Inferring Compressible Fluid Dynamics From Vent Discharges During Volcanic Eruptions

Observations at numerous volcanoes reveal that eruptions are often accompanied by continual radio frequency (CRF) emissions. The source of this radiation, however, has remained elusive until now. Through experiments and the analysis of field data, we show that CRF originates from proximal discharges driven by the compressible fluid dynamics associated with individual volcanic explosions. Blasts produce flows that expand supersonically, generating regions of weakened dielectric strength in close proximity to the vent. As erupted material—charged through fragmentation, friction, or other electrification process—transits through such a region, pyroclasts remove charge from their surfaces in the form of small interparticle spark discharges or corona discharge. Discharge is maintained as long as overpressured conditions at the vent remain. Beyond describing the mechanism underlying CRF, we demonstrate that the magnitude of the overpressure at the vent as well as the structure of the supersonic jet can be inferred in real time by detecting and locating CRF sources. Key Points The production of vent discharges is tightly coupled to the compressible fluid dynamics associated with individual volcanic explosions The supersonic (Prandtl‐Meyer) expansion of an overpressured jet controls the location and timing of vent discharges The magnitude of the vent overpressure, as well as the structure of an overpressured jet, can be inferred from the location of vent discharges