Brian A. Tinsley

Influence of solar wind on the global electric circuit, and inferred effects on cloud microphysics,temperature, and dynamics in the troposphere

There are at least three independent ways in which the solar wind modulates the flow of current density (Jz) in the global electric circuit. These are (A) changes in the galactic cosmic ray energy spectrum, (B) changes in the precipitation of relativistic electrons from the magnetosphere, and (C) changes in the ionospheric potential distribution in the polar caps due to magnetosphere-ionosphere coupling. The current density Jz flows between the ionosphere and the surface, and as it passes through conductivity gradients it generates space charge concentrations dependent on Jz and the conductivity gradient. The gradients are large at the surfaces of clouds and space charge concentrations of order 1000 to 10,000 elementary charges per cm3 can be generated at cloud tops. The charge transfers to droplets, many of which are evaporating at the cloud-clear air interface. The charge remains on the residual evaporation nuclei with a lifetime against leakage of order 1000 sec, and for a longer period the nuclei also retain coatings of sulfate and organic compounds adsorbed by the droplet while in the cloud.The charged evaporation nuclei become well mixed with more droplets in many types of clouds with penetrative mixing. The processes of entrainment and evaporation are also efficient for these clouds. The collection of such nuclei by nearby droplets is greatly increased by the electrical attraction between the charge on the particle and the image charge that it creates on the droplet. This process is called electroscavenging. Because the charge on the evaporation nuclei is derived from the original space charge, it depends on Jz, giving a rate of electroscavenging responsive to the solar wind inputs.There may be a number of ways in which the electroscavenging has consequences for weather and climate. One possibility is enhanced production of ice. The charged evaporation nuclei have been found to be good ice forming nuclei because of their coatings, and so in supercooled clouds droplet freezing can occur by contact ice nucleation, as the evaporation nuclei are electroscavenged. Although quantitative models for the all the cloud microphysical processes that may be involved have not yet been produced, we show that for many clouds, especially those with broad droplet size distributions, relatively high droplet concentrations, and cloud top temperatures just below freezing, this process is likely to dominate over other primary ice nucleation processes. In these cases there are likely to be effects on cloud albedo and rates of sedimentation of ice, and these will depend on Jz.For an increase in ice production in thin clouds such as altocumulus or stratocumulus the main effect is a decrease in albedo to incoming solar radiation, and in opacity to outgoing longwave radiation. At low latitudes the surface and troposphere heat, and at high latitudes in winter they cool. The change in meridional temperature gradient affects the rate of cyclogenesis, and the amplitude of planetary waves. For storm clouds, as in winter cyclones, the effect of increased ice formation is mainly to increase the rate of glaciation of lower level clouds by the seeder-feeder process. The increase in precipitation efficiency increases the rate of transfer of latent heat between the air mass and the surface. In most cyclones this is likely to result in intensification, producing changes in the vorticity area index as observed. Cyclone intensification also increases the amplitude of planetary waves, and shifts storm tracks, as observed.In this paper we first describe the production of space charge and the way in which it may influence the rate of ice nucleation. Then we review theory and observations of the solar wind modulation of Jz, and the correlated changes in atmospheric temperature and dynamics in the troposphere. The correlations are present for each input, (A, B, and C), and the detailed patterns of responses provide support for the inferred electrical effects on the physics of clouds, affecting precipitation, temperature and dynamics.

Apparent tropospheric response to MeV‐GeV particle flux variations: A connection via electrofreezing of supercooled water in high‐level clouds?

The ionization production by MeV-GeV particles (mostly galactic cosmic rays) in the lower atmosphere has-well defined variations on a day-to-day time scale related to solar activity, and on the decadal time scale related to the sunspot cycle. New results based on an analysis of 33 years of northern hemisphere meteorological data show clear correlations of winter cyclone intensity (measured as the changes in the area in which vorticity is above a certain threshold) with day-to-day changes in the cosmic ray flux. Similar correlations are also present between winter cyclone intensity, the related storm track latitude shifts, and cosmic ray flux changes on the decadal time scale. These point to a mechanism in which atmospheric electrical processes affect tropospheric thermodynamics, with a requirement for energy amplification by a factor of about 107 and a time scale of hours. A process is hypothesized in which ionization affects the nucleation and/or growth rate of ice crystals in high-level clouds by enhancing the rate of freezing of thermodynamically unstable supercooled water droplets which are known to be present at the tops of high clouds. The electrofreezing increases the flux of ice crystals that can glaciate midlevel clouds. In warm core winter cyclones the consequent release of latent heat intensifies convection and extracts energy from the baroclinic instability to further intensify the cyclone. As a result, the general circulation in winter is affected in a way consistent with observed variations on the interannual/decadal time scale. The effects on particle concentration and size distributions in high-level clouds may also influence circulation via radiative forcing.

Correlations of Atmospheric Dynamics With Solar Activity Evidence for a Connection via the Solar Wind, Atmospheric Electricity, and Cloud Microphysics

We respond to several criticisms of the view that there is a physical linkage between solar activity and the dynamics of the troposphere and lower stratosphere, and we provide further evidence in support of a mechanism for such a linkage involving atmospheric electricity and cloud microphysics. The main criticisms are (1) that the decadal time scale variations in stratified data result from aliasing introduced by the sampling process and are not responses to a decadal time scale physical input; (2) that the observed correlations are due to chance coincidence or an atmospheric periodicity that is not uniquely related to solar variability; and (3) that there are no plausible mechanisms that can amplify one of the weak solar-varying inputs in the region where the correlations are found. We show that the aliasing criticism is inadequate because the real quasi-biennial oscillation departs from an ideal sine wave in a way that reduces aliasing effects to insignificant levels. The nonuniqueness of identification of the 11-year solar cycle as the period of the arctic forcing for the Arctic winter stratospheric temperatures is a problem only for the short 33-year record of polar temperatures; in much longer time series of unstratified climate data the periods of 11 and 22 years are prominent. Highly unique signatures of solar wind forcing of tropospheric dynamics exist on the day-to-day time scale via two independent inputs to atmospheric electricity. These are (1) through changes in tropospheric ion production as a result of solar wind modulation of galactic cosmic rays and (2) through changes in the potential difference between the polar ionospheres and the surface, forced by the solar wind By component. The product of the cosmic ray flux and the ionospheric potential determines the vertical air-earth electrical current. In the presence of clouds of large horizontal extent, this current determines the rate of polarization charging of the clouds via the accumulation of positive electrostatic charges on droplets near cloud tops. The observed correlations, and theoretical and laboratory results for the effects of electrostatic charges on droplets and aerosols on the rates of ice nucleation, are consistent with the postulate that for certain regions and seasons and atmospheric levels the large-scale atmospheric electrical parameters have significant effects on the rates of initial ice nucleation. In such cases the chain of consequences includes changes in the rates of precipitation, net latent heat release, vertical motions, atmospheric vorticity, and ultimately in the general circulation. Much more work is required before the mechanism can be considered to have a secure basis in laboratory experiment and quantitative atmospheric modeling.

Effects of Image Charges on the Scavenging of Aerosol Particles by Cloud Droplets and on Droplet Charging and Possible Ice Nucleation Processes

Abstract Previous calculations of the rate at which falling droplets in clouds collide with aerosols have led to the conclusion that except in thunderclouds any electrical charges on the aerosols or droplets have little effect on the collision rate. However, it had been assumed that the aerosols would have only a few elementary charges on them, whereas it is now known that at the tops of nonthunderstorm clouds the evaporating droplets may have several hundred elementary charges on them and that much of this charge remains on the residual aerosol for 5 min or so after the evaporation. Also, most previous calculations neglected image charge forces that provide strong attraction at close range even when droplet and aerosol have charges of the same sign and of comparable magnitude. The authors present numerical calculations showing that electrical effects dominate collision rates for charged evaporation aerosols. The calculations are for the size range of 0.1- to 1.0-μm radius with the collision efficiency co...

Correlations of atmospheric dynamics with solar wind‐induced changes of air‐Earth current density into cloud tops

We analyze reported correlations between solar activity and weather and climate and show that in six independent data sets there is a correlation of measured changes in atmospheric dynamics with measured or inferred changes in vertical atmospheric air-earth current density. The current density changes are due to external modulation of the global electric circuit by the solar wind. We describe the several ways in which the solar wind modulates the global circuit, and the observations that support a simple model of the circuit, with two return paths in parallel. One return path is at low latitudes with relatively constant impedance and the other is at high latitudes and is responsive to solar wind modulation. The six independent data sets exhibiting the correlations include meteorological and air-earth current density changes on the 10 to 12-year solar cycle as well as on the day-to-day timescales of Forbush decreases of galactic cosmic ray flux and of heliospheric current sheet crossings. The geographic locations include northern and southern high latitudes as well as the tropics. In regions where these correlations arc found, there exists free energy in the form of supercooled water droplets near the tops of clouds that are unstable with respect to precipitation. Laboratory data and models suggest that electrostatic charge accumulating on supercooled droplets and aerosols near cloud tops affects the probability of ice nucleation and droplet freezing, enhancing the rate of growth and sedimentation of ice crystals. This proposed mechanism is also an explanation for another longstanding meteorological problem, the discrepancy between measurements at cloud tops of initial concentrations of ice and of concentrations of ice-forming nuclei. For light cloud cover the effect of increases in ice nucleation and sedimentation can he to reduce cloud opacity and albedo. For storm cloud systems the effect can be to enhance precipitation rates and latent heat release intensifying the storm. In several cases, measured or inferred storm intensification (or weakening) is directly related to measured or inferred increases (or decreases) of air-earth current density. Thus electrical effects on cloud microphysics may serve as connecting links between the observed or inferred increases in air-earth current density and the observed changes in atmospheric dynamics. In cases where thunderstorm electric fields are generated there are additional cloud microphysical effects that might contribute to the correlations. We discuss the present uncertainties regarding solar wind effects on the distribution of air-earth current density in the glohal electric circuit and regarding the relevant cloud microphysics. Much work is required to quantify these effects and evaluate their importance relative to competing processes.

Determination of F region height and peak electron density at night using airglow emissions from atomic oxygen

Measurements of the vertical column emission rates of atomic oxygen emissions arising from radiative recombination, ion-ion recombination, and dissociative recombination in the nighttime F region are sufficient to remotely sense the F layer height and peak plasma density. For example, measurements can be made of O I 1356 A and [O I] 6300 A, with vertical column emission rates J1356 and J6300. To a very good approximation the peak electron density is proportional to (J1356)1/2, and second-order dependence on height and exospheric temperature is very small. To a good approximation the ratio (J1356)1/2/J6300 is a single-valued function of the layer height.

Neutral atom precipitation-a review

Abstract The production of energetic neutral atoms by charge exchange of ring current ions with neutral hydrogen in the geocorona was predicted many years ago, and there are now a number of measurements of the effect of the impact of these energetic atoms on the thermosphere. Theoretical models of the process have been developed. The latitude variation of the precipitating flux depends very much on the pitch angle distribution of the ions in the ring current, and on the L shell on which they are located. The production of a belt of trapped particles at low altitude near the magnetic equator may occur when neutral atoms re-ionize and become trapped on impacting the thermosphere, and this belt has been found in particle measurements near the equator and is enhanced during periods of magnetic activity. A region of enhanced optical emission due to precipitating neutrals is found in the thermosphere near the magnetic equator in both disturbed and quiet times, implying a low L value and/or pancake pitch angle distribution for the ring current particles that give rise to these neutrals. An isotropic pitch angle distribution is present in parts of the ring current at time during magnetic storms. This gives rise to neutral atom precipitation at all latitudes, and particularly of particles near 90° pitch angle in the region of SAR arc occurrence, about 10° in dip latitude equatorward of the isotropic region. The rate of energy deposition and the rate of production of ionization in the thermosphere depend on the ion species present in the ring current; their energy spectra, and on the distributions of the ions with L value and pitch angle. The rate of energy deposition may at times reach 10 −2 to 10 −1 mWm −2 , sufficient for significant heating and wind generation. The rate of production of ionization in the thermosphere at night may be much greater than that of other low latitude night-time ionization sources.

Electroscavenging in clouds with broad droplet size distributions and weak electrification

Abstract Electrical, thermophoretic, diffusophoretic and gravitational forces have been included in a self-consistent way in new trajectory calculations of the scavenging of charged aerosol particles by cloud droplets. The important effect of the electrical force is a short-range attraction due to the interaction between the charge on the aerosol particle and the image charge that it induces on the droplet. It is stronger than phoretic forces with only a few tens of elementary charges on aerosol particles of radii 0.1 to 1.0 μm, interacting with cloud droplets of radii greater than 15 μm, and for relative humidity of order 98%. Under these conditions the electrical forces result in collision efficiencies typically an order of magnitude greater than that due to phoretic forces alone. The short-range attraction is insensitive to the sign or magnitude of the charge on the droplet, for larger droplets in weakly electrified clouds. Under typical conditions, aerosol particles arise from evaporation of charged droplets (evaporation residues) and retain the droplet charge, which decays with a time constant of order 15 min. During the time the charge is retained the electrically enhanced scavenging occurs. Some of the evaporation residues may act as ice-forming or condensation nuclei, so there are implications for contact ice nucleation, droplet size distributions, precipitation and cloud cover, as well as for cloud chemistry. We call this electrically enhanced scavenging electroscavenging, and it is strongest for broad droplet size distributions (extending past 15-μm radius). Applying calculated electroscavenging rates to charged evaporation residues, and assuming a fraction of them to have ice nucleation properties consistent with the limited measurements and theory available, we compare for illustrative purposes the rates of primary production of ice, in the form of droplets frozen by contact nucleation, with the concentrations of ice particles from deposition nucleation. Using a set of measured droplet size distributions that are broad or bimodal we find, for temperatures between 0 and −15 °C and in regions of clouds where mixing and evaporation are occurring, that the two production rates are of comparable magnitude.

Atmospheric Ionization and Clouds as Links Between Solar Activity and Climate

Observations of changes in cloud properties that correlate with the 11-year cycles observed in space particle fluxes are reviewed. The correlations can be understood in terms of one or both of two microphysical processes; ion mediated nucleation (IMN) and electroscavenging. IMN relies on the presence of ions to provide the condensation sites for sulfuric acid and water vapors to produce new aerosol particles, which, under certain conditions, might grow into sizes that can be activated as cloud condensation nuclei (CCN). Electroscavenging depends on the buildup of space charge at the tops and bottoms of clouds as the vertical current density (J z ) in the global electric circuit encounters the increased electrical resistivity of the clouds. Space charge is electrostatic charge density due to a difference between the concentrations of positive and negative ions. Calculations indicate that this electrostatic charge on aerosol particles can enhance the rate at which they are scavenged by cloud droplets. The aerosol particles for which scavenging is important are those that act as in-situ ice forming nuclei (IFN) and CCN. Both IMN and electroscavenging depend on the presence of atmospheric ions that are generated, in regions of the atmosphere relevant for effects on clouds, by galactic cosmic rays (GCR). The space charge depends, in addition, on the magnitude of J z . The magnitude of J z depends not only on the GCR flux, but also on the fluxes of MeV electrons from the radiation belts, and the ionospheric potentials generated by the solar wind, that can vary independently of the GCR flux.

Field aligned airglow observations of transequatorial bubbles in the tropical F-region

Abstract It is possible to form images of the tropical F -region ionization structures, variously labelled as ‘bubbles’, ‘plumes’, or ‘depletions’, in a plane perpendicular to the magnetic field by observing the airglow emissions associated with them in a field aligned direction. Structures which are present at altitudes from 250 km to more than 700 km above the dip equator map down to the 250–350 km region, where recombination and associated airglow emissions occur, ranging from the equator to dip latitudes of 15° or more. The structures can be viewed in a field aligned direction from sites in the range 17°–23° dip latitude. Measurements with high angular resolution (as small as 0.1° in the meridian) could show structures as small as 2 km. It is possible to make simultaneous measurements in both 6300 and 7774 A recombination emissions, from which the height h max of the peak plasma concentration n ( e ) max on the field line can be estimated from a ratio of the emission rates. It is possible to make maps of n ( e ) max and h max either by raster scanning the sky in the two emissions or by imaging them onto an imaging detector. Useful data can be obtained from one site over a range of 20° in dip latitude and 10° in dip longitude. Observations in the same magnetic meridian as a backscatter radar system are desirable, as also are observations from near magnetic conjugate points. Imaging characteristics for the observation sites in the range of dip latitude 17°–23° have been calculated.