On the Disturbance of the Atmospheric Potential Gradient caused by the Smoke-cloud of the Volcano Yake-yama

摘要

The Volcano Yake-yama, Niigata prefecture, erupted at 8h in the morning of Feb. 5th, 1949. The cloud of the eruption-smoke was carried away by the west-northwesterly monsoon to the Kashimanada and the volcanic ashes fell on the northern part of Kanto district. On this day, the Benndorf's electrometer of the Kakioka Magnetic Observatory recorded a large bag-type disturbance exceeding 1200 volts per meter between 11h and 12h. After 12h, the value of the atmospheric potential gradient remained as a small negative value, and after 16h, it returned to its normal value.The author considered the model distribution of the electric charge to explain the disturbance of the potential gradient. We have the aerological data which confirm that the height of the smoke-cloud was lower than 4.5km. Therefore, we took the height as 4km and estimated the distribution of the electric charge. It is natural to consider the distribution of electric charge in the cloud as spindle-type. We assume a column of point charges which have the following electric charges respectively, 2, 3, 5, 7, 8, 9, 10, 10, 10, 10, 10, 9, 8, 7, 5, 3, 2, and the distance between the consecutive charges is assumed to be 4km, and the speed of the smoke-cloud to be 72km per hour. The distribution of the potential gradient under such assumption is shown with figure in the text. The sum of the electric charge which makes the spindle-type distribution is -0.55 coul. It is nearly the same order of magnitude with the case of the Volcano Asama-yama, in which the electric charge was estimated as -1.4 coul or -0.13 coul.