Probing the interior of volcanic plumes with W and L-band radiation

Abstract

Probing the interior dynamics of eruptive columns using electrostatic processes generated within the flows themselves has garnered much interest in the recent years [Arason et al., 2011; Behnke et al., 2012; James et al., 1998; Méndez Harper et al., 2016; Miura et al., 1996; Thomas et al., 2009]. Indeed, large eruptions are often accompanied by brilliant displays of lightning, testifying to the high potentials that can be accumulated by a diverse set of electrification mechanisms. Unfortunately, lightning on its own cannot be used as a general remote sensing tool because not all volcanic eruptions produce spark discharges. As pointed out by McNutt et al., 2010, only 30-35% of volcanoes maintain lightning storms. The absence of lightning in two thirds of all eruptions indicates that most volcanoes produce flows with 1) inefficient or limited granular charging processes or 2) dynamics that do not promote the charge separation that sets up coherent electric fields needed for lightning. Yet, even if the prerequisites for spark discharges are not met, it is difficult to argue for plumes which are completely electrostatically neutral. The problems permeating passive electromagnetic sensing may be overcome through the use of active methods which involve interrogating charged volcanic plumes with electromagnetic radiation. The scattering of electromagnetic waves has been a common method to retrieve the physical properties of collections of particles, specifically those which cannot be accessed directly. In the present work, we numerically and experimentally explore the manner in which electrostatic charge on particles affects the propagation of electromagnetic waves through volcanic plumes. We show that, for the range of complex dielectric constants measured in volcanic ash, the extinction efficiency of a charged particle is significantly larger than that associated with an equivalent neutral particle for L-band radiation. Conversely, we find that charging on sub-millimeter-sized particles does not directly affect the propagation of W-band radiation. However, we find that changes to the dynamics of a particle system resulting from charging can be detected indirectly with millimeter-wave light. Thus, this work represents the first effort to characterize how electromagnetic radiation can be used to assess charging in volcanic plumes.