The Effect of Electrostatic Charge on the Propagation of GPS (L‐band) Signals Through Volcanic Plumes

Abstract

Recent field studies have shown that the presence of ash in the atmosphere can produce measurable attenuation of Global Positioning System (GPS) signals (Aranzulla et al., 2013, https://doi.org/10.1007/s10291-012-0294-4; Larson, 2013, https://doi.org/10.1002/grl.50556; Larson et al., 2017, https://doi.org/10.1016/j.jvolgeores.2017.04.005). The ability to detect plumes using GPS is appealing because many active volcanoes are already instrumented with high‐quality receivers. However, analyses using a Ralyeigh approximation have shown that the large attenuations cannot be explained by the scattering and absorption associated with ash or hydrometeors alone. Here, we show that the extinction of GPS signals, which fall into the L‐band of the electromagnetic spectrum, may be exacerbated significantly by excess surface charge on pyroclasts. Indeed, volcanic eruptions are often accompanied by a range of electrostatic processes, leading, in some cases, to spectacular lightning storms. We use a modified Mie scattering model to demonstrate that electrostatic effects can increase the extinction of L‐band radiation by up to an order of magnitude, producing attenuations consistent with those observed in the field. Thus, future work involving GPS as a tool to remotely probe plumes must take into account the electrification of ash in radiative transfer models. Additionally, we propose that the sensitivity of GPS to particle charging may catalyze the development of new techniques to explore electrostatic processes in plumes, especially if GPS measurements are complemented with millimeter‐wave RADAR measurements.

Origin of large-amplitude oscillations of dust particles in a plasma sheath

“…Dusty or complex plasmas, consisting of microscopic particles immersed in a weakly ionized gas, are ubiquitous in the natural universe and play important roles in man-made systems [1][2][3][4][5][6]. When exposed to a plasma environment, particles gain charge by collecting discrete ions and electrons on their surfaces.…”

Modeling GPS Signal Propagation Through Volcanic Plumes

“…However, at diameters below 1 mm, the absorption cross-section is significantly increased. This can be seen in Figure 3, which shows the percent difference between the absorption and scattering cross-sections as a function of particle diameter and charge up to the breakdown voltage of 2.7 E-5 C/m 2 , the theoretical maximum charge possible under standard atmospheric conditions (Méndez Harper et al, 2019).…”

“…The model also did not consider constituents of the plume other than ash, or charging of the ash particles. Charged particles were modeled by Méndez Harper et al (2019), who incorporated a full Mie model (Mie, 1978) of both the absorption and scattering cross-sections. This study found a significant increase in attenuation for smaller particles.…”

“…This is not necessarily true for a particle in an ash plume, which can experience electrification due to radioactive decay (Nicoll et al, 2019), tribocharging (Méndez Harper & Dufek, 2016), and fractocharging (James et al, 2000), as well electrification due to interactions between ash particles and hydrometeors (Mather & Harrison, 2006). Méndez Harper et al (2019) took this effect into account and modeled charged particle absorption and scattering of ash particles at GPS frequencies. This built on the work of Klacka and Kocifaj (2007), which modified the Mie coefficients to include a charged surface boundary condition.…”

“…The impact of tephra scattering and absorption on GPS has been previously discussed by Larson et al (2017); however, given the assumptions made for particle size, the predicted attenuation was nearly an order of magnitude less than what is seen in observations. Méndez Harper et al (2019) extended this analysis by exploring the increase in absorption of a GPS signal due to a surface charge on the tephra particles. While including this effect did result in an increase in predicted absorption for some tephra sizes, the particle distribution of the plume was not modeled as part of this effort, and additional research is required to understand if charge has a significant impact on overall received GPS signal strength.…”