Atmospheric Electrical Properties of Volcanic Ash Particles in the Eruption of Mt. Usu Volcano, 1977

Abstract

During the volcanic activities of the Mt. Usu volcano, Hokkaido Island, Japan, in August 1977, atmospheric electrical observations, especially atmospheric electric field strength (potential gradient), charges on volcanic ash particles together with spatial concentration and size of ash particles were carried out at the school grounds of Sobetsu Junior High School, Sobetsu Town. The township is located approximately 5km east-northeastward from the crater of Mt. Usu, and the observations were made from August 9 to August 15, 1977.
Although the atmospheric electric field strength showed undisturbed values from a calm night to the early morning, immediately after a sea breeze commenced blowing in the morning the atmospheric electric field showed an abrupt negative shift and distinct changes were seen as shown in Figs. 3∼5 and 11, an the other hand, when the volcanic ash particles fell the atmospheric electric field changed violently to positive alternating to negative again and vice versa incessantly. The values of the electric field exceeded ±15, 000 V⋅m-1 when coagulated large ash particles fell. The inverse relationship between the polarities of the electric field and the net falling ash particle current held.
In laboratory experiments with volcanic ash particles, relatively smaller ash particles were charged more negatively than larger ash particles. This came from friction. It is considered that our experimental results adequately explains the negative atmospheric electric field recorded in dust storms of volcanic ash swept away from the ground surface.