75 research outputs found
The importance of the weak: Interaction modifiers in artificial spin ices
The modification of geometry and interactions in two-dimensional magnetic
nanosystems has enabled a range of studies addressing the magnetic order,
collective low-energy dynamics, and emergent magnetic properties, in e.g.
artificial spin ice structures. The common denominator of all these
investigations is the use of Ising-like mesospins as building blocks, in the
form of elongated magnetic islands. Here we introduce a new approach: single
interaction modifiers, using slave-mesospins in the form of discs, within which
the mesospin is free to rotate in the disc plane. We show that by placing these
on the vertices of square artificial spin ice arrays and varying their
diameter, it is possible to tailor the strength and the ratio of the
interaction energies. We demonstrate the existence of degenerate ice-rule
obeying states in square artificial spin ice structures, enabling the
exploration of thermal dynamics in a spin liquid manifold. Furthermore, we even
observe the emergence of flux lattices on larger length-scales, when the energy
landscape of the vertices is reversed. The work highlights the potential of a
design strategy for two-dimensional magnetic nano-architectures, through which
mixed dimensionality of mesospins can be used to promote thermally emergent
mesoscale magnetic states.Comment: 17 pages, including methods, 4 figures. Supplementary information
contains 16 pages and 15 figure
Insulation effects of Icelandic dust and volcanic ash on snow and ice
In the Arctic region, Iceland is an important source of dust due to ash production from volcanic eruptions. In addition, dust is resuspended from the surface into the atmosphere as several dust storms occur each year. During volcanic eruptions and dust storms, material is deposited on the glaciers where it influences their energy balance. The effects of deposited volcanic ash on ice and snow melt were examined using laboratory and outdoor experiments. These experiments were made during the snow melt period using two different ash grain sizes (1 phi and 3.5 phi) from the Eyjafjallajokull 2010 eruption, collected on the glacier. Different amounts of ash were deposited on snow or ice, after which the snow properties and melt were measured. The results show that a thin ash layer increases the snow and ice melt but an ash layer exceeding a certain critical thickness caused insulation. Ash with 1 phi in grain size insulated the ice below at a thickness of 9-15 mm. For the 3.5 phi grain size, the insulation thickness is 13 mm. The maximum melt occurred at a thickness of 1 mm for the 1 phi and only 1-2 mm for 3.5 phi ash. A map of dust concentrations on Vatnajokull that represents the dust deposition during the summer of 2013 is presented with concentrations ranging from 0.2 up to 16.6 g m(-2).Peer reviewe
Icelandic Inland Wetlands: Characteristics and Extent of Draining
Iceland has inland wetland areas with soils exhibiting both Andosol and Histosol properties which are uncommon elsewhere on Earth. They are generally fertile, with higher bird-nest densities than in similar wetlands in the neighboring countries, with nutrients released by rapid weathering of aeolian materials of basaltic nature. Icelandic inland wetlands cover about 9000 km2 constituting 19.4 % of the vegetated surfaces of the island. The wetland soils are often 1–3 m thick and store 33 to >100 kg C m−2. They have been subjected to broad-scale subsidy-driven draining for agricultural purposes. About 47 % of Icelandic inland wetlands are impacted by drainage. The ditch network extends about 30,000 km, mainly in lowland areas, where about 70 % of the wetland areas are impacted. There are >1 million wetland patches, most of them <1 ha. Much of the wetlands impacted from drainage are not used for intensive agriculture such as haymaking, however some are used for grazing. There is a need to prioritize the protection of undrained wetlands and their restoration based on a broad range of factors.Peer Reviewe
Iceland is an episodic source of atmospheric ice-nucleating particles relevant for mixed-phase clouds.
Ice-nucleating particles (INPs) have the potential to remove much of the liquid water in climatically important mid- to high-latitude shallow supercooled clouds, markedly reducing their albedo. The INP sources at these latitudes are very poorly defined, but it is known that there are substantial dust sources across the high latitudes, such as Iceland. Here, we show that Icelandic dust emissions are sporadically an important source of INPs at mid to high latitudes by combining ice-nucleating active site density measurements of aircraft-collected Icelandic dust samples with a global aerosol model. Because Iceland is only one of many high-latitude dust sources, we anticipate that the combined effect of all these sources may strongly contribute to the INP population in the mid- and high-latitude northern hemisphere. This is important because these emissions are directly relevant for the cloud-phase climate feedback and because high-latitude dust emissions are expected to increase in a warmer climate
Thermodynamics of elementary excitations in artificial magnetic square ice
We investigate the thermodynamics of artificial square spin ice systems
assuming only dipolar interactions among the islands that compose the array.
The emphasis is given on the effects of the temperature on the elementary
excitations (magnetic monopoles and their Dirac strings). By using Monte Carlo
techniques we calculate the specific heat, the density of poles and their
average separation as functions of temperature. The specific heat and average
separation between monopoles and antimonopoles exhibit a sharp peak and a local
maximum, respectively, at the same temperature,
(here, is the strength of the dipolar interaction and is the
Boltzmann constant). As the lattice size is increased, the amplitude of these
features also increases but very slowly. Really, the specific heat and the
maximum in the average separation between oppositely charged
monopoles increase logarithmically with the system size, indicating that
completely isolated charges could be found only at the thermodynamic limit. In
general, the results obtained here suggest that, for temperatures , these systems may exhibit a phase with separated monopoles, although
the quantity should not be larger than a few lattice spacings for
viable artificial materials.Comment: 7 pages, 9 figure
Complex refractive index and single scattering albedo of Icelandic dust in the shortwave part of the spectrum
Icelandic dust can impact the radiative budget in high-latitude regions directly by affecting light absorption and scattering and indirectly by
changing the surface albedo after dust deposition. This tends to produce a positive radiative forcing. However, the limited knowledge of the
spectral optical properties of Icelandic dust prevents an accurate assessment of these radiative effects. Here, the spectral single scattering
albedo (SSA) and the complex refractive index (m=n-ik) of Icelandic dust from five major emission hotspots were retrieved between
370–950 nm using online measurements of size distribution and spectral absorption (βabs) and scattering (βsca)
coefficients of particles suspended in a large-scale atmospheric simulation chamber. The SSA(λ) estimated from the measured
βabs and βsca increased from 0.90–0.94 at 370 nm to 0.94–0.96 at 950 nm in Icelandic dust from the
different hotspots, which falls within the range of mineral dust from northern Africa and eastern Asia. The spectral complex refractive index was
retrieved by minimizing the differences between the measured βabs and βsca and those computed using the Mie theory for
spherical and internally homogeneous particles, using the size distribution data as input. The real part of the complex refractive
index (n(λ)) was found to be 1.60–1.61 in the different samples and be independent of wavelength. The imaginary part (k(λ)) was
almost constant with wavelength and was found to be around 0.004 at 370 nm and 0.002–0.003 at 950 nm. The estimated complex
refractive index was close to the initial estimates based on the mineralogical composition, also suggesting that the high magnetite content observed
in Icelandic dust may contribute to its high absorption capacity in the shortwave part of the spectrum. The k(λ) values retrieved for Icelandic dust
are at the upper end of the reported range for low-latitude dust (e.g., from the Sahel). Furthermore, Icelandic dust tends to be more absorbing
towards the near-infrared. In Icelandic dust, k(λ) between 660–950 nm was 2–8 times higher than most of the dust samples sourced
in northern Africa and eastern Asia. This suggests that Icelandic dust may have a stronger positive direct radiative forcing on climate that has
not been accounted for in climate predictions.</p
Modeling Historic Rangeland Management and Grazing Pressures in Landscapes of Settlement
Defining historic grazing pressures and rangeland management is vital if early landscape threshold crossing and long–term trajectories of landscape change are to be properly understood. In this paper we use a new environmental simulation model, Búmodel, to assess two contrasting historical grazing landscapes in Mývatnssveit Iceland for two key periods—the colonization period (ca. Landnám, A.D. 872–1000) and the early eighteenth century A.D. Results suggest that there were spatial and temporal variations in productivity and grazing pressure within and between historic grazing areas and indicate that land degradation was not an inevitable consequence of the livestock grazing introduced with settlement. The results also demonstrate the significance of grazing and livestock management strategies in preventing overgrazing, particularly under cooler climatic conditions. The model enables detailed consideration of historic grazing management scenarios and their associated landscape pressures
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