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Lightning occurred associated with the ash clouds of all three eruptions of Mt. Spurr Volcano in 1992. Lightning was detected on seismograms as simultaneous spikes and simultaneous gain-ranging (a feature that normally lowers the gain at a station when the signal level begins to saturate). Spikes had typical durations of 0.04–0.05 s. Using uniform criteria we found 28 lightning flashes in the June 27th eruption, 29 in the August 18th eruption, and three in the September 17th eruption. We measured peak voltages on station RSO, 94 km SSW, to determine the relative strengths of lightning, and found that the August lightning was strongest, June weakest, and September intermediate. Based on relative signal strengths at different stations, we found evidence for different lightning geometries between the June and August eruptions, during which prevailing winds blew the ash clouds to the north and east, respectively. For all three eruptions the first lightning was recorded 21–26 min after the onset of the eruption, suggesting that charge separation occurred in the convecting cloud rather than at the vent. Data recorded by a Bureau of Land Management lightning detection system for the August eruption showed negative polarities for the first 12 recorded flashes and a positive polarity for the last. This suggests a charge separation based on particle size, in which negative charge is found for larger particles which fall first, and positive charge remains on smaller particles which remain suspended longer. All three eruptions had similar durations of 3.5–4 h, and tephra volumes of 44–56 million cubic meters. The August eruption, however, produced stronger volcanic tremor, 30 cm2 reduced displacement as compared with 16 cm2 for June, and greater gas, 400±120 kt SO2 for August and 200±60 kt for June. Thus lightning strength correlates with both tremor amplitude and magmatic gas content. The August eruption occurred during the lightest winds, so the ash cloud and charge separation were vertically oriented and favored cloud-to-ground lightning. The September eruption occurred during the coldest and driest atmospheric conditions, which may explain the small amount of lightning. In general, volcanic lightning is important because it can help confirm that explosive eruptions are in progress, although the value of the information may be limited by the long delay from eruption onset to first lightning and the variability of eruptive and atmospheric conditions. |
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