Testing solar forcing of pervasive Holocene climate cycles

The temporal and spatial extent of Holocene climate change is an area of considerable uncertainty, with solar forcing recently proposed to be the origin of cycles identified in the North Atlantic region. To address these issues we have developed an annually resolved record of changes in Irish bog tree populations over the last 7468 years which, together with radiocarbon‐dated bog and lake‐edge populations, extend the dataset back to ～9000 yr ago. The Irish trees underpin the internationally accepted radiocarbon calibration curve, used to derive a proxy of solar activity, and allow us to test solar forcing of Holocene climate change. Tree populations and age structures provide unambiguous evidence of major shifts in Holocene surface moisture, with a dominant cyclicity of 800 yr, similar to marine cycles in the North Atlantic, indicating significant changes in the latitude and intensity of zonal atmospheric circulation across the region. The cycles, however, are not coherent with changes in solar activity (both being on the same absolute timescale), indicating that Holocene North Atlantic climate variability at the millennial and centennial scale is not driven by a linear response to changes in solar activity. Copyright © 2005 John Wiley & Sons, Ltd.     

The dark side of Iapetus

This paper presents new photometric and spectrophotometric observations of the dark (leading) hemisphere of Saturn's satellite Iapetus. Spectrophotometry from 0.3–1.0 um (May 1979) shows the dark hemisphere to be very red, similar to a few asteroids and the Moon, but with no spectral features attributable to olivine or pyroxene. Near-infrared spectrophometry in the regions 1.4–2.5 um (May 1981) and 3.0–3.8 um (February 1981) reveals water ice absorption bands, probably resulting from the polar caps intruding onto the dark hemisphere. The reflectance of Iapetus is unlike that of carbonaceous chondrites or C-type asteroids and most closely resembles the reflectance (and low albedo) of carbonaceous (organic) residue from the Murchison C2 carbonaceous chondrite. The Murchison material has the same red slope and a probable spectral feature near 0.6 um seen in Iapetus data. Three hypotheses for the formation of the dark hemisphere are discussed in light of the observational data. The favored hypothesis is that debris from Phoebe or other unknown outer satellites of Saturn impacts the dark hemisphere of Iapetus as Poynting-Robertson drag causes the debris to spiral toward Saturn. The high-velocity impacts preferentially remove ice from the satellite's surface, causing enrichment of included carbonaceous material intrinsic to Iapetus. The reflectance of Phoebe itself is significantly different from that of Iapetus, suggesting that relatively little Phoebe debris lies on the dark hemisphere. There remains the possibility that the impacting debris originates from another body of composition similar to the Murchison residue and that this material is exposed on the surface of Iapetus.     

A comparison of the thick-target model with stereo data on the height structure of solar hard x-ray bursts

The height structure of a thick-target solar hard X-ray source is predicted for a beam injected vertically downward with a power-law spectrum and dominated by Coulomb collisional energy losses. This structure is characterised by the ratio of hard X-ray flux from an upper part of the source to that from the entire source, and is essentially a function only of the atmospheric column density ΔN (cm^?2) in the upper region. These predictions are compared with the flux ratios at 150 keV and 350 keV observed by two spacecraft for five events in which the solar limb occults part of the source for one spacecraft. In three events the occulting levels h ranged from 0 to 2500 km. For these the theoretical and observed ratios are found to be comparable for values of ΔN in reasonable accord with those found at these altitudes by optical and UV spectroscopic modelling of flare chromospheres. In one event the occultation ratio was observed to rise after the burst peak and it is found that this rise is consistent with an increase in ΔN due to conductively driven chromospheric evaporation. However the energy dependence of the occultation ratio is not consistent with that predicted by the model and it is concluded that non-collisional losses must be significant in beam dynamics. In the other two events, the occultation level h was ? 25 000 km. For these the energy dependence of the occultation ratio is comparable with the model predictions. However the values of ΔN required demand extremely high coronal densities and/or acceleration altitudes. Furthermore, the one observed evolution of the occultation ratio is entirely inconsistent with the model. It is concluded that in these, bremsstrahlung emissions other than that from a beam must be important.     

Effects of grazing by Daphnia on algal carotenoids: Implications for paleolimnology

Reverse-phase thin layer chromatography was used to quantify algal carotenoid degradation resulting from grazing by Daphnia magna in suspensions of lyophilized Oscillatoria utermöhlii, O. limnetica, Anabaena flos-aquae and Synechococcus sp. Three samples were removed every 24 h from 0 h to 144 h and the carotenoid, carbon and nitrogen content of the remaining algal-fecal mixture was determined. Grazing by Daphnia reduced carotenoid concentrations by 30%–80% after 144 h, with myxoxanthophyll being the most labile pigment in experiments with filamentous cyanophytes. However, as a group, the carotenoids were more resistant to the combined effects of grazing by Daphnia and bacterial action than were particulate algal carbon (78% to 82% lost by 144 h) or nitrogen (86%–90%) during the period in which easily digestible material remained. The maximal extent of carotenoid enrichment relative to particulate carbon ranged from 150% to 300% of initial pigment concentrations but declined with repeated coprophagy. On the basis of these results and published pigment budgets, we conclude both that fecal transportation of pigments may be an important determinant of carotenoid accumulation rates and that carotenoid stratigraphies should record predator-mediated changes in zooplankton community structure, especially in oligotrophic conditions.     

The change in Sq(H) amplitude on Abnormal Quiet Days

Summary. Analysis of geomagnetic data has shown that the superposed northward magnetic field, which reduces the S _q( H ) amplitude at northern mid-latitude stations on Abnormal Quiet Days, and increases the amplitude at stations on the equatorward side of the S _q focus, builds up in amplitude over four to five days before the AQD occurs, and subsides over a similar period after the AQD. It is inferred indirectly that the azimuthal component B_y of the interplanetary magnetic field varies similarly. Data for the opposite meridian show that the imposed field reverses to a southward direction at lower latitudes. The inferred currents to account for these fields are believed to flow in the ionosphere, but to arise from magnetospheric electric fields induced by the solar wind-transported IMF.     

Models of radar imaging of the ocean surface waves

A number of models which would explain ocean wave imagery taken with a synthetic aperture imaging radar are analyzed analytically and numerically. Actual radar imagery is used to support some conclusions. The models considered correspond to three sources of radar backscatter cross section modulation: tilt modulation, roughness variation, and the wave orbital velocity. The effect of the temporal changes of the surface structure, parametric interactions, and the resulting distortions are discussed.