Earth‐current measurements at the College‐Fairbanks Polar‐Year Station
Authors: Rooney W. J. , Sherman K. L. 
Published: February 2008

Continuous registration of earth‐current potentials was begun in September 1932 at the College‐Fairbanks (Alaska) Polar‐Year Station and continued until the end of March 1934. The potentials measured were those between two pairs of electrodes, one pair 1.28 km apart on a due south‐north line and the other 1.21 km apart on a due west‐east line. The electrodes were grids of lead buried at a depth of about one meter. The measuring apparatus consisted of two Leeds and Northrup type‐R galvanometers and a la Cour photographic recorder. The current‐sensitivity of the galvanometers, about 1×10−10 ampere per millimeter deflection on the drum, permitted the use of resistances of 2.0 megohms in series with them, so that the total circuit‐resistances were not markedly affected by variations in the contact‐resistances of the electrodes. The total external resistance of each circuit was Jess than 300 ohms during the summer and, although it increased considerably during the winter months due to increased contact‐resistances at the electrodes as the soil about them froze, it never reached as much as one per cent of the total circuit‐resistance in the case of the west‐east line or as much as three per cent in the case of the south‐north line.

The records for a full year, from October 1, 1932, to September 30, 1933, have been reduced. The most striking feature of the records is the comparatively great short‐period fluctuations, often oscillatory in character, which are found to occur almost exclusively at night, and the contrasting smoothness of the traces during the daylight hours. The amplitude of these short‐period oscillations frequently reaches one volt per kilometer or more. Corresponding in time to these short sharp pulsations at College disturbances less conspicuous in intensity and abruptness are invariably found in the records from Tucson (Arizona), and in the case of strong isolated disturbances the effect is noticeable also in the records from Huancayo (Peru). They are not at all local in character, but rather due to an intensification of widespread electrical disturbances.

The mean diurnal‐variation curve for the northward component shows a fairly smooth double oscillation in potential gradient with maxima at about 5h and 16h local time, and minima just before noon and midnight. Its mean diurnal range is about 14 mv/km if all days are included and about 11 mv/km on calm days only. The same type of diurnal variation persists throughout the year with a considerably smaller range, 7 to 9 mv/km during the winter, and a corresponding increase, 17 to 22 mv/km, in the summer.

The diurnal‐variation curve for the eastward component during the winter is essentially the same as the northward curve except for a reversal in sign and a somewhat smaller amplitude. The diurnal‐variation curve obtained during the summer tends more toward a single‐period curve, the mean curve for the year being intermediate between the two. There is some evidence that the records of the eastward component were affected somewhat by contact‐potentials at the electrodes during the summer.

A comparison of the earth‐current and auroral records at College shows considerable agreement between aurorae and disturbances in the earth‐current records.