Electric charge within volcanic plumes on Earth and Io
Field, experimental and theoretical investigations have been carried out into the electrification of volcanic plumes. At Sakurajima volcano, Japan, ground-level perturbations of the atmospheric electric potential gradient generated by particulate plumes were recorded. Simultaneous measurements, made at up to five sites around the volcano, are shown to be capable of detecting even very small quantities of volcanic particles, and tracking a continuously produced (over a period of several hours), dilute plume as it was dispersed by the wind. Data collected during one small Vulcanian eruption suggest that charges, about 1 C in magnitude, were generated and then separated, with the positive charge centre residing above the negative charge centre. Experiments have been carried out in order to assess fracto-emission (the release of ions, neutral atoms and various frequencies of electromagnetic radiation from fracture surfaces) as a charging mechanism for the electrification of silicate particles in plumes. Silicate particles, generally less than 100 µm in diameter, were produced by colliding pumice samples together and were shown to have net specific charges of approximately 10-5 to 10-6 C kg-1 (similar to previous field measurements on ashfall). Most (but not all) of the experimental samples produced net negatively charged ash, and demonstrated a positive-above-negative charge separation. Additional charge thought to have been released in the form of ions was also detected, supporting fracto-emission as the charging mechanism for volcanic plumes. The magnitude of the net ash charge is shown to be a function of the particle size, with relative humidity having only a small effect. The polarity of the net charge appears to be a function of a property of individual pumice types, possibly their geochemistry. Experiments during which the particles were separated by their polarity indicate that the net charges reflect only a small imbalance in much larger, individual particle charges (10-3 C kg-1) of both polarities. The particle charges are shown to vary with the sample impact velocity, the number of impacts carried out within an experiment and, to a lesser degree, relative humidity. Variation within the results appears to be partly an effect of particle aggregation within the experiment. Investigation of any effects that this particle charging may have on the plumes observed on Io (one of Jupiter’s moons) suggests that luminous discharges and particle aggregation are both probable in this environment. It is shown that the distinct asymmetries observed in plumes are unlikely to be a result of electromagnetic interactions, but probably represent decompression features and asymmetric vent or crater morphologies.