arXiv subjects

K. L. Aplin

Publications and source records attributed to K. L. Aplin.

13 recordsLinked to original sources

arXiv2020

Atmospheric Electricity at the Ice Giants

Lightning was detected by Voyager 2 at Uranus and Neptune, and weaker electrical processes also occur throughout planetary atmospheres from galactic cosmic ray (GCR) ionisation. Lightning is an indicator of convection, whereas electrical processes away from storms modulate cloud formation and chemistry, particularly if there is little insolation to drive other mechanisms. The ice giants appear to be unique in the Solar System in that they are distant enough from the Sun for GCR-related mechanisms to be significant for clouds and climate, yet also convective enough for lightning to occur. This paper reviews observations (both from Voyager 2 and Earth), data analysis and modelling, and considers options for future missions. Radio, energetic particle and magnetic instruments are recommended for future orbiters, and Huygens-like atmospheric electricity sensors for in situ observations. Uranian lightning is also expected to be detectable from terrestrial radio telescopes.

physics.space-ph↗

arXiv2017

Solar-driven variation in the atmosphere of Uranus

Long-term measurements (1972-2015) of the reflectivity of Uranus at 472 and 551 nm display variability that is incompletely explained by seasonal effects. Spectral analysis shows this non-seasonal variability tracks the 11-year solar cycle. Two mechanisms could cause solar modulation, (a) nucleation onto ions or electrons created by galactic cosmic rays (GCR), or (b) UV-induced aerosol colour changes. Ion-aerosol theory is used to identify expected relationships between reflectivity fluctuations and GCR flux, tested with multiple regression and compared to the linear response predicted between reflectivity and solar UV flux. The statistics show that 24%of the variance in reflectivity fluctuations at 472 nm is explained by GCR ion-induced nucleation, compared to 22% for a UV-only mechanism. Similar GCR-related variability exists in Neptune's atmosphere, hence the effects found at Uranus provide the first example of common variability in two planetary atmospheres driven through energetic particle modulation by their host star.

physics.ao-ph↗

arXiv2013

Lord Kelvin's atmospheric electricity measurements

Lord Kelvin (William Thomson) made important contributions to the study of atmospheric electricity during a brief but productive period from 1859-1861. By 1859 Kelvin had recognised the need for "incessant recording" of atmospheric electrical parameters, and responded by inventing both the water-dropper instrument for measuring the atmospheric Potential Gradient (PG), and photographic data logging. The water-dropper was widely adopted internationally and is still in use today. Following theoretical considerations of electric field distortion by local topography, Kelvin developed a portable electrometer, using it to investigate PG on the Scottish island of Arran. During these environmental measurements, Kelvin may have unwittingly detected atmospheric PG changes during solar activity in August/September 1859 associated with the "Carrington event". Kelvin's atmospheric electricity work presents an early representative study in quantitative environmental physics, through the application of mathematical principles to an environmental problem, the design and construction of bespoke instrumentation for real world measurements and the limitations of the original theoretical view revealed by experimental work.

physics.hist-ph↗

arXiv2012

Cosmic ray modulation of infra-red radiation in the atmosphere

Cosmic rays produce molecular cluster ions as they pass through the lower atmosphere. Neutral molecular clusters such as dimers and complexes are expected to make a small contribution to the radiative balance, but atmospheric absorption by charged clusters has not hitherto been observed. In an atmospheric experiment, a thermopile filter radiometer tuned to a 9.15μm absorption band, associated with infra-red absorption of molecular cluster ions, was used to monitor changes following events identified by a cosmic ray telescope sensitive to high energy (>400MeV) particles, principally muons. The change in longwave radiation in this absorption band due to molecular cluster ions is 7 mWm^-2. The integrated atmospheric energy change for each event is 2Jm^-2, representing an amplification factor of 10^12 compared to the estimated energy density of a typical air shower. This absorption is expected to occur continuously and globally, but calculations suggest that it has only a small effect on climate.

physics.ao-ph↗

arXiv2012

Electrical properties of volcanic ash samples from Eyjafjallajökull and Grímsvötn

Volcanic ash is known to charge electrically, producing some of the most spectacular displays of lightning in nature. Here we investigate the electrical characteristics of ash from two different Icelandic volcanoes - Eyjafjallajökull in 2010 and Grímsvötn in 2011. Laboratory tests investigated the charge transferred to a conducting plate due to fall of volcanic ash through an insulating cylinder. Ash from the Eyjafjallajökull eruption was found to charge slightly positively, whilst Grímsvötn ash was substantially negatively charged. Measurement of the volumetric ratio of particle diameters showed the Eyjafjallajökull ash to have a bimodal distribution, and the Grímsvötn ash a monomodal distribution. Previous experiments with single-material particle systems show that smaller particles charge negatively and larger ones positively. Since charge is carried by individual particles, the charging is likely to be dominated by the number size distribution, therefore the large negative charge of the Grímsvötn ash is likely to be related to a large number of small particles within the number size distribution of the ash.

physics.geo-ph↗

arXiv2011

Use of coated silicon field emitters as neutralisers for fundamental physics missions in space

Spacecraft neutralisers are required as part of the ion propulsion system for accurate station keeping in fundamental physics missions. This paper describes the use of thin layers of insulating materials as coatings for the gated silicon field emitter array structure used in a spacecraft neutraliser. These thin coatings are postulated to reduce power consumption and reduce overheating. The power consumption and lifetime of aluminium nitride and amorphous hydrogenated diamond-like carbon coatings have been tested by current-voltage and endurance tests. Diamond-like carbon coatings were promising, performing better in endurance tests than uncoated samples, but further work is required to characterise the coating's physical properties and its effects on field emission. The thermal conductivity of the coating material had little effect on measured sample lifetimes. Aluminium nitride had reduced power consumption compared to diamond-like carbon coated and uncoated samples. A thin (~5 nm) layer of aluminium nitride was found to be optimal, meeting European Space Agency specifications for the neutraliser engineering model.

physics.space-ph↗

arXiv2006

Ion-aerosol interactions in the lower atmosphere of Venus

All planetary atmospheres are electrified to some extent by cosmic ray ionisation, and Venus is no exception. There is increasing awareness that ion-aerosol interactions could modulate terrestial radiative processes, and this possibility will be investigated for the Venusian atmosphere. The likelihood of a Venusian global atmospheric electric circuit will also be discussed.

physics.space-ph↗

arXiv2005

Aspirated capacitor measurements of air conductivity and ion mobility spectra

Measurements of ions in atmospheric air are used to investigate atmospheric electricity and particulate pollution. Commonly studied ion parameters are (1) air conductivity, related to the total ion number concentration, and (2) the ion mobility spectrum, which varies with atmospheric composition. The physical principles of air ion instrumentation are long-established. A recent development is the computerised aspirated capacitor, which measures ions from (a) the current of charged particles at a sensing electrode, and (b) the rate of charge exchange with an electrode at a known initial potential, relaxing to a lower potential. As the voltage decays, only ions of higher and higher mobility are collected by the central electrode and contribute to the further decay of the voltage. This enables extension of the classical theory to calculate ion mobility spectra by inverting voltage decay time series. In indoor air, ion mobility spectra determined from both the novel voltage decay inversion, and an established voltage switching technique, were compared and shown to be of similar shape. Air conductivities calculated by integration were: 5.3 +- 2.5 fS/m and 2.7 +- 1.1 fS/m respectively, with conductivity determined to be 3 fS/m by direct measurement at a constant voltage. Applications of the new Relaxation Potential Inversion Method (RPIM) include air ion mobility spectrum retrieval from historical data, and computation of ion mobility spectra in planetary atmospheres.

physics.ins-det↗

arXiv2005

Effect of the troposphere on surface neutron counter measurements

Surface neutron counter data are often used as a proxy for atmospheric ionisation from cosmic rays in studies of extraterrestrial effects on climate. Neutron counter instrumentation was developed in the 1950s and relationships between neutron counts, ionisation and meteorological conditions were investigated thoroughly using the techniques available at the time; the analysis can now be extended using modern data. Whilst surface neutron counts are shown to be a good proxy for ionisation rate, the usual meteorological correction applied to surface neutron measurements, using surface atmospheric pressure, does not completely compensate for tropospheric effects on neutron data. Residual correlations remain between neutron counts, atmospheric pressure and geopotential height, obtained from meteorological reanalysis data. These correlations may be caused by variations in the height and temperature of the atmospheric layer at ~100hPa. This is where the primary cosmic rays interact with atmospheric air, producing a cascade of secondary ionising particles.

physics.ao-ph↗

arXiv2005

Absorption of infra-red radiation by atmospheric molecular cluster-ions

Protonated water clusters are a common species of atmospheric molecular cluster-ion, produced by cosmic rays throughout the troposphere and stratosphere. Under clear-sky conditions or periods of increased atmospheric ionisation, such as solar proton events, the IR absorption by atmospheric ions may affect climate through the radiative balance. Fourier Transform Infrared Spectrometry in a long path cell, of path length 545m, has been used to detect IR absorption by corona-generated positive molecular cluster-ions. The column concentration of ions in the laboratory spectroscopy experiment was estimated to be ~10^13 m-2; the column concentration of protonated atmospheric ions estimated using a simple model is ~10^14 m-2. Two regions of absorption, at 12.3 and 9.1 um are associated with enhanced ion concentrations. After filtering of the measured spectra to compensate for spurious signals from neutral water vapour and residual carbon dioxide, the strongest absorption region is at 9.5 to 8.8 um (1050 to 1140 cm-1) with a fractional change in transmissivity of 0.03 plus/minus 0.015, and the absorption at 12.5 to 12.1 um (800 to 825 cm-1) is 0.015 plus/minus 0.008.

physics.ao-ph↗

arXiv2005

Nineteenth century air pollution variations in Paris inferred from Eiffel Tower potential gradient measurements

Early surface measurements of atmospheric Potential Gradient were made in many European cities in the nineteenth century (C19th). The data was usually obtained at hourly resolution, and good accounts of the calibration of the instruments are also often available. The PG measurements made by Chauveau on the Eiffel Tower, soon after its completion in the 1890s, are particularly notable. Atmospheric electrical proxy techniques in combination with simple boundary layer meteorology are used to determine air pollution levels. The C19th PG measurements in both polluted and clean Parisian air present a unique resource for European air pollution and atmospheric composition studies.

physics.ao-ph↗

arXiv2005

A novel technique to determine atmospheric ion mobility spectra

Detailed tropospheric ion measurements are needed to improve understanding of the electrical microphysics affecting clouds. Additionally, atmospheric ion mobility spectra can be used to identify ion growth processes leading to condensation nucleus formation. However these measurements are rare, particularly in the troposphere where the majority of clouds form. Developments in the operating theory of the classical instrument for ion measurement, the aspirated cylindrical capacitor, are described, which enable ion mobility spectrum information to be extracted from the rate of voltage decay of the aspirated capacitor in air. In this paper, data from historical balloon-borne ion counter ascents will be reanalysed to extract new ion mobility spectra from simple voltage time series. Such data recovery will increase the amount of atmospheric ion spectra available for analysis.

physics.ao-ph↗

arXiv2005

Instrumentation for atmospheric ion measurements

Small ions are part of the atmospheric aerosol spectrum, and study of ion-aerosol interactions is fundamental in atmospheric physics. Air ion physics and instrumentation are reviewed, including the historical context. A miniaturised Gerdien condenser for ion measurement, operating in situ to minimise inlet errors, is described. Two operating modes using independent current and voltage decay measurements are employed. A more sophisticated self-calibrating and fully programmable ion mobility spectrometer (PIMS) based on the same principles, is also discussed. Detailed analysis of error terms and application of new technology is demonstrated to greatly improve its capability. Self-consistent experimental approaches, including ionisation and ion concentration instruments, are used for calibration. In developing and characterising the individual components of the PIMS, favourable and unfavourable operating regimes are identified: this approach can also be applied to other aspiration ion counting techniques. Use of a sophisticated programmable electrometer permits compensation for leakage terms. Electrically-charged aerosol particles have been found to complicate the ion measurements. Consequently, conventional ion-aerosol theory, which neglects the particulate concentration, is thought to be incomplete. The polymodal ion mobility spectrum is also found to influence the instrument's operation. Development of the novel programmable ion instrumentation, in conjunction with consideration of the ion mobility spectrum yields an improved and flexible approach to in situ atmospheric ion measurements.

physics.ao-ph↗