A statistical study of Galactic SNRs using the PMN survey

The Parkes–MIT–NRAO (PMN) radio survey has been used to generate a quasi all-sky study of Galactic Supernova Remnants (SNRs) at a common frequency of 4.85 GHz (λ=6 cm). We present flux densities estimated for the sample of 110 Southern Galactic SNRs (up to δ=−65°) observed with the Parkes 64-m radio telescope and an additional sample of 54 from the Northern PMN (up to δ=+64°) survey undertaken with the Green Bank 43-m (20 SNRs) and 91-m (34 SNRs) radio telescopes. Out of this total sample of 164 selected SNRs (representing 71% of the currently 231 known SNRs in the Green catalogue) we consider 138 to provide reliable estimates of flux density and surface brightness distribution. This sub-sample represents those SNRs which fall within carefully chosen selection criteria which minimises the effects of the known problems in establishing reliable fluxes from the PMN survey data. Our selection criteria are based on a judicious restriction of source angular size and telescope beam together with careful evaluation of fluxes on a case by case basis. Direct comparison of our new fluxes with independent literature values gives excellent overall agreement. This gives confidence in the newly derived PMN fluxes when the selection criteria are respected. We find a sharp drop off in the flux densities for Galactic SNRs beyond 4 Jy and then a fairly flat distribution from 5 to 9 Jy, a slight decline and a further flat distribution from 9 to 20 Jy though the numbers of SNR in each Jy bin are low. We also re-visit the contentious Σ–D (radio surface brightness–SNRs diameter) relation to determine a new power law index for a sub-sample of shell type SNRs which yields β=−2.2±0.6. This new evaluation of the Σ–D relation, applied to the restricted sample, provides new distance estimates and their Galactic scale height distribution. We find a peak in the SNR distribution between 7–11 kpc with most restricted to ±100 pc Galactic scale height.     

Scaling-law equilibria for calcium in canopy-type models of the solar chromosphere

Scaling laws for resonance line formation are used to obtain approximate excitation and ionization equilibria for a three-level model of singly ionized calcium. The method has been developed for and is applied to the study of magnetograph response in the 8542 Å infrared triplet line to magnetostatic canopies which schematically model diffuse, nearly horizontal fields in the low solar chromosphere. For this application, the method is shown to be efficient and semi-quantitative, and the results indicate the type and range of effects on calcium-line radiation which result from reduced gas pressure inside the magnetic regions.Operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.     

The Canadian Meteor Orbit Radar Meteor Stream Catalogue

The Canadian Meteor Orbit Radar is a multi-frequency backscatter radar which has been in routine operation since 1999, with an orbit measurement capability since 2002. In total, CMOR has measured over 2 million orbits of meteoroids with masses greater than 10 μg, while recording more than 18 million meteor echoes in total. We have applied a two stage comparative technique for identifying meteor streams in this dataset by making use of clustering in radiants and velocities without employing orbital element comparisons directly. From the large dataset of single station echoes, combined radiant activity maps have been constructed by binning and then stacking each years data per degree of solar longitude. Using the single-station mapping technique described in Jones and Jones (Mon Not R Astron Soc 367:1050–1056, 2006) we have identified probable streams from these single station observations. Additionally, using individual radiant and velocity data from the multi-station velocity determination routines, we have utilized a wavelet search algorithm in radiant and velocity space to construct a list of probable streams. These two lists were then compared and only streams detected by both techniques, on multiple frequencies and in multiple years were assigned stream status. From this analysis we have identified 45 annual minor and major streams with high reliability.     

Energetic electron signatures of Saturn's smaller moons: Evidence of an arc of material at Methone

We present several energetic charged particle microsignatures of two Lagrange moons, Telesto and Helene, measured by the MIMI/LEMMS instrument. These small moons absorb charged particles but their effects are usually obscured by Tethys and Dione, the two larger saturnian satellites that occupy the same orbits. The scales and structures of these microsignatures are consistent with standard models for electron absorption from asteroid-sized moons in Saturn's radiation belts. In the context of these observations, we also examine the possibility that the 3 km Satellite Methone is responsible for two electron microsignatures detected by Cassini close to this moon's orbit. We infer that a previously undetected arc of material exists at Methone's orbit (R/2006 S5), we speculate how such a structure could form and what its physical characteristics and location could be. The origin of this arc could be linked to a possible presence of a faint ring produced by micrometeoroid impacts on Methone's surface, to E-ring dust clump formation at that distance or to temporary dust clouds produced by enceladian activity that spiral inwards under the effect of non-gravitational forces.     

Aerial electromagnetic sounding of the lithosphere of Venus

Electromagnetic (EM) investigation depths are larger on Venus than Earth due to the dearth of water in rocks, in spite of higher temperatures. Whistlers detected by Venus Express proved that lightning is present, so the Schumann resonances ～10–40 Hz may provide a global source of electromagnetic energy that penetrates ～10–100 km. Electrical conductivity will be sensitive at these depths to temperature structure and hence thermal lithospheric thickness. Using 1D analytic and 2D numerical models, we demonstrate that the Schumann resonances—transverse EM waves in the ground-ionosphere waveguide—remain sensitive at all altitudes to the properties of the boundaries. This is in marked contrast to other EM methods in which sensitivity to the ground falls off sharply with altitude. We develop a 1D analytical model for aerial EM sounding that treats the electrical properties of the subsurface (thermal gradient, water content, and presence of conductive crust) and ionosphere, and the effects of both random errors and biases that can influence the measurements. We initially consider specified 1D lithospheric thicknesses 100–500 km, but we turn to 2D convection models with Newtonian temperature-dependent viscosity to provide representative vertical and lateral temperature variations. We invert for the conductivity-depth structure and then temperature gradient. For a dry Venus, we find that the error on temperature gradient obtained from any single local measurement is ～100%—perhaps enough to distinguish “thick” vs. “thin” lithospheres. When averaging over thousands of kilometers, however, the standard deviation of the recovered thermal gradient is within the natural variability of the convection models, <25%. A “wet” interior (hundreds of ppm H₂O) limits EM sounding depths using the Schumann resonances to <20 km, and errors are too large to estimate lithospheric properties. A 30-km conductive crust has little influence on the dry-interior models because the Schumann penetration depths are significantly larger. We conclude that EM sounding of the interior of Venus is feasible from a 55-km high balloon. Lithospheric thickness can be measured if the upper-mantle water content is low. If H₂O at hundreds of ppm is present, the deeper, temperature-sensitive structure is screened, but the “wet” nature of the upper mantle, as well as structure of the upper crust, is revealed.     

Does Magnetic Flux Submerge at Flux Cancelation Sites?

Simultaneous measurements of the magnetic fields in the photosphere and chromosphere were used to investigate if magnetic flux is submerging at sites between adjacent opposite polarity magnetic network elements in which the flux is observed to decrease or `cancel'. These data were compared with chromospheric and coronal intensity images to establish the timing of the emission structures associated with these magnetic structures as a function of height. We found that most of the cancelation sites show either that the bipole is observed longer in the photosphere than in the chromosphere and corona (44%) or that the timing difference of the disappearance of the bipole between these levels of the atmosphere is unresolved. The magnetic axis lengths of the structures associated with the cancelation sites are on average slightly smaller in the chromosphere than the photosphere. These observations suggest that magnetic flux is retracting below the surface for most, if not all, of the cancelation sites studied.     

Reproductive cues in Panulirus ornatus

Two experiments were performed to assess the effect of photoperiod and temperature on spawning of Panulirus ornatus. In experiment 1, sexually mature lobsters taken from the wild during summer were held at one of two photoperiods, winter (13 Light: 11 Dark) and summer (14.5 Light:9.5 Dark). Additionally, lobsters were also exposed to either summer (29°C) or winter (24°C) average water temperatures. Spawning was significantly greater when animals were exposed to summer photoperiod than to winter photoperiod, irrespective of temperature. Although a higher percentage of lobsters spawned when placed under a higher temperature, this trend was not statistically significant. In experiment 2, sexually mature lobsters were taken from the wild during winter and exposed to the same two photoperiods as in experiment 1, at a summer equivalent temperature of 29°C. Breeding started earlier and was more successful at the summer photoperiod. Time to first breeding was 17 weeks after exposure to summer photoperiod, compared with less than 1 week in experiment 1, and did not occur until individuals had moulted. Moulting occurred in 81% of lobsters, primarily after an increase in temperature to 29°C. The time between moulting and mating was varied and there was no significant difference in moult frequency between the two experimental photoperiods. After the lobsters had moulted, breeding success was reached earlier if photoperiod was lengthened. Results suggest photoperiod is the primary cue for the onset of gonad maturity and mating activity, with temperature playing a less important role. Physiological rest and possibly a moult may be required between breeding seasons before spawning can occur. Furthermore, temperature may be an important cue for pre‐reproduction moulting.