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Behnke, S. A.; Edens, H. E.; Thomas, R. J.; Smith, C. M.; McNutt, S. R.; Eaton, A. R. van; Cimarelli, C.; Cigala, V. (2018): Investigating the Origin of Continual Radio Frequency Impulses During Explosive Volcanic Eruptions. In: Journal of Geophysical Research-Atmospheres, Vol. 123, No. 8: pp. 4157-4174
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Abstract

Volcanic lightning studies have revealed that there is a relatively long-lasting source of very high frequency radiation associated with the onset of explosive volcanic eruptions that is distinct from radiation produced by lightning. This very high frequency signal is referred to as continual radio frequency (CRF) due to its long-lasting nature. The discharge mechanism producing this signal was previously hypothesized to be caused by numerous, small (10-100m) leader-forming discharges near the vent of the volcano. To test this hypothesis, a multiparametric data set of electrical and volcanic activity occurring during explosive eruptions of Sakurajima Volcano in Japan was collected from May to June 2015. Our observations show that a single CRF impulse has a duration on the order of 160ns (giving an upper limit on discharge length of 10m) and is distinct from near-vent lightning discharges that were on the order of 30m in length. CRF impulses did not produce discernible electric field changes and occurred in the absence of a net static electric field. Lightning mapping data and infrared video observations of the eruption column showed that CRF impulses originated from the gas thrust region of the column. These observations indicate that CRF impulses are not produced by small, leader-forming discharges but rather are more similar to a streamer discharge, likely on the order of a few meters in length. Plain Language Summary This paper investigates the origin of a radio frequency (RF) signal previously termed continual RF that has been detected from the ash plumes of explosive volcanic eruptions. Continual RF had been hypothesized to be caused by many very small lightning-like discharges on the order of 100m in length occurring at the vent of the volcano. To test this hypothesis, we conducted a multiparametric observational experiment at Sakurajima Volcano in Japan. We measured continual RF during eruptions of Sakurajima in 2015 and found that continual RF is a collection of short duration RF impulses, on the order of 160ns in duration. These RF impulses are distinct from those typically produced by lightning. We also observed small, 30-m lightning discharges occurring near the volcanic vent that were distinct from the source of continual RF. Our observations show that continual RF is caused by an electrical discharge that is more simple than even a very small (approximate to 100m) lightning discharge. We conclude that continual RF is caused by numerous, small, electrical discharges similar to streamer discharges, on the order of a few meters in length.