176 research outputs found
Direct and inverse cascades in the geodynamo
The rapid rotation of planets causes cyclonic thermal turbulence in their
cores which may generate the large-scale magnetic fields observed outside the
planets. We consider the model which enables us reproduce the typical features
of small-scale geostrophic flows in physical and wave spaces. We present
estimates of kinetic and magnetic energy fluxes as a function of the wave
number. The joint existence of forward and inverse cascades are demonstrated.
We also consider the mechanism of magnetic field saturation at the end of the
kinematic dynamo regime
Nonlinearity in a dynamo
Using a rotating flat layer heated from below as an example, we consider
effects which lead to stabilizing an exponentially growing magnetic field in
magnetostrophic convection in transition from the kinematic dynamo to the full
non-linear dynamo. We present estimates of the energy redistribution over the
spectrum and helicity quenching by the magnetic field. We also study the
alignment of the velocity and magnetic fields. These regimes are similar to
those in planetary dynamo simulations.Comment: Accepted to Geophys. Astrophys. Fluid Dyna
Historical geomagnetic observations from Prague observatory (since 1839) and their contribution to geomagnetic research
Shortly after the introduction of the physical unit for the magnetic field in 1832 and the invention of the bifilar apparatus in 1837, both being extraordinary scientific achievements that took place in Göttingen, the Clementinum observatory in Prague became one of the first places where systematic observations of the horizontal intensity of the geomagnetic field began. Karl Kreil was decisively responsible for this. In this paper, we focus on the very beginnings of geomagnetic observations in Prague, dating from the middle of 1839. We describe the archival materials with data that exist from that time, how the main instrument for observing magnetic storms – the bifilar magnetometer – worked and how it was calibrated, and the first magnetic survey in Bohemia. This study indicates the importance of historical geomagnetic observation materials to modern science, such as space weather research.</p
Using time- and temperature-dependent Preisach models to investigate the limitations of modelling isothermal remanent magnetization acquisition curves with cumulative log Gaussian functions
Effects of anisotropy in geostrophic turbulence
The Boussinesq model of convection in a flat layer with heating from below is
considered. We analyze the effects of anisotropy caused by rapid rotation in
physical and wave spaces and demonstrate the suppression of energy transfer by
rotation. We also examine the structure of the wave triangle in nonlinear
interaction. The range of parameters is adapted to the models of convection in
the geodynamo
Wybrane parametry zachorowalnoÅ›ci na nowotwory żeÅ„skich narzÄ…dów pÅ‚ciowych w regionie Polski poÅ‚udniowo-wschodniej w latach 1963-2003
Zachorowalność na nowotwory złośliwe okrężnicy w regionie Polski południowo-wschodniej w latach 1963-2005
Optically-triggered deterministic spiking regimes in nanostructure resonant tunnelling diode-photodetectors
This work reports a nanostructure resonant tunnelling diode-photodetector (RTD-PD) device and demonstrates its operation as a controllable, optically-triggered excitable spike generator. The top contact layer of the device is designed with a nanopillar structure (500 nm in diameter) to restrain the injection current, yielding therefore lower energy operation for spike generation. We demonstrate experimentally the deterministic optical triggering of controllable and repeatable neuron-like spike patterns in the nanostructure RTD-PDs. Moreover, we show the device's ability to deliver spiking responses when biased in either of the two regions adjacent to the negative differential conductance region, the so-called 'peak' and 'valley' points of the current–voltage (I–V) characteristic. This work also demonstrates experimentally key neuron-like dynamical features in the nanostructure RTD-PD, such as a well-defined threshold (in input optical intensity) for spike firing, as well as the presence of spike firing refractory time. The optoelectronic and chip-scale character of the proposed system together with the deterministic, repeatable and well controllable nature of the optically-elicited spiking responses render this nanostructure RTD-PD element as a highly promising solution for high-speed, energy-efficient optoelectronic artificial spiking neurons for novel light-enabled neuromorphic computing hardware
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