Optical aspects of winter lightning discharges triggered by the rocket‐wire technique in Hokuriku district of Japan

Abstract

Optical aspects of winter lightning flashes triggered by the rocket‐wire technique from 1977 to 1982 in Hokuriku district of Japan are described. Thirty‐nine flashes out of 45 successfully triggered ones were photographed by high‐speed cameras with framing speed of around 1000 frames/s. Each flash starts with an upward leader from the tip of the ascending steel wire, regardless of the polarity of the cloud charge. Observations made at a distance of 70 m from the launch site gave clear images of the initial development of upward leaders. Positive leaders started at a speed of around 0.1×10 5 m/s with a number of branches, while negative one started at a speed of 0.5 to 1×10 5 m/s with a few branches. The established flashes kept their brightness for 19 ms up to 706 ms and four flashes for more than 600 ms. The long life flashes are apt to occur when the electric field on the ground keeps high value for a long period. After inspecting darkness of the channel images in the film, the time variation of luminous intensity of the flashes are classified into four patterns.