Oscillations of coronal electron traps inferred from hard X-ray data

The work of Brown and Hoyng (1975) on the betatron acceleration of hard X-ray source electrons trapped in a vibrating flux tube is generalised to include Fermi acceleration by the varying transverse field. This development can explain the trajectory of bursts in a plot of equivalent thick-target electron flux versus spectral index as inferred from observations obtained by ESRO TD1A. Specifically the loops observed in this , diagram, unexplained in Brown and Hoyng's anslysis are accounted for by a changing phase relationship between the varying field strength f(t) and magnetic scale length g(t)- Application of the formalism to the detailed TD1A observations of the large events of 1972, August 4 and 7, allows inference of the evolution of f(t), g(t) in these events.Now at: Space Sciences Laboratory, University of California, Berkeley, U.S.A.     

Anomalous lithospheric structure of Northern Juan de Fuca plate — a consequence of oceanic rift propagation?

Anomalous crustal and upper mantle structure of northern Juan de Fuca plate is revealed from wide-angle seismic and gravity modelling. A 2-D velocity model is produced for refraction line II of the 1980 Vancouver Island Seismic Project (VISP80). The refraction data were recorded on three ocean bottom seismometers (OBSs) deployed at the ends and middle of a 110 km line oriented parallel to the North American continental margin. The velocity model is constructed via ray tracing and conforms to first-arrival amplitude observations and travel time picks of direct, converted and reflected phases. Between sub-sediment depths of 3 to 11 km, depths normally associated with the lower crust and upper oceanic mantle, the final model shows that compressional-wave velocities decrease significantly from southeast to northwest along the profile. At sub-sediment depths of 11 km at the northwestern end of the profile, P-wave velocities are as low as 7.2 km/s. A complementary 2-D gravity model using the geometry of the velocity model and velocity–density relationships characteristic of oceanic crust is produced. The high densities required to match the gravity field indicate the presence of peridotites containing 25–30% serpentine by volume, rather than excess gabbroic crust, within the deep low velocity zone. Anomalous travel time delays and unusual reflection characteristics observed from proximal seismic refraction and reflection experiments suggest a broader zone of partially serpentinized peridotites coincident with the trace of a pseudofault. We propose that partial serpentinization of the upper mantle is a consequence of slow spreading at the tip of a propagating rift.     

Hydrogeology of a montane headwater groundwater system downgradient of a coal-mine waste rock dump: Elk Valley,British Columbia,Canada

Steelmaking-coal waste rock placed in mountain catchments in the Elk Valley, British Columbia, Canada, drain constituents of interest (CIs) to surface water downgradient of the waste rock dumps. The role of groundwater in transporting CIs in the headwaters of mountain catchments is not well understood. This study characterizes the physical hydrogeology of a portion of a 10-km² headwater catchment (West Line Creek) downgradient of a 2.7-km² waste rock dump placed over a natural headwater valley-bottom groundwater system. The study site was instrumented with 13 monitoring wells. Drill core samples were collected to determine subsurface lithology and geotechnical properties. The groundwater system was characterized using field testing and water-level monitoring. The valley-bottom sediments were composed of unconsolidated glacial and meltwater successions (<64 m thick) deposited as a series of cut and fill structures overlying shale bedrock. An unconfined basal alluvial aquifer located above fractured bedrock was identified as the primary conduit for groundwater flow toward Line Creek (650 m from the toe of the dump). Discharge through the basal alluvial aquifer was estimated using the geometric mean hydraulic conductivity (±1 standard deviation). These calculations suggest groundwater discharge could account for approximately 15% (ranging from 2 to 60%) of the total water discharged from the watershed. The residence time from the base of the waste rock dump to Line Creek was estimated at <3 years. The groundwater system was defined as a snowmelt (i.e., nival) regime dominated by direct recharge (percolation of precipitation) across the catchment.     

The resistances of the photosphere and of a flaring coronal loop

Solar Physics - We consider two aspects of solar flares from the point of view of circuit theory. First, we show that the so-called “dynamo models”, which invoke an analogy between the...     

Mass motions in a heated flare filament

Energy transport in a hot flare plasma is examined with particular reference to the influence of fluid motion. On the basis of dimensional considerations the dynamical timescale of the flare plasma is shown to be comparable to the timescale for energy loss by conduction and radiation. It is argued that mass motion is likely to have a profound influence on the evolution of the flare.The detailed response of a flare filament to a localized injection of energy is then analyzed. Radiative, conductive and all dynamical terms are included in the energy equation. Apart from greatly enhancing the rate of propagation of the thermal disturbance through space, mass motion is found to be significant in transferring energy through the moving fluid.Finally the predicted thermal structure is discussed and it is concluded that the presence of mass motions in the flare may be inferred from the form of the soft X-ray differential emission measure.     

Rapid sunspot motion during a major solar flare

A major solar flare on 15 November, 1991 produced a striking perturbation in the position and shape of the sunspot related most closely to the flare. We have studied these perturbations by use of the aspect-sensor images from the Soft X-ray Telescope on board YOHKOH, and with ground-based data from the Mees Solar Observatory. The perturbation occurred during the impulsive phase of the flare, with a total displacement on the order of 1 arc sec. The apparent velocity of approximately 2 km s^–1 exceeds that typically reported for sunspot proper motions even in flare events. We estimate that the magnetic energy involved in displacing the sunspot amounted to less than 4 × 10³⁰ ergs, comparable to the radiant energy from the perturbed region. Examination of the Mees Observatory data shows that the spot continued moving at lower speed for a half-hour after the impulsive phase. The spot perturbation appears to have been a result of the coronal restructuring and flare energy release, rather than its cause.     

Unravelling the evolution of an Alpine to post-glacially active fault in the Swiss Alps

The Gemmi fault is a prominent NW–SE striking lineament that crosses the Gemmi Pass in the central Swiss Alps. A multidisciplinary investigation of this structure that included geological mapping, joint profiling, cathodoluminescence and scanning electron microscopy, stable isotope measurements, luminescence- and U-TH-dating, 3D ground penetrating radar (GPR) surveying and trenching reveals a history of fault movements from the Miocene to the Holocene. The main fault zone comprises a 0.5–3 m thick calcite cataclasite formed during several cycles of veining and brittle deformation. Displaced Cretaceous rock layers show an apparent dextral slip of 10 m along the fault.A detailed study of a small sediment-filled depression that crosses the fault provides evidence for a post-glacial reactivation of the fault. A trench excavated across the fault exposed a Late-Glacial-age loess layer and late Holocene colluvial-like slope-wash deposits that showed evidence for fault displacement of a few centimeters, indicating a recent strike-slip reactivation of the fault. Focal mechanisms of recent instrumentally recorded earthquakes are consistent with our findings that show that the fault at the Gemmi Pass, together with other parallel faults in this area, may be reactivated in today's stress field. Taking together all the observations of its ancient and recent activity, the Gemmi fault can be viewed as a window through geological space and time.     

Assessing the contribution of off‐fault deformation to slip‐rate estimates within the Taupo Rift,New Zealand,using 3‐D ground‐penetrating radar surveying and trenching

We demonstrate that conventional palaeoseismic trenching and mapping techniques that do not account for the effects of off‐fault deformation can significantly underestimate a fault’s slip rate. Using combined interpretations of 3‐D ground‐penetrating radar (GPR) and palaeoseismic trench data, we show that drag folding and hangingwall and footwall horizontal‐axis rotations have accommodated up to 41% of total extension across a normal fault within the Taupo Rift, New Zealand, over the past 24.6 ± 1.0 cal. ka BP. Our results may explain why geologically determined fault‐slip rates for the central and southern Taupo Rift are anomalously low when compared with geodetic estimates. We suggest that a combination of GPR surveying and palaeoseismic trenching may help resolve differences between geodetically and geologically determined strain rates observed across active extensional regimes worldwide.     

The height distribution of flare hard X-rays in thick and thin target models

The current controversy between thick and thin target models of hard X-ray flares is reviewed and it is concluded that the most promising method of distinguishing them is in terms of the differences in the distribution of the source with height in the solar atmosphere. Quantitative predictions are made of this height distribution for both models and the results discussed in relation to observations of hard X-ray emission from flares behind the limb. It is concluded that the thick target model is as compatible with such events as the thin target whereas the latter is in general much less satisfactory in terms of energy requirements and of flare observations at other wavelengths. Other source models are also briefly considered.