Evolution of cold dry snow and firn plays important roles in glaciology; however, the physical formulation of a densification law is still an active research topic. We forced eight firn-densification models and one seasonal-snow model in six different experiments by imposing step changes in temperature and accumulation-rate boundary conditions; all of the boundary conditions were chosen to simulate firn densification in cold, dry environments. While the intended application of the participating models varies, they are describing the same physical system and should in principle yield the same solutions. The firn models all produce plausible depth-density profiles, but the model outputs in both steady state and transient modes differ for quantities that are of interest in ice core and altimetry research. These differences demonstrate that firn-densification models are incorrectly or incompletely representing physical processes. We quantitatively characterize the differences among the results from the various models. For example, we find depth-integrated porosity is unlikely to be inferred with confidence from a firn model to better than 2 m in steady state at a specific site with known accumulation rate and temperature. Firn Model Intercomparison Experiment can provide a benchmark of results for future models, provide a basis to quantify model uncertainties and guide future directions of firn-densification modeling.

The resonant interaction of a narrow electron beam spiraling in a cold homogeneous magnetized plasma with electromagnetic cylindrical waves is considered. The conclusions derived in recent papers about the formation, in the presence of effective electromagnetic energy dissipation out of the radially bounded beam, of dynamically stable and longtime living bunches of resonant electrons in the developed stage of their nonlinear interaction with the waves are discussed in this paper for more general physical cases and for wider ranges of characteristic parameters. General analytical formulas for the excited electromagnetic fields, the radiated electromagnetic power flux and the linear growth rate as well as relevant numerical simulations results are presented.

A modified electron whistler dispersion law is derived in the cold-plasma approximation for analytical treatment and simplified numerical calculations of wave propagation in a wide range of ratios ωc/ωp of electron gyro- to plasma frequencies if the wave frequency is much less than ωp. The net contribution of ions to the wave dispersion law is expressed through the value of the lower-hybrid resonance frequency ωlhr only. This approximate dispersion law is valid in a wide frequency domain, that is, from the range of ωlhr until the domain where the contribution of ions can be neglected. A comparison of geometrical-optics ray trajectories calculated by the use of modified and total cold-plasma electron whistler dispersion laws is presented for the case of the Earth's plasma environment. Computer simulations of dynamical spectra of whistler waves excited by lightning discharges and registered in remote regions of the Earth's plasmasphere reveal good numerical stability of the developed ray-tracing code.

Undoped and Si-doped GaN films were grown by low pressure MOCVD on (0001) sapphire substrates. The angular distribution of the X-ray diffraction corresponding to the (0002), (0004), (100), (200), and (114) reflections has been measured by means of double- and triple -crystal diffractometry with Mo Kα1 and Cu Kα1 radiation under conditions of symmetrical and asymmetrical Bragg- and Laue-geometry. In our experiments a non-coplanar geometry was also applied. On the basis of the performed studies, five independent components of the tensor of microdistortion were evaluated and the average grain-size in two directions was determined. The type, position, and density of dislocations were established as well. The role of dislocations in strain relaxation and their influence on the optical and electrical properties are discussed.

In this study, both single undoped GaN epilayers and GaN-based device structures was treated by electrochemical etching in the dilute water solution of KOH or NaOH. Our investigations showed that in the undoped GaN epilayers grown by MOCVD the electrical and optical properties were nonuniform in depth. In this case, high defective and high conductive sublayer adjacent to the substrate was revealed by the electrochemical etching. This high conductive region was proved to condition the results of Hall effect measurements. Electrolyte etching of i-n GaN-based device structures grown by HVPE gave rise to significant increasing of the electroluminescence intensity. Influence of electrochemical etching on luminescence properties of the device structure is discussed.

We have measured the thermal conductivity λ and the compressibility of highly pure C70 in the range 90 to 450 K under pressures up to 1.2 GPa. Our results for the thermal conductivity indicate molecular rotation in C70 above 280 K at zero pressure. The phase boundary for the rotationally disordered phase has an approximate slope dT/dp = 75 K GPa−1. The bulk modulus B increases linearly with increasing p above 0.1 GPa, with an extrapolated zero pressure value at 296 K of B(0) = 8.3 GPa. Unexpected anomalies are found in both B and λ near 100 MPa, and we tentatively suggest that orientational ordering is responsible.