No abstract

Soon after the onset of an eruption, model forecasts of ash dispersal are used to mitigate the hazards to aircraft, infrastructure, and communities downwind. However, it is a significant challenge to constrain the model inputs during an evolving eruption. Here we demonstrate that volcanic lightning may be used in tandem with satellite detection to recognize and quantify changes in eruption style and intensity. Using the eruption of Calbuco volcano in southern Chile on 22 and 23 April 2015, we investigate rates of umbrella cloud expansion from satellite observations, occurrence of lightning, and mapped characteristics of the fall deposits. Our remote sensing analysis gives a total erupted volume that is within uncertainty of the mapped volume (0.56 ± 0.28 km3 bulk). Observations and volcanic plume modeling further suggest that electrical activity was enhanced both by ice formation in the ash clouds >10 km above sea level and development of a low‐level charge layer from ground‐hugging currents.

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 (10–100 m) 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 160 ns (giving an upper limit on discharge length of 10 m) and is distinct from near‐vent lightning discharges that were on the order of 30 m 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.

[1] On 3 August 2010 an extensive lightning flash was triggered over Langmuir Laboratory in New Mexico. The upward positive leader propagated into the storm's midlevel negative charge region, extending over a horizontal area of 13 Â 13 km and 7.5 km altitude. The storm had a normalpolarity tripolar charge structure with upper positive charge over midlevel negative charge. Lightning Mapping Array (LMA) observations were used to estimate positive leader velocities along various branches, which were in the range of 1-3 Â 10 4 m s À1 , slower than in other studies. The upward positive leader initiated at 3.4 km altitude, but was mapped only above 4.0 km altitude after the onset of retrograde negative breakdown, indicating a change in leader propagation and VHF emissions. The observations suggest that both positive and negative breakdown produce VHF emissions that can be located by time-of-arrival systems, and that not all VHF emissions occurring along positive leader channels are associated with retrograde negative breakdown.

[1] Using three-dimensional lightning mapping observations, the initial leaders of intracloud flashes have been found to start at a median speed of about 1.6 Â 10 5 m s À1 and to decelerate during the first 10-15 ms of the discharge. The results disagree with the predictions that the speed should increase with time as the developing leader shorts out an increasing potential difference in the storm. The observations can be explained if the flash initiation region is preconditioned in some manner to give a high initial speed and if the preconditioning decays with time and/or decreases with distance to give the observed speed decrease. Such preconditioning could be the result of ionization and excited molecules produced by energetic electron avalanches.