K. L. Aplin’s research while affiliated with University of Bristol and other places

Here we outline a novel modular approach to separating charge contributions for a better understanding of triboelectric charge measurements.

The triboelectric charging of granular materials remains a poorly understood phenomenon with a wide range of scientific and industrial applications, from volcanic lightning to pharmaceutical manufacturing. The Faraday cup is the most commonly used apparatus for studying triboelectric charging, yet current methods of interpreting measurements are overly simplistic, often conflating charging due to particle-particle interactions with other charging mechanisms. In this study, we present a modular approach for interpreting Faraday cup measurements, which allows for more detailed exploration of triboelectric phenomena. The approach involves fitting approximated charge distribution shapes to experimental Faraday cup data, using measured size distributions alongside simplified models of charge distribution and particle dynamics. This modular framework is adaptable, allowing for fine-tuning at each step to suit specific application cases, making it broadly applicable to any insulating granular material. As a case study, we examine volcanic ash samples from Gr\'imsv\"otn and Atitl\'an volcanoes, finding that the Gr\'imsv\"otn ash exhibited a higher proportion of charge due to particle-particle interactions. Experimental validation with sieved volcanic ash fractions revealed that larger particle sizes showed stronger particle-particle charging. Additionally, non-particle-particle charging was found to scale with particle size as $\propto d_p^{-0.85 \pm 0.03}$, approximately following the particles' effective surface area.

Extreme space weather events can have serious impacts on critical infrastructure, including Global Navigation Satellite Systems (GNSS). The use of GNSS, particularly as sources of accurate timing signals, is becoming more widespread, with one example being the measurement of electricity grid frequency and phase information to aid grid management and stability. Understanding the likelihood of extreme space weather impacts on GNSS timing signals is therefore becoming vital to maintain national electricity grid resilience. This study determines critical intensity thresholds above which the complete failure of a GNSS based timing system may occur. Solar radio bursts are identified as a simple example to investigate in more detail. The probability of occurrence of an extreme space weather event with an intensity equal to or greater than the critical intensity is estimated. Both a power law and extreme value theory were used to evaluate recurrence probabilities based on historical event frequencies. The probability was estimated to be between 3%–12% per decade to cause the complete failure of any GNSS‐based timing system.

NASA's InSight mission launched in May 2018 and landed on the surface of Mars in November of the same year. With a final mission duration of 1440 sols, this spacecraft provided a wealth of scientific data. Amongst its payloads was the first and only sensor to measure the magnetic field from the surface of Mars-the InSight Fluxgate Magnetometer. On Mars, triboelectrically atmospheric dust presents challenges for solar power generation on the surface of the planet. Dust is key to the Martian climate, absorbing sunlight and altering atmospheric properties. There is also speculation of electrostatic discharge and a global electric circuit on Mars, though this is unconfirmed as yet. By combining the magnetometer dataset with pressure, wind velocity, solar panel current, and seismic data from several of the other sensors on InSight, the magnetic field emissions of dust devils can be studied. A strong source of magnetic noise was found during periods when the battery was fully charged and vortices during these times were excluded from further analysis. The identification of this noise source may help in understanding the natural background electromagnetic variability at the Martian surface. This paper presents an overview of trends observed in the statistical analysis of nearly 7500 events. Following this, a preliminary analysis of the magnetic field of dust devil events is compared to the background. In a typical sol (taking sol 239 as an example), the background magnetic field at InSight's landing site is-925 nT, with a mean surface pressure of between 600 and 750 Pa-and the 3 dust devil events on the sol yield a median 1 nT magnetic peak approximately 15 s after the pressure peak. Expanding this analysis to the full set of data, the analysis of 1470 dust devil events yields a +0.08 nT vertical magnetic field peak just after the closest approach (T+25 s) of the dust devil, with a median core pressure drop of 2.27 Pa.

This paper provides key insights from the test campaign of a student CubeSat payload, along with actionable recommendations for future missions. The University of Bristol, UK, is developing a volcano imaging CubeSat payload-PROVE Pathfinder. This 2U payload consists of a visual and a thermal infrared camera, and their power and control electronics. In April 2023, the team conducted an environmental testing campaign at the ESA CubeSat Support Facility in Transinne, Belgium, under ESA's Fly Your Satellite! Testing Opportunities scheme. This test campaign aimed to structurally qualify the payload to survive a nominal worst-case launch scenario, guided by relevant space testing standards. This paper presents the structural modelling and test design process employed for the payload. Through finite element modelling, the study calculated the anticipated structural response under a varying launch environment, thereby informing the test strategy. The results of the testing campaign are presented. Challenges in mode identification within the response dynamics are acknowledged, stemming from the proximity of frequency peaks indicating that they are excited simultaneously. In addition to this, where possible, evidence-based reasons for frequency/amplitude shifts in modes are provided. Furthermore, the paper offers recommendations for future vibration testing campaigns of CubeSat payloads, drawing upon lessons learned from this campaign. By describing the methodology and challenges encountered, it is hoped that this study helps other student teams who are contemplating the qualification of CubeSat payloads.

Dust devils – rotating vortices of lofted particulates formed by convection from solar heating - are a prevalent aspect of Martian weather.Within a dust devil, particles become charged though triboelectrification, and observations of terrestrial dust devils have revealed electric and magnetic fields. Understanding the behaviour of these aeolian events is vital for mission planning and duration, for example, with the Insight lander as a prime example of a mission cut short due to obscuration of solar panels by lofted dust, which is highly likely to be charged. Charged dust devils on Mars strongly imply the existence of electric discharges due to the low breakdown field. The existence of discharges would in turn imply a global electric circuit on the planet – with implications for organic chemical formation and methane losses. This work presents a laboratory-based experimental set up for investigation into both electric and magnetic fields and the feasibility of using Martian analogue dust as alternative to in-situ measurements. In a dust devil, there are two main driving motions – a vertical separation of small and large particles, and the characteristic tight spiralling motion. The experimental set-up splits these components, allowing investigation into each. It is made up of a carefully designed cylindrical tank into which dusts or powders can be dropped to allow triboelectric charging as they fall under gravity. The base of the tank has a Faraday cup for charge measurement and there is also a field mill looking into the tank from the top. The rotational component is achieved using a paddle arrangement for samples placed in the base of the tank. Electric and magnetic fields have successfully been detected from charged dust with this system.

Measuring the electric field is a central goal in electrostatics research, such as the study of electrostatics in atmospheric processes. It serves as a key indicator for various atmospheric phenomena, including the presence of lightning, dust or charged clouds. Traditionally, electric field measurements have been conducted from static platforms, with limited mobile measurements from airborne platforms such as balloons or, more recently, Unmanned Aerial Vehicles (UAVs). Here, we explore the potential of terrestrial robots to measure the electric field with some level of autonomy, such as during supervised navigation between user-defined waypoints. We mount a field mill on a four-wheeled rover and use a Global Navigation Satellite System (GNSS) to track the position of its measurements during an outdoor survey. The robot has a depth camera for 3D terrain mapping to contextualise local field measurements. We present plans for future research, including the use of semi-autonomous identification and exploration of electrostatic ‘hotspots’; and deployment of multiple robots (e.g., six) in a ‘sparse swarm’ configuration. We consider opportunities to employ such a robot system in environmental science or space research, for instance on Mars or the moon, where an understanding of electrostatic processes could be significant for future space missions.