482 research outputs found
On the Asymmetry Between Upward and Downward Field-Aligned Currents Interacting With the Ionosphere
The paper presents results from the numerical study of the magnetosphere-ionosphere interactions driven by the large-scale electric field in the magnetically conjugate, high-latitude regions of northern and southern hemispheres. Simulations of the two-fluid MHD model demonstrate that these interactions can lead to a generation of a system of small-scale, intense field-aligned currents with a significant difference in size and amplitude between the upward and downward currents. In particular, in both hemispheres, the downward currents (where the electrons are flowing from the ionosphere) become more narrow and intense than the adjacent upward currents. At high latitudes, the field-aligned currents are closely related to the discrete auroral arcs. The fact that this mechanism produces very narrow and intense downward currents embedded into the broader upward current regions makes it relevant to the explanation of the “black” auroral arcs appearing as narrow, dark strips embedded in the broad luminous background
Orbital structure and magnetic ordering in stoichiometric and doped crednerite CuMnO2
The exchange interactions and magnetic structure in layered system CuMnO2
(mineral crednerite) and in nonstoichiometric system Cu1.04Mn0.96O2, with
triangular layers distorted due to orbital ordering of the Mn3+ ions, are
studied by ab-initio band-structure calculations, which were performed within
the GGA+U approximation. The exchange interaction parameters for the Heisenberg
model within the Mn-planes and between the Mn-planes were estimated. We explain
the observed in-plane magnetic structure by the dominant mechanism of the
direct d-d exchange between neighboring Mn ions. The superexchange via O ions,
with 90 degree Mn-O-Mn bonds, plays less important role for the in-plane
exchange. The interlayer coupling is largely dominated by one exchange path
between the half-filled 3z^2-r^2 orbitals of Mn3+. The change of interlayer
coupling from antiferromagnetic in pure CuMnO2 to ferromagnetic in doped
material is also explained by our calculations
Realization of anisotropic compass model on the diamond lattice of Cu in CuAlO
Spin-orbit (SO) Mott insulators are regarded as a new paradigm of magnetic
materials, whose properties are largely influenced by SO coupling and featured
by highly anisotropic bond-dependent exchange interactions between the
spin-orbital entangled Kramers doublets, as typically manifested in
iridates. Here, we propose that a very similar situation can be realized in
cuprates when the Cu ions reside in a tetrahedral environment, like in
spinel compounds. Using first-principles electronic structure calculations, we
construct a realistic model for the diamond lattice of the Cu ions in
CuAlO and show that the magnetic properties of this compound are
largely controlled by anisotropic compass-type exchange interactions that
dramatically modify the magnetic ground state by lifting the spiral spin-liquid
degeneracy and stabilizing a commensurate single- spiral
Effects of the Hall Conductivity in Ionospheric Heating Experiments
We investigate the role of Hall conductivity in ionospheric heating experiments. Ionosphericheating by powerful X-mode waves changes the Hall and Pedersen conductances in theEandDregions,which lead to the generation of ultra-low frequency (ULF)/extremely-low frequency/very low frequencywaves, when the electric field exists in the ionosphere. The importance of the Hall currents in themagnetosphere-ionosphere interactions, carried by ULF waves and field-aligned currents, has beenconsistently overlooked in studies devoted tothe active experiments. Simulations of the three-dimensionaltwo-fluid magnetohydrodynamic (MHD) model, presented in this paper, demonstrate that the Hallconductivity changes (1) the growth rate and the amplitude of ULF waves generated by the heating and (2)the orientation and the direction of propagation of the generated waves. These findings provide insight inthe experiments where the waves were generated with a geometric modulation technique and suggest anew and more efficient approach for conducting such experiments in the future
On the Propagation of Whistler-Mode Waves in the 2 Magnetic Ducts
This paper studies extremely-low frequency (ELF) whistler-mode waves’ behavior within small-scale magnetic field irregularities in the Earth’s magnetosphere, known as magnetic ducts. Based on the magnetic fields’ magnitude inside and outside these ducts, they are categorized as high-magnetic ducts (HBD) and low-magnetic ducts (LBD). Using the whistler-mode dispersion relation analysis, our primary focus is to show that LBDs are prone to leak electromagnetic energy outside the duct. We further investigate the hypothesis that whistlers can propagate within LBDs without any signal loss when the width of the duct corresponds to an integer multiple of the perpendicular wavelengths of the waves inside it. This condition offers a straightforward and effective method for identifying non-leaking eigenmodes of LBDs. Our analysis of this non-leaking condition reveals that every LBD possesses a finite number of non-leaking eigenmodes directly proportional to the duct’s width and the magnitude of the ambient magnetic field within it. The analytical results are then validated using two-dimensional, time-dependent simulations of the electron-Magnetohydrodynamics (EMHD) model. Also, we model the non-leaking propagation of an ELF whistler-mode wave observed inside the LBD by the NASA Magnetospheric Multiscale mission (MMS) satellite
Excitation of zero-frequency magnetic field-aligned currents by ionospheric heating
Time-dependent, three-dimensional numerical simulations of the reduced MHD model describing shear Alfve ́n waves in the magnetosphere provide an interesting prediction superficially similar to results of several iono- spheric heating experiments conducted at high altitudes. In these experiments, heating of the ionospheric F-region with a constant/zero-frequency beam of HF waves causes luminous structures in the ionosphere in the form of a ring or a solid spot with a characteristic size comparable to the size of the heated spot. Simulations suggest that spots/rings or similar optical appearance might be associated with a magnetic field- aligned current system produced by the ionospheric heat- ing. Two of the most interesting features of this current system are (1) strong localization across the ambient mag- netic field and (2) distinctive non-symmetrical luminous sig- natures (ring/spot) in magnetically conjugate locations in the ionosphere
Whistler-Mode Waves in Magnetic Ducts
Observations from the NASA MMS satellites show packages of ELF whistler-mode waves localized inside the small-scale irregularities of the magnetic field. These irregularities are formed by the narrow field-aligned channels where the magnitude of the background magnetic field inside the channel is greater or less than outside. By analogy with the classical density ducts, we introduce the high-B duct (HBD), where the magnitude of the field inside the channel is greater than the outside, and the low-B duct (LBD), where the magnitude of the field inside the channel is less than the outside. We investigate the guiding of the ELF whistler-mode waves by high-B and low-B ducts. We derive the analytical criteria for the wave ducting by these ducts and confirm them with two-dimension, time-dependent simulations of the electron-MHD model. Also, we model ELF whistler mode waves observed inside the high-B and low-B ducts by MMS satellites
Role of local geometry in spin and orbital structure of transition metal compounds
We analyze the role of local geometry in the spin and orbital interaction in
transition metal compounds with orbital degeneracy. We stress that the tendency
observed for the most studied case (transition metals in O octahedra with
one common oxygen -- common corner of neighboring octahedra and with metal--oxygen--metal bonds), that ferro-orbital ordering renders
antiferro-spin coupling, and, {\it vice versa}, antiferro-orbitals give
ferro-spin ordering, is not valid in general case, in particular for octahedra
with common edge and with M--O--M bonds. Special attention is
paid to the ``third case'', neighboring octahedra with common face (three
common oxygens) -- the case practically not considered until now, although
there are many real systems with this geometry. Interestingly enough, the
spin--orbital exchange in this case turns out to be to be simpler and more
symmetric than in the first two cases. We also consider, which form the
effective exchange takes for different geometries in case of strong spin--orbit
coupling.Comment: 31 pages, 9 figures, submitted to JET
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