23 research outputs found
ESR study of the spin ladder with uniform Dzyaloshinskii-Moria interaction
Evolution of the ESR absorption in a strong-leg spin ladder magnet
(CHN)CuBr (abbreviated as DIMPY) is studied from 300K to
400mK. Temperature dependence of the ESR relaxation follows a staircase of
crossovers between different relaxation regimes. We ague that the main
mechanism of ESR line broadening in DIMPY is uniform Dzyaloshinskii-Moria
interaction (K) with an effective longitudinal component along
an exchange bond of Cu ions within the legs resulting from the low crystal
symmetry of DIMPY and nontrivial orbital ordering. The same
Dzyaloshinskii-Moriya interaction results in the lifting of the triplet
excitation degeneracy, revealed through the weak splitting of the ESR
absorption at low temperatures.Comment: 13 pages, submitted to PRB, Fig.3 update
Public political narratives: developing a neglected source through the exploratory case of Russia in the Putin-Medvedev era
The place of narrative in political science is an issue that resurfaces with regularity, usually focusing on the questions of generalisability, evidence and causality which lie at the heart of the discipline. Most such debate concerns the use of narrative by political scientists. Far less attention has been devoted to the use of narrative by political actors, despite its relative ubiquity. Even where such attention has been given, it concentrates less on the narrative per se, and more on its performance and impact. However, the nature of public political narratives means that analysis of them facilitates a holistic understanding of their narrators' politics. A public political narrative consists of a sequential account given by political actors connecting selected, specific developments so as to impose a desired order on them. Taking contemporary Russia as an exploratory case, narrative analysis draws out the motivations, world view and inconsistencies within the Putin-Medvedev regime. Recurring motifs and symbols identify the regime's political priorities, explaining policy choices and revealing future concerns. Narrative has a predictive aspect, identifying likely policy responses to unexpected events. Narratives capture time, and shifts in their temporalities indicate changes in self-conceptualisation and political priorities. Temporal appropriations include or exclude particular agential and causal explanations. The relationship between their plots and subplots represents a political signalling process. Public political narratives provide temporally and spatially specific exceptionalist accounts, but their combinatory quality also facilitates comparative analysis. The approach essayed here provides methodological generalisability, arguing that the neglect of public political narratives merits correcting
Steep latitudinal gradients of thermospheric composition during magnetic storms: a possible formation mechanism
Mass spectrometer satellite observations show that a narrow region with steep latitudinal gradients of neutral composition is formed in the subauroral winter thermosphere during magnetic storms. In order to analyze the relative importance of individual terms in the continuity equation for atomic oxygen, a two-dimensional model was used to simulate the thermospheric disturbance formation in response to intense Joule heating imposed in the auroral oval. Such an approach allowed three characteristic zones to be distinguished in the high-latitude thermosphere at heights of about 250 km. It was shown that vertical transport is of greatest importance within the local heating region. Horizontal transport dominates at subauroral latitudes near the mid-night edge of the auroral oval. Propagation of the disturbances to middle latitudes is prohibited near the noon edge of the oval by a strong counteraction of a poleward meridional wind. Here is a "relaxation zone" defined as the region which is spread to the equator from the boundary between the local heating area and the subauroral zone in the noon sector LT. It is at this boundary that composition distributions with steep latitudinal gradient are formed within the first few hours of Joule heating source action. Perturbations transported to middle latitudes during the periods when the meridional wind is directed equatorward begin to relax in this zone with a characteristic time scale of about 7 h, independent of season. However, in winter, composition at subauroral latitudes recovers to unperturbed N<sub>2</sub>/O values before the wind again turns equatorward, giving rise to a distribution with steep latitudinal gradient recovering. In summer, a complete relaxation cannot be reached due to a shorter time interval with poleward wind and a larger disturbance amplitude. These two factors result in an effective smoothing of the initial steep gradient and a more regular latitudinal distribution of composition is observed in the summer thermosphere
Steep latitudinal gradients of thermospheric composition during magnetic storms: a possible formation mechanism
Mass spectrometer satellite observations show
that a narrow region with steep latitudinal gradients of neutral composition is
formed in the subauroral winter thermosphere during magnetic storms. In order to
analyze the relative importance of individual terms in the continuity equation
for atomic oxygen, a two-dimensional model was used to simulate the
thermospheric disturbance formation in response to intense Joule heating imposed
in the auroral oval. Such an approach allowed three characteristic zones to be
distinguished in the high-latitude thermosphere at heights of about 250 km. It
was shown that vertical transport is of greatest importance within the local
heating region. Horizontal transport dominates at subauroral latitudes near the
mid-night edge of the auroral oval. Propagation of the disturbances to middle
latitudes is prohibited near the noon edge of the oval by a strong counteraction
of a poleward meridional wind. Here is a "relaxation zone" defined as
the region which is spread to the equator from the boundary between the local
heating area and the subauroral zone in the noon sector LT. It is at this
boundary that composition distributions with steep latitudinal gradient are
formed within the first few hours of Joule heating source action. Perturbations
transported to middle latitudes during the periods when the meridional wind is
directed equatorward begin to relax in this zone with a characteristic time
scale of about 7 h, independent of season. However, in winter, composition at
subauroral latitudes recovers to unperturbed N2/O values before the
wind again turns equatorward, giving rise to a distribution with steep
latitudinal gradient recovering. In summer, a complete relaxation cannot be
reached due to a shorter time interval with poleward wind and a larger
disturbance amplitude. These two factors result in an effective smoothing of the
initial steep gradient and a more regular latitudinal distribution of
composition is observed in the summer thermosphere
Neutral gas composition changes and E×B vertical plasma drift contribution to the daytime equatorial F2-region storm effects
Theoretical model calculations along with
ground-based observations from Huancayo ionosonde station and ESRO-4 gas
analyzer data, were used to estimate the contribution of neutral gas composition
changes and E×B vertical plasma drift to the
observed F2-layer storm effects at the geomagnetic equator. Atomic oxygen
concentration increase may give the main contribution to the positive NmF2
effect when drift velocity changes are small, but negative storm effects, on the
other hand, are related mostly to vertical drift variations
Daytime F2-layer positive storm effect at middle and lower latitudes
Daytime F2-layer positive storm effects at
middle and lower latitudes in the winter thermosphere are analyzed using AE-C,
ESRO-4 neutral gas composition data, ground-based ionosonde observations and
model calculations. Different longitudinal sectors marked by the storm onset as
'night-time' and 'daytime' demonstrate different F2-layer positive storm
mechanisms. Neutral composition changes in the 'night-time' sector with
increased [O] and [N2] absolute concentrations, while (N2/O)storm/(N2/O)quiet\approx1
at F2-layer heights, are shown to contribute largely to the background NmF2
increase at lower latitudes lasting during daytime hours. Storm-induced surges
of the equatorward wind give rise to an additional NmF2
increase above this background level. The mid-latitude F2-layer positive storm
effect in the 'daytime' sector is due to the vertical plasma drift increase,
resulting from the interaction of background (poleward) and storm-induced (equatorward)
thermospheric winds, but not to changes of [O] and [N2]
concentrations
Daytime F2-layer positive storm effect at middle and lower latitudes
Daytime F2-layer positive storm effects at middle and lower latitudes in the winter thermosphere are analyzed using AE-C, ESRO-4 neutral gas composition data, ground-based ionosonde observations and model calculations. Different longitudinal sectors marked by the storm onset as 'night-time' and 'daytime' demonstrate different F2-layer positive storm mechanisms. Neutral composition changes in the 'night-time' sector with increased [O] and [N<sub>2</sub>] absolute concentrations, while (N<sub>2</sub>/O)<i><sub>storm</sub></i>/(N<sub>2</sub>/O)<i><sub>quiet</sub></i>approx1 at F2-layer heights, are shown to contribute largely to the background N<i><sub>m</sub></i>F2 increase at lower latitudes lasting during daytime hours. Storm-induced surges of the equatorward wind give rise to an additional N<i><sub>m</sub></i>F2 increase above this background level. The mid-latitude F2-layer positive storm effect in the 'daytime' sector is due to the vertical plasma drift increase, resulting from the interaction of background (poleward) and storm-induced (equatorward) thermospheric winds, but not to changes of [O] and [N<sub>2</sub>] concentrations
Monthly median f0F2 and M(3000)F2 ionospheric model over Europe
Monthly median f0F2 and M(3000)F2 ionospheric model, MQMF2, based on the multiquadric (MQ) method of spatial interpolation and a new ionospheric index MF2 describes the monthly median f0F2 and M(3000)F2 over Europe for any UT moment, month and level of solar activity. The multiquadric method allows a surface to be drawn strictly over a given set of points unlike many other currently used ionosphere mapping methods. A non-linear dependence of f0F2 and M(3000)F2 on solar activity level (expressed by MF2 index) is used to establish local models for each ionosonde station. Observations on 28 ionosondes for f0F2 and 19 for M(3000)F2 over 10-30 years were used for model derivation. The MQMF provides better accuracy than the CCIR model in retrospective mode over Europe. Long-term f0F2 prediction with the help of MF2 index for the rising part of solar cycle 22 is shown to provide better prediction accuracy than the CCIR model based on sunspot number R12. MQMF2 is implemented as a code for PC AT-386/387 or compatible, providing tables, plots and maps
ESR study of the spin ladder with uniform Dzyaloshinskii-Moriya interaction
©2015 American Physical Society. Evolution of the ESR absorption in a strong-leg spin ladder magnet (C7H10N)2CuBr4 (abbreviated as DIMPY) is studied from 300 K to 400 mK. Temperature dependence of the ESR relaxation follows a staircase of crossovers between different relaxation regimes. We argue that the main mechanism of ESR line broadening in DIMPY is uniform Dzyaloshinskii-Moriya interaction (|D|=0.31 K) with an effective longitudinal component along an exchange bond of Cu ions within the legs resulting from the low crystal symmetry of DIMPY and nontrivial orbital ordering. The same Dzyaloshinskii-Moriya interaction along with other weaker anisotropic spin-spin interactions results in the lifting of the triplet excitation degeneracy, revealed through the weak splitting of the ESR absorption at low temperatures