Experimental determination of layer cloud edge charging from cosmic ray ionisation

The cloud-air transition zone at stratiform cloud edges is an electrically active region where droplet charging has been predicted. Cloud edge droplet charging is expected from vertical flow of cosmic ray generated atmospheric ions in the global electric circuit. Experimental confirmation of stratiform cloud edge electrification is presented here, through charge and droplet measurements made within an extensive layer of supercooled stratiform cloud, using a specially designed electrostatic sensor. Negative space charge up to 35 pC m−3 was found in a thin (<100 m) layer at the lower cloud boundary associated with the clear air-cloud conductivity gradient, agreeing closely with space charge predicted from the measured droplet concentration using ion-aerosol theory. Such charge levels carried by droplets are sufficient to influence collision processes between cloud droplets

AIDS Surveillance in Africa

Surveillance of the AIDS pandemic in Africa has always posed formidable problems for epidemiologists. Diagnostic accuracy-according to the case definitions for AIDS used in industrialised countries-is impossible to achieve in all but a few places with the right diagnostic facilities. Responding to the urgent need for surveillance, the World Health Organisation drew up a clinical case definition (the WHO/Bangui definition), which depended on clinical criteria without the need for serological verification.'2 Judged by its use, the WHO/Bangui definition has been successful-52 African countries have reported cases ofAIDS using mainly this definition.3 Some countries have modified it to fit local circumstances, removing a defining symptom here, adding the need for an extra sign there, and many now accept or encourage a positive result of an HIV test as supportive evidence. (At least one, COte d'Ivoire, requires such a result.2) Inevitably the definition has its limitations, and two papers in this week's journal discuss these at length (p 11852, p 11894). Because of limited laboratory facilities published evaluations of the WHO/Bangui definition have been mainly restricted to groups of sick patients using HIV positivity as the reference standard. The definition's sensitivity and specificity have been calculated as being between 60% and 90%2 5- useful for purposes of surveillance, but leaving uncertainty over whether this surveillance tool is intended to monitor trends in cases of AIDS or HIV infection. Other problems exist with the WHO/Bangui definition. Because many doctors lack diagnostic facilities they use the definition for diagnosis. The title "clinical case definition" encourages this confusion. The misuse is disturbing as the probability that a patient who fulfils the WHO/Bangui definition tests positive for HIV may fall well below 50% when seroprevalence is low.5 Another problem of using the definition is the delayed and incomplete picture that it gives of the spread of infection.6 Far preferable for surveillance of infection is the unlinked anonymous testing for HIV of sentinel groups attending health services67 (such as pregnant women and people with sexually transmitted diseases), which has now begun in several African countries89 using the same methods as in industrialised countries.'° Where does this leave the WHO/Bangui definition? De Cock and colleagues rehearse the overwhelming case for AIDS reporting to continue and suggest a thoughtful redesign of the definition, which includes the requirement for a positive HIV test result.3 Insisting on positive test results in all circumstances, however, is impractical: HIV tests are already limited and are lioely to become more so as AIDS funding to Africa inevitably falls. As a provisional solution to the problem of surveillance the WHO/Bangui definition has been useful, but the time has come for its reappraisal

Observations of Saharan dust layer electrification

Electrification of atmospheric dust influences the coagulation, wet removal and fall speeds of dust particles. Alignment of dust particles can also occur in fair weather atmospheric electrical conditions if the particles are charged. However, very few electrical measurements made in elevated dust layers exist. Balloon-borne charge and particle instrumentation have been used to investigate the electrical properties of elevated Saharan dust layers. Soundings from the Cape Verde Islands, which experience frequent Saharan dust outbreaks, intercepted several dust layers. Two balloon soundings during summer 2009 detected dust particles in layers up to 4 km altitude. Simultaneous electrical measurements showed charge inside the dust layers, with a maximum measured charge density of 25 pC m − 3, sufficient to influence wet removal processes

Electrical charging of ash in Icelandic volcanic plumes

The existence of volcanic lightning and alteration of the atmospheric potential gradient in the vicinity of near-vent volcanic plumes provides strong evidence for the charging of volcanic ash. More subtle electrical effects are also visible in balloon soundings of distal volcanic plumes. Near the vent, some proposed charging mechanisms are fractoemission, triboelectrification, and the so-called "dirty thunderstorm" mechanism, which is where ash and convective clouds interact electrically to enhance charging. Distant from the vent, a self-charging mechanism, probably triboelectrification, has been suggested to explain the sustained low levels of charge observed on a distal plume. Recent research by Houghton et al. (2013) linked the self-charging of volcanic ash to the properties of the particle size distribution, observing that a highly polydisperse ash distribution would charge more effectively than a monodisperse one. Natural radioactivity in some volcanic ash could also contribute to self-charging of volcanic plumes. Here we present laboratory measurements of particle size distributions, triboelectrification and radioactivity in ash samples from the Gr\'{i}msv\"{o}tn and Eyjafjallaj\"{o}kull volcanic eruptions in 2011 and 2010 respectively, and discuss the implications of our findings.Comment: XV Conference on Atmospheric Electricity, 15-20 June 2014, Norman, Oklahoma, US

Testing the Hubble Law with the IRAS 1.2 Jy Redshift Survey

We test and reject the claim of Segal et al. (1993) that the correlation of redshifts and flux densities in a complete sample of IRAS galaxies favors a quadratic redshift-distance relation over the linear Hubble law. This is done, in effect, by treating the entire galaxy luminosity function as derived from the 60 micron 1.2 Jy IRAS redshift survey of Fisher et al. (1995) as a distance indicator; equivalently, we compare the flux density distribution of galaxies as a function of redshift with predictions under different redshift-distance cosmologies, under the assumption of a universal luminosity function. This method does not assume a uniform distribution of galaxies in space. We find that this test has rather weak discriminatory power, as argued by Petrosian (1993), and the differences between models are not as stark as one might expect a priori. Even so, we find that the Hubble law is indeed more strongly supported by the analysis than is the quadratic redshift-distance relation. We identify a bias in the the Segal et al. determination of the luminosity function, which could lead one to mistakenly favor the quadratic redshift-distance law. We also present several complementary analyses of the density field of the sample; the galaxy density field is found to be close to homogeneous on large scales if the Hubble law is assumed, while this is not the case with the quadratic redshift-distance relation.Comment: 27 pages Latex (w/figures), ApJ, in press. Uses AAS macros, postscript also available at http://www.astro.princeton.edu/~library/preprints/pop682.ps.g

Rapid Bidirectional Switching of Synaptic NMDA Receptors

SummarySynaptic NMDA-type glutamate receptors (NMDARs) play important roles in synaptic plasticity, brain development, and pathology. In the last few years, the view of NMDARs as relatively fixed components of the postsynaptic density has changed. A number of studies have now shown that both the number of receptors and their subunit compositions can be altered. During development, the synaptic NMDARs subunit composition changes, switching from predominance of NR2B-containing to NR2A-containing receptors, but little is known about the mechanisms involved in this developmental process. Here, we report that, depending on the pattern of NMDAR activation, the subunit composition of synaptic NMDARs is under extremely rapid, bidirectional control at neonatal synapses. This switching, which is at least as rapid as that seen with AMPARs, will have immediate and dramatic consequences on the integrative capacity of the synapse

Bell-inequality violation with a triggered photon-pair source

Here we demonstrate, for the first time, violation of Bell's inequality using a triggered quantum dot photon-pair source without post-selection. Furthermore, the fidelity to the expected Bell state can be increased above 90% using temporal gating to reject photons emitted at times when collection of uncorrelated light is more probable. A direct measurement of a CHSH Bell inequality is made showing a clear violation, highlighting that a quantum dot entangled photon source is suitable for communication exploiting non-local quantum correlations.Comment: 14 pages, 4 figure