Acoustic wavefield and Mach wave radiation of flashing arcs in strombolian explosion measured by image luminance

Abstract

Explosive activity often generates visible flashing arcs in the volcanic plume considered as the evidence of the shock-front propagation induced by supersonic dynamics. High-speed image processing is used to visualize the pressure wavefield associated with flashing arcs observed in strombolian explosions. Image luminance is converted in virtual acoustic signal compatible with the signal recorded by pressure transducer. Luminance variations are moving with a spherical front at a 344.7-m/s velocity. Flashing arcs travel at the sound speed already 14-m above the vent and are not necessarily the evidence of a supersonic explosive dynamics. However, seconds later, the velocity of small fragments increases, and the spherical acousto-luminance wavefront becomes planar recalling the Mach wave radiation generated by large scale turbulence in high-speed jet. This planar wavefront forms a Mach angle of 55° with the explosive jet axis, suggesting an explosive dynamics moving at M o -=-1.22 Mach number. Key Points New image processing uses image luminance to measure the full acoustic wavefieldFlashing arcs are not necessarily the evidence of the shock-front propagationAcousto-luminance has plane wavefront recalling Mach wave radiation pattern