The generation of light filaments in air has attracted intense interest during the last decade due to possible applications for lightning protection and atmospheric remote sensing. The properties of plasma filaments and their characteristics were investigated in numerous publications. Of primary interest among them are the concentration of plasma in a filament and its decay. Although several methods were used to measure these parameters in the filament, including time-resolved diffractometry [1], longitudinal interferometry [2], and plasma channel conductivity, the published data differ essentially. We report results of experimental measurements of plasma density decay in the filament using a new original method based on transverse scattering of probe terahertz (THz) pulses. The control of the plasma decay by applying an external electric field is also demonstrated.
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