R. Giles Harrison

Empirical evidence for a nonlinear effect of galactic cosmic rays on clouds

Galactic cosmic ray (GCR) changes have been suggested to affect weather and climate, and new evidence is presented here directly linking GCRs with clouds. Clouds increase the diffuse solar radiation, measured continuously at UK surface meteorological sites since 1947. The ratio of diffuse to total solar radiation—the diffuse fraction (DF)—is used to infer cloud, and is compared with the daily mean neutron count rate measured at Climax, Colorado from 1951–2000, which provides a globally representative indicator of cosmic rays. Across the UK, on days of high cosmic ray flux (above 3600×102 neutron counts h−1, which occur 87% of the time on average) compared with low cosmic ray flux, (i) the chance of an overcast day increases by (19±4) %, and (ii) the diffuse fraction increases by (2±0.3) %. During sudden transient reductions in cosmic rays (e.g. Forbush events), simultaneous decreases occur in the diffuse fraction. The diffuse radiation changes are, therefore, unambiguously due to cosmic rays. Although the statistically significant nonlinear cosmic ray effect is small, it will have a considerably larger aggregate effect on longer timescale (e.g. centennial) climate variations when day-to-day variability averages out.

Enhancement of contact nucleation by scavenging of charged aerosol particles

Abstract The influence of interparticle electrical forces on the particle collection rates of charged water drops are investigated using a particle–droplet trajectory model. For the charge levels expected naturally on supercooled water drops and aerosol active as contact nuclei, the charge-enhanced collection efficiency leads to an increased capture of ice nuclei (IN), causing the freezing probability by contact nucleation to be increased over neutral collisions. The collection efficiency depends on the aerosol charge, but not on the sign or charge carried by water drops. A threefold increase in the collection efficiency is found for aerosols carrying up to 10 elementary charges, collecting submicron aerosol. Modest charges on aerosol particles (APs) arising from natural asymmetries in ion concentration are sufficient to increase collision rates with suitable freezing nuclei.

The Carnegie Curve

The Earth’s fair weather atmospheric electric field shows, in clean air, an average daily variation which follows universal time, globally independent of the measurement position. This single diurnal cycle variation (maximum around 19UT and minimum around 03UT) is widely known as the Carnegie curve, after the geophysical survey vessel of the Carnegie Institution of Washington on which the original measurement campaigns demonstrating the universal time variation were undertaken. The Carnegie curve’s enduring importance is in providing a reference variation against which atmospheric electricity measurements are still compared; it is believed to originate from regular daily variations in atmospheric electrification associated with the different global disturbed weather regions. Details of the instrumentation, measurement principles and data obtained on the Carnegie’s seventh and final cruise are reviewed here, also deriving new harmonic coefficients allowing calculation of the Carnegie curve for different seasons. The additional harmonic analysis now identifies changes in the phasing of the maximum and minimum in the Carnegie curve, which shows a systematic seasonal variation, linked to the solstices and equinoxes, respectively.

Enhancement of cloud formation by droplet charging

Charge affects the activation of cloud droplets by reducing the minimum supersaturation at which haze droplets begin to grow. Although the droplet charge required to enhance activation is substantial, we show that sufficient charging occurs at the edges of layer clouds because the fair-weather current in the global atmospheric electrical circuit flows through a discontinuity in conductivity. Our theory predicts that droplet neutralization will cause a transient cooling of cloud base. This hypothesis was tested during a period of extreme solar activity, when we detected transient current bursts at the surface beneath a layer of cloud. We attribute these to bursts of ion production, which would cause transient droplet neutralization in the cloud and an associated increase in droplet critical supersaturation. We observed transient decreases in downward long-wave radiation measurements coincident with the transient current bursts. As the vertical current density passing through stratiform clouds is a global phenomenon, there are many regions in which a charge enhancement effect on cloud formation can potentially occur; we find that the effect of charge-enhanced activation on surface radiation in the present-day climate could be as large as 0.1 W m−2.

Cloud Formation and the Possible Significance of Charge for Atmospheric Condensation and Ice Nuclei

Cloud formation in the atmosphere is related to the presence of water vapour, cloud condensation nuclei (CCN) and ice nuclei (IN). Ionisation in the atmosphere is caused by a variety of sources, but the contribution from cosmic rays is always present and is modulated by the solar cycle. Methods of investigating the variability in ionisation are described. The mechanisms proposed by which (1) ionisation could influence cloud formation, and (2) by which changes to the CCN and IN could occur are discussed. Direct formation of sulphate CN is conceivable in atmospheric air by radioactivity, and charging of molecular clusters leads to greater collisions rates than for neutral clusters. Modification of the ice nucleation efficiency of aerosol could also have atmospheric effects through latent heat release. However in both cases definitive atmospheric experimental work is lacking and therefore any link between solar variability and clouds remains unproven.

Atmospheric condensation nuclei formation and high-energy radiation

Abstract Considerable controversy exists over a proposed link between cosmic radiation and clouds in the apparent absence of a microphysical mechanism between ionisation and particle formation. New atmospheric experimental data which supports previous laboratory evidence of radiolytic particle formation is presented, showing increases in surface condensation nuclei (CN) which correlate positively with increases in surface ionisation. Correlations between physically-displaced Geiger counters are used to attribute some of the ionisation events to cosmic radiation, above a background noise level determined by Monte Carlo simulations of the detector system. When cosmic ionisation events are more frequent, ionisation maxima are followed by increases and then decreases in CN concentration. Some of the variability in atmospheric CN may therefore be attributable to processes initiated by cosmic ionisation.

Cloud base height and cosmic rays

Cosmic rays modify current flow in the global atmospheric electrical circuit. Charging at horizontal layer cloud edges has been observed to be consistent with global circuit vertical current flow through the cloud, which can modify the properties of small and pure water droplets. Studies have been hampered by the absence of cloud edge observations, hence cloud base height information is investigated here. Cloud base height measured at the Lerwick Observatory, Shetland, UK, is analysed using threshold tests and spectral analysis. The cloud base height distributions for low cloud (cloud base less than 800 m) are found to vary with cosmic ray conditions. Further, 27 day and 1.68 year periodicities characteristic of cosmic ray variations are present, weakly, in the cloud base height data of stratiform clouds, when such periodicities are present in neutron monitor cosmic ray data. These features support the idea of propagation of heliospheric variability into layer clouds, through the global atmospheric electric circuit.

Observed atmospheric electricity effect on clouds

The atmospheres fair weather electric field is a permanent feature, arising from the combination of distant thunderstorms, Earths conducting surface, a charged ionosphere and cosmic ray ionization. Despite its ubiquity, no fair weather electricity effect on clouds has been hitherto demonstrated. Here we report surface measurements of radiation emitted and scattered by extensive thin continental cloud, which, after ~2 min delay, shows changes closely following the fair weather electric field. For typical fluctuations in the fair weather electric field, changes of about 10% are subsequently induced in the diffuse short-wave radiation. These observations are consistent with enhanced production of large cloud droplets from charging at layer cloud edges.

Femtoampere current reference stable over atmospheric temperatures

Calibration of ultralow current ammeters deployed in atmospheric ion counters requires stable current references operating at high source impedance. Using standard precision components, this current reference generates equal bipolar currents of nominally ±500 fA, cyclically for 32 s each. The currents were found to be stable to 2 fA over the temperature range −20 to 20 °C, typical of atmospheric conditions. The output current is delivered via a capacitor, and by arranging for the capacitor to be guarded at ground potential when the system is not generating a current, the reference can be permanently connected to an electrometer with minimal leakage.

Energetic Charged Particles Above Thunderclouds

The French government has committed to launch the satellite TARANIS to study transient coupling processes between the Earth’s atmosphere and near-Earth space. The prime objective of TARANIS is to detect energetic charged particles and hard radiation emanating from thunderclouds. The British Nobel prize winner C.T.R. Wilson predicted lightning discharges from the top of thunderclouds into space almost a century ago. However, new experiments have only recently confirmed energetic discharge processes which transfer energy from the top of thunderclouds into the upper atmosphere and near-Earth space; they are now denoted as transient luminous events, terrestrial gamma-ray flashes and relativistic electron beams. This meeting report builds on the current state of scientific knowledge on the physics of plasmas in the laboratory and naturally occurring plasmas in the Earth’s atmosphere to propose areas of future research. The report specifically reflects presentations delivered by the members of a novel Franco-British collaboration during a meeting at the French Embassy in London held in November 2011. The scientific subjects of the report tackle ionization processes leading to electrical discharge processes, observations of transient luminous events, electromagnetic emissions, energetic charged particles and their impact on the Earth’s atmosphere. The importance of future research in this area for science and society, and towards spacecraft protection, is emphasized.