supported previous conclusions that potential-gradient 

 has a large diurnal variation which proceeds according 

 to universal time. Necessarily, then, the air-earth cur- 

 rent density varies through the day according to universal 

 time. To examine this variation, all complete diurnal- 

 variation series of current density in the table in section 

 VI, except the series for November 13 to 14, 1928, which 

 was disturbed, and the four series obtained in October and 

 November, 1929, were assembled into three groups and 

 an average diurnal-variation curve was drawn for each 

 group. The four October and November series omitted 

 were included in a fourth group based on continuous 

 registration of conductivity as well as potential-gradient 

 during those months, which made more than twenty days 

 of data in each element available (see tables in sections 

 Vn and VTn and the curves in fig. 5) for computation of 

 air-earth current density. Assembling the current-den- 

 sity data into four groups has divided the material sea- 

 sonally and geographically as shown in table 1. 



Diurnal-variation curves for the four groups are 

 shown in figure 14, and their similarity to diurnal -varia- 

 tion curves for potential-gradient for the same periods 

 (fig. 1) is obvious. The displacement of the time of max- 

 imum in the months April to September to a later hour 

 than is found for the months of October to February, and 

 the smaller amplitude of the diurnal variation in the for- 

 mer months than in the latter, are features of the diurnal 

 variation in current density which have become recog- 

 nized as important features of the potential-gradient also, 

 as pointed out by Wait and Sverdrup (2) in their discus- 

 sion of a mechanism for explaining the universal charac- 

 ter of the diurnal variation of the potential-gradient. 



It is seen, then, that the air-earth current density 

 changes through the Greenwich day in a systematic and 



145 



