Ice Targets Research Articles

Thunderstorm electrification: The effect of cloud droplets

Further laboratory studies have been made of the charge transferred when ice crystals collide and then separate from an ice target. Previous results indicated increased charge transfer when cloud droplets were present. The new data show that the size distribution of the droplets is important. When the cloud droplets are too small to collide with the growing target, it charges negatively; for the same liquid water content but with larger droplets that hit the target, it charges positively. The results suggest that the relative growth rate and the surface states of both the interacting bodies may be important in controlling the sign of the charge transfer. It is suggested that these results may explain the observation of negative charge centers in thunderstorms at temperatures warmer than −20°C.

Ice conductivity restraints on the inductive theory of thunderstorm electrification

According to the inductive theory of thunderstorm electrification, a small ice crystal bouncing off the underside of a larger hydrometeor falling in the presence of an electric field should remove some of the field‐induced polarization charge from the surface of the larger particle; subsequent gravitational separation of the two particles should then result in a rapid growth of the electric field. However, because of the finite surface conductivity of the ice, it is not obvious that the polarization charges can flow along the ice surface in the short interaction time. We report laboratory experiments on the effect of an external electric field on the charge transferred during individual collisions of small (typically, 100‐μm) ice spheres with larger ice targets. When the surface conductivity of the ice was artificially increased by using 10−2 M NaCl, the full theoretical charge transfer was observed, but at 10−5 M NaCl, no field‐dependent charge transfer could be detected. Airborne measurements of cloud and precipitation purity have shown that 10−4 M NaCl is occasionally reached in low‐level stratiform clouds in polluted regions, but even with this degee of contamination the inductive mechanism operates at less than 20% of its potential efficiency at −10°C; at lower temperatures this fraction is reduced still further. We conclude that the inductive mechanism cannot account for the observed electrification of thunderstorms unless other contaminants are having an unexpectedly large effect on the conductivity.

Ice crystal interactions with a riming target: Charge transfer and collection efficiencies

Recent laboratory studies have shown that the sign and magnitude of electric charge transferred to a riming pellet of soft hail when ice crystals collide then separate are sensitive functions of the cloud liquid water content and the temperature. Certain differences between these results, obtained with crystal/rimer relative velocities of ∼3 m s−1, and those of other workers at ∼10 m s−1 have been reconciled in the present work in terms of a dependence of the sign of the charge transfer on velocity. By measuring the charge transferred to two ice targets of different size, an estimate has been made of the product of collision and separation probabilities of crystals interacting with a 5‐mm‐diameter target.

Impact experiments on ice

The results of cratering and fragmentation experiments on pure ice are reported. The projectiles used are cylindrical aluminum, poly‐carbonate, teflon, and pyrophyllite fired at velocities between 110 m/sec and 680 m/sec, with kinetic energies at impact between 2 and 500 joules. Crater diameters (pit diameters) in the ice were about two times larger than craters in the same energy range in basalts. The ratios of (pit diameter)/(spall diameter) are about three and the ratios of (depth of crater)/(spall diameter) are between 0.1 and 0.3 which are close to the depth/diameter ratios observed in basalts. The crater diameter in ice is also well expressed as a single function of the ‘late‐stage effective energy’ defined recently by Mizutani et al. [1983]. The specific energy for complete destruction of ice target is about 50 J/kg which is two orders of magnitude smaller than that of basalt. The present experimental data on cratering and fragmentation of ice show that the impacts associated with the largest craters on Callisto and Mimas must have severely fractured the whole satellites, and that those giant impacts with the kinetic energy of 1023 to 1027 joules probably affected significantly the evolution of the satellites.

Laboratory studies of the charging of soft‐hail during ice crystal interactions

AbstractA laboratory investigation of electric charge transfer during the impact of vapour‐grown ice crystals and supercooled water droplets upon a simulated soft‐hailstone target has shown that the magnitude of the charge transferred to the riming surface when crystals separate from it is a function of temperature, crystal dimension, relative velocity, liquid water content, and impurity content of the water droplets and hence the impurity content of the riming target. The sign of the charge transfer depends on temperature, liquid water content and droplet and rime impurity content.In the absence of crystals, no charge transfer was detected during riming. In the absence of supercooled water droplets, crystals impacting at 10ms1 on an evaporating rime target produced a small negative charge on the rime of less than − 0.25fC per separating crystal. When the target surface grew by vapour diffusion it gained a small positive charge during such interactions. Much larger charges and completely different charge transfer behaviour was noted during riming. The target became positively charged at high liquid water contents and temperatures above a critical value, but negatively charged at lower temperatures or with lower liquid water contents. The critical sign reversal temperature at a liquid water content of 1 gm−3 was about − 20°C. At − 10°C with a liquid water content of 2gm−3, a 125 μm crystal impacting at 3ms−1 charged the target by +101C upon separation. The charge transfer increased sharply with impact speed and crystal size. Warming the positively charging rime to cause it to evaporate failed to reverse the sign of the charge transfer. Experiments with impurities showed that the sign reversal temperature increased if the droplets contained contaminants at concentrations found in cloud water.It is suggested that there are two distinct charge transfer processes during crystal interactions with an ice target, the dominant one requiring the presence of supercooled water droplets. Careful control and knowledge of the microphysical properties of the clouds used in these experimental simulations has permitted an examination of charge transfer under many of the conditions used in previous studies. The results provide an understanding of the differences and a reconciliation between some of the previously disparate findings in terms of the two distinct charge transfer regimes.

Correction to ‘Role of relaxation and contact times in charge separation during collision of precipitation particles with ice targets’ by Gerardo Wolfgang Gross

Correction to ‘Role of relaxation and contact times in charge separation during collision of precipitation particles with ice targets’ by Gerardo Wolfgang Gross

Role of relaxation and contact times in charge separation during collision of precipitation particles with ice targets

The relaxation time of charge transfer and the contact time are two of the parameters controlling charge separation when ice particles collide with one another or with liquid water droplets. For maximum efficiency of charge separation, the relaxation time should be of the same order as the contact time. The relaxation time of charge transport in ice was systematically determined as a function of impurity content and temperature. The contact time for ice/ice collisions was estimated from theory. Depending on the impurity species and their concentration, the relaxation time may be increased or decreased by orders of magnitude with respect to pure ice. Only particles of the order of 10−2 m radius seem to have contact times sufficiently long to give appreciable charge separation by ion transport in collisions between pure or slightly doped (0.1 ppm chloride) ice particles in a dry environment. Charge separation mechanisms involving liquid water and those operating by the transfer of electrons from surface states largely relax these conditions. These results have implications for cloud electrification models involving collision mechanisms.

Performance Evaluation of an Operational Spaceborne Scatterometer

Study results are presented showing performance capability of a spaceborne scatterometer to measure ocean surface wind speed and direction operationally. In addition, a research mode is described which will allow development of improved radar signatures for ocean, sea ice, and land targets. The study results show that a scatterometer can meet the operational user requirement of ±2 m/s wind-speed accuracy (or ±10 percent, whichever is greater) and ±20° wind-direction accuracy over most of the expected ocean surface conditions. The six-beam scatterometer design evaluated is shown to be skillful (>90 percent correct) in specifying the correct wind-vector solution (with a 1800° ambiguity) from the multiple solutions derived. Further improvement must rely on meteorological and pattern-recognition techniques which are being studied.

The role of volatiles and lithology in the impact cratering process

A survey of published descriptions of 32 of the largest, least eroded terrestrial impact structures reveals that the amount of melt at craters in crystalline rocks is approximately 2 orders of magnitude greater than at craters in sedimentary rocks. In this paper we present a model for the impact process and examine whether this difference in melt abundance is due to differences in the amount of melt generated in various target materials or due to differences in the fate of the melt during late stages of the impact. The model consists of a theoretical part for the early stages of impact, based on a Birch‐Murnaghan equation of state, a penetration scheme after Shoemaker (1963), and an attenuation model modified from Gault and Heitowit (1963), and a descriptive part for the later stages of impact, based on field observations at the large terrestrial craters. The impacts of iron, stone, permafrost, and ice meteorites 1 km in diameter into crystalline, carbonate, dry sandstone, ice‐saturated sand, and ice targets are modeled for velocities of 6.25, 17, and 24.6 km/s. Tables of calculated crater volume, depth of penetration of the meteorite, equivalent scaled depth of burst, radii to various peak pressure isobars, volume of silicate melt, and volume of water vapor (or, in the case of carbonate, carbon dioxide vapor) are presented. Simple algebraic expressions for pressure attenuation are derived: for the near field, dX/dR = 3Xn/R(1 ‐ n), where X is the pressure normalized to an averaged bulk modulus for the target rocks, R is the radius normalized to the radius of the cavity in which energy is initially deposited, and n is the pressure derivative of the bulk modulus. For the far field the pressure attenuation is given by dX/dR ∼−3X/R. For most materials considered, n = 4–6, and therefore the near‐field attenuation is proportional to R−3.65 ‐ R−4 and the far‐field attenuation is proportional to R−3. The calculations show that the volume of material shocked to pressures sufficient for melting should not be significantly different in sedimentary and crystalline rocks. Hence we conclude that shock melt is formed in the early stages of the cratering process by impacts into rocks rich in volatiles but is destroyed by the cratering process. We propose that the melt is finely dispersed by the great expansion of shocked volatiles upon release from high pressure and that suevite units are the product of this process. The fragmented silicates produced by this process may react penecontemporaneously with the hot volatiles to produce hydrated minerals such as clays. This process may produce hydrothermally altered minerals in planetary regoliths, such as the Martian regolith. The dispersion of shock melt by volatile expansion may also account for the apparent lack of lunarlike melt sheets on the surface of Mars. Because large amounts of volatiles vaporize during impact and are transferred from depth either into space, into the atmosphere, or onto near‐surface ejecta by condensation, repeated impact degasses a planet, depleting some layers in volatiles and, unless the volatiles escape the planet, enriching others.

Continuously produced, high specific activity 11C for studies of photosynthesis, transport and metabolism

A continuous supply of high specific activity 11CO 2 was produced for studies of photosynthesis, transport, metabolic utilization, and storage in plants. The reaction 12C( p, d) 11C from a 35 MeV proton beam on a 300 cm 3 gaseous 12CO 2 target yielded 7.7 × 10 5 atoms/μA/sec in a flow of 1 cm 3/sec, and yielded 2.1 × 10 9 11C atoms/μA/sec from a dry ice target. The resultant 11CO was purified (in air), then oxidized to 11CO 2, and 12CO 2 was added in known amounts to produce the mixture air/ 12CO 2/ 11CO 2 of known concentration and specific activity. By analyzing pulse-fronts and steady-state labelling levels in an individual plant it is possible to extract considerably more information about uptake rates, product pool sizes and turnover rates, transport velocities and concentrations, metabolic sink strengths, and other parameters, than would be attainable by pulse methods alone. The high yields of 11C and potentially of 13N by these and related methods would also be of use in radiopharmaceuticals and biomedical tracers.

A laboratory study of charge transfer accompanying the collision of ice crystals with a simulated hailstone

AbstractIn a series of experiments, conducted inside a cold‐room, clouds of small ice crystals of mean diameter 10μm were drawn at velocities between 1.5 and 18ms−1 past simulated hailstones. Two of these ice targets, side by side but under different conditions, were subjected to crystals from the same clouds simultaneously, so that the effects of various parameters on charge transfer could be isolated. The concentrations, sizes and shapes of the ice crystals were determined.A target heated radiatively with respect to the crystals became charged more negatively than an unheated target. A colder target became charged more positively. However, the magnitude of the charging did not depend on the temperature difference between target and crystals, provided this was greater than about 1 K. Attempts to find evidence of discharge‐limited charging produced negative results. A strong velocity dependence of charging currents was found, but subsidiary experiments on the collection of crystals by the target suggested that this was largely due to the variation in the probability of separation of the crystals, which appeared to be close to zero at the lowest velocities employed. The effects of doping the targets with HF and NH3, and of surface roughness, were investigated and were found to be small.These results were found to be in reasonable qualitative and quantitative agreement with the predictions of the charging mechanisms proposed by Auf der Maur and Buser, involving differences in the work functions of ice surfaces of different nature.It was estimated that the charge, q', transferred in a rebounding collision between an ice crystal and a heated ice target is about 900 aC. It is shown that this is roughly compatible with the general pattern of results of other workers (with one exception) if the charging increases with the square of the diameter, d, of the small particles. There are good theoretical reasons for such a relationship, which may be described by the equation q'(aC)=5d2, for 10μm⩽d⩽200 μm. Charge transfer of this magnitude is estimated to be just sufficient to account for the electrification of thunderclouds.

A laboratory study of charge transfer accompanying the collision of ice crystals with a simulated hailstone

In a series of experiments, conducted inside a cold-room, clouds of small ice crystals of mean diameter 10μm were drawn at velocities between 1.5 and 18ms−1 past simulated hailstones. Two of these ice targets, side by side but under different conditions, were subjected to crystals from the same clouds simultaneously, so that the effects of various parameters on charge transfer could be isolated. The concentrations, sizes and shapes of the ice crystals were determined. A target heated radiatively with respect to the crystals became charged more negatively than an unheated target. A colder target became charged more positively. However, the magnitude of the charging did not depend on the temperature difference between target and crystals, provided this was greater than about 1 K. Attempts to find evidence of discharge-limited charging produced negative results. A strong velocity dependence of charging currents was found, but subsidiary experiments on the collection of crystals by the target suggested that this was largely due to the variation in the probability of separation of the crystals, which appeared to be close to zero at the lowest velocities employed. The effects of doping the targets with HF and NH3, and of surface roughness, were investigated and were found to be small. These results were found to be in reasonable qualitative and quantitative agreement with the predictions of the charging mechanisms proposed by Auf der Maur and Buser, involving differences in the work functions of ice surfaces of different nature. It was estimated that the charge, q', transferred in a rebounding collision between an ice crystal and a heated ice target is about 900 aC. It is shown that this is roughly compatible with the general pattern of results of other workers (with one exception) if the charging increases with the square of the diameter, d, of the small particles. There are good theoretical reasons for such a relationship, which may be described by the equation q'(aC)=5d2, for 10μm⩽d⩽200 μm. Charge transfer of this magnitude is estimated to be just sufficient to account for the electrification of thunderclouds.

The stopping power of heavy ice for low-energy (10-30 keV) deuterons

Measured yields for the D(d,n)T reaction on a thick heavy ice target have been used to obtain the stopping power for deuterons in the energy range 10-30 keV. The results agree with other solid target measurements but are substantially lower than gas target values.

Electrification Associated with Droplet Accretion on Ice

Droplets in the size range of 40 to 120 µm diameter were accelerated in a small wind tunnel and accreted onto an ice target connected to an electrometer that measured the charging effect. The experiments were performed with environment temperatures between −6 and −14C and impact velocities between 1 and 30 m sec−1, both in the absence of an external field and with fields up to 2.5 × 105 V m−1. It was found that in the absence of field, significant charging occurs only above a threshold velocity, of the order of 10 m sec−1 for 100-µm droplets. The sign and magnitude of the effect were critically dependent on the liquid composition, the sign being negative for NACl solutions and positive for (NH4)2SO4 solutions and pure water. The magnitude increased with droplet size and with impact velocity, reaching a maximum in the order of 1–3 fC (average charge per droplet) at about 25 m sec−1, for 100-µm droplets. No evidence of ice splintering was found, and the effect appeared associated with splashing. An explanation based on the mechanism of double layer shearing (which implies that the Workman-Reynolds effect is active) is proposed. External fields induce charge separations proportional to their intensity, with proportionality constants increasing with droplet size and impact velocity; this effect becomes predominant for fields in the order of 104 V m−1 or higher. Consequences of the experiments for the theory of thunderstorm electrification are drawn. A separate series of experiments, dealing with the electrification associated with the splashing of 370- and 500-µm droplets, is reported in an Appendix.

The effects of Ammonia on the electrification of freezing and splashing water drops

Experiments were undertaken in which 2 mm diameter water drops contaminated with NaCl and/or (NH 4 ) 2 CO 3 were allowed to fall through a cold environment and impinge on an ice target. Charge deposited on the target was measured as a function of environment temperature (0°C to ?30°C) and concentration of the solution (10 ?6 M to 10 ?2 M). Maximum negative charge of 2.04 esu/g was produced by 10 ?4 M NaCl at an environment temperature of ?20°C. Maximum positive charge of 2.79 esu/g was produced by 10 ?3 M (NH 4 ) 2 CO 3 at an environment temperature of ?30°C. It was possible to suppress and even reverse the usual negative charge of salt by mixing with ammonia, suggesting the possibility of modifying thunderstorm electricity. All solutions weaker than 10 ?5 M gave complictated waveforms with a net negative charge. DOI: 10.1111/j.2153-3490.1972.tb01573.x

The effects of Ammonia on the electrification of freezing and splashing water drops

Experiments were undertaken in which 2 mm diameter water drops contaminated with NaCl and/or (NH 4 ) 2 CO 3 were allowed to fall through a cold environment and impinge on an ice target. Charge deposited on the target was measured as a function of environment temperature (0°C to ?30°C) and concentration of the solution (10 ?6 M to 10 ?2 M). Maximum negative charge of 2.04 esu/g was produced by 10 ?4 M NaCl at an environment temperature of ?20°C. Maximum positive charge of 2.79 esu/g was produced by 10 ?3 M (NH 4 ) 2 CO 3 at an environment temperature of ?30°C. It was possible to suppress and even reverse the usual negative charge of salt by mixing with ammonia, suggesting the possibility of modifying thunderstorm electricity. All solutions weaker than 10 ?5 M gave complictated waveforms with a net negative charge. DOI: 10.1111/j.2153-3490.1972.tb01573.x

Accurately defined neutron beams from D(d,n) 3He at Ed = 50 keV, using the associated particle method

Accurately collimated fast neutron beams of small angular width and known absolute intensity and energy were produced by bombarding heavy ice targets with 50 keV deuterons and operating the neutron detector in coincidence with a semiconductor counter detecting the 3He recoil nuclei. The measured beam profile agreed with the theoretically calculated profile. The beam was used to measure the absolute neutron detection efficiency and the scintillation pulse spectrum of a plastic scintillation counter bombarded by 2.55 MeV neutrons.

The sticking probability of water on ice

A direct method to determine the sticking probability, or condensation coefficient, α c , of water molecules on ice has been developed. A high flux molecular beam of water vapour is impinged on an ice target supported by a cooled copper sheet suspended from a quartz spring. The sticking probability is obtained from the increase in weight of the target. The value found for α c is 0·83 ± 0·15 for the temperature range of −115° to −140°C.

Total cross sections of V, In, I and Bi for 3.2–5.2 MeV neutrons

The total neutron cross sections of V, In, I and Bi have been measured by the transmission method for neutrons of energies between 3.2 and 5.2 MeV. Neutrons of about keV energy spread were produced by bombarding a heavy ice target with deuterons. Broad resonances were observed in all nuclei that suggest flactuations in the level density of the compound nucleus.

Ca 40(n,α o) Reaction energy spectra and differential cross sections

Experiments were recently carried on using silicon junctions as target- detectors to measure the energy spectra of charged particles from (n,x) reactions in Si28 and Si29. The maximum Q value for charged-particle emission in (n,x) reactions from silicon isotopes, contained in the detector, is --0.02 Mev for the Si29 (n, alpha o) reaction. The Ca40(n alpha o) reaction was investigated in order to extend the measurements for the energy spectra for the (n, charged particles) reaction. The Q value for the Ca40(n, alpha o) reaction is 1.75 Mev, which is in a region where the Coulomb barrier is not too high to prevent the alpha emission. The neutron source was a liquid-air-cooled D2O ice target bombarded with deuterons accelerated by a Van de Graaff accelerator.