Electrical Properties Of Volcanic Ash Samples From Grimsvoetn

Recent Icelandic eruptions in 2012 and 2011 were associated with sustained charging of the ash plume (Harrison et al., 2010) and significant volcanic lightning (Bennett et al., 2010; Arason et al., 2012). The sustained charging suggests that some charging of the plume is independent of the eruption process, in addition to other electrification mechanisms such as triboelectric or fractoemission processes at the vent, the ‘dirty thunderstorm’ mechanism if ice forms in the plume, and the internal radioactivity of the plume (Mather and Harrison, 2006; James et al., 2008). We investigate the triboelectric charging of volcanic material using a charged particle apparatus, which allows the charged particles to fall under gravity in a screened metallic cylinder. This apparatus comprises two induction rings connected to sensitive electrometers, which detect charge induced by the falling ash, and a Faraday cup to measure the total ash charge. The release mechanism has been designed to facilitate only self charging of the ash, as is expected in the atmospheric plume. Previous work on triboelectric charging of single-material particle systems has shown that the charging is likely to be determined by the number size distribution (Lacks and Levandovsky, 2007). We present a mass distribution measurement for a sample of volcanic ash provided by the Icelandic Meteorological Office collected 70 km from the crater of the 2011 GrÃ_msvötn eruption. This shows a high proportion of fine particles compared with the volumetric size distribution reported in Piper et al. (2012). Optical microscopy of the different size fractions of the GrÃ_msvötn sample indicates that the composition varies with size. In addition to charge transfer as a function of size, the different substances will triboelectrically interact with each other, which complicates the nature of the charge interactions. To separate the dependence of particle charging on size from composition, particle charging experiments were performed with pumice (a much more homogeneous material than ash) and compared to the results obtained with the volcanic ash samples.