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Meteoric ablation in the Earth's atmosphere produces particles of nanometer size and larger. These particles can become charged and influence the charge balance in the D region (60–90 km) and the incoherent scatter observed with radar from there. Radar studies have shown that, if enough dust particles are charged, they can influence the received radar spectrum below 100 km, provided the electron density is sufficiently high (>109 m 3). Here, we study an observation made with the EISCAT VHF radar on 9 January 2014 during strong particle precipitation so that incoherent scatter was observed down to almost 60 km altitude. We found that the measured spectra were too narrow in comparison to the calculated spectra. Adjusting the collision frequency provided a better fit in the frequency range of ± 10–30 Hz. However, this did not lead to the best fit in all cases, especially not for the central part of the spectra in the narrow frequency range of ±10 Hz. By including a negatively charged dust component, we obtained a better fit for spectra observed at altitudes of 75–85 km, indicating that dust influences the incoherent-scatter spectrum at D-region altitudes. The observations at lower altitudes were limited by the small number of free electrons, and observations at higher altitudes were limited by the height resolution of the observations. Inferred dust number densities range from a few particles up to 104 cm -3 , and average sizes range from approximately 0.6 to 1 nm. We find an acceptable agreement with the dust profiles calculated with the WACCM-CARMA (Whole Atmosphere Community Climate Model-Community Aerosol Radiation Model for Atmospheres) model. However, these do not include charging, which is also based on models. [ABSTRACT FROM AUTHOR]

Using our experimental data for ns pulsed second harmonic generation (SHG) by periodically poled stoichiometric LiTaO3 (PPSLT) crystals, we consider in detail the mechanism underlying laser-induced damage in ferroelectric crystals. This mechanism involves generation and heating of free electrons, providing an effective kinetic pathway for electric breakdown and crystal damage in ns pulsed operation via combined two-photon absorption (TPA) and induced pyroelectric field. In particular, a temperature increase in the lattice of ≈1 K induced initially by ns SHG and TPA at the rear of operating PPSLT crystal is found to induce a gradient of spontaneous polarization generating a pyroelectric field of ≈10 kV/cm, accelerating free electrons generated by TPA to an energy of ≈10 eV, followed by impact ionization and crystal damage. Under the damage threshold for ns operation, the impact ionization does not lead to the avalanche-like increase of free electron density, in contrast to the case of shorter ps and fs pulses. However, the total number of collisions by free electrons, ≈1018 cm-3 (generated during the pulse and accelerated to the energy of ≈10 eV), can produce widespread structural defects, which by entrapping electrons dramatically increase linear absorption for both harmonics in subsequent pulses, creating a positive feedback for crystal lattice heating, pyroelectric field and crystal damage. Under pulse repetition, defect generation starting from the rear of the crystal can propagate towards its center and front side producing damage tracks along the laser beam and stopping SHG. Theoretical analysis leads to numerical estimates and analytical approximation for the threshold laser fluence for onset of this damage mechanism, which agree well with our (i) experiments for the input 1064 nm radiation in 6.8 kHz pulsed SHG by PPSLT crystal, (ii) pulsed low frequency 532 nm radiation transmission experiments, and also (iii) with the data published for other nonlinear crystals and operated wavelengths. [ABSTRACT FROM AUTHOR]

We present a detailed investigation of the formation of a secondary sodium layer at altitudes of 79-85 km below the main sodium layer based on sodium lidar and airglow imager measurements made at Ramfjordmoen near Tromsø, Norway on the night of 19 December 2014. The airglow imager observations of OH emission revealed four passing frontal systems that resembled mesospheric bores which typically occur in ducting regions of the upper mesosphere. For about 1.5 hours, the lower altitude sodium layer had densities similar to that of the main layer with a peak around 90 km. The lower altitude sodium layer weakened and disappeared soon after the fourth front had passed. The fourth front had weakened in intensity by the time it approached the region of lidar beams and disappeared soon afterwards. The column integrated sodium densities increased gradually during formation of the lower altitude sodium layer. Temperatures measured with the lidar indicate that there was 10 a strong thermal duct structure between 87 and 93 km. Furthermore, the temperature was enhanced below 85 km. Horizontal wind magnitudes estimated from the lidar showed strong wind shears above 93 km. We conclude that the combination of an enhanced stability region due to the temperature profile and intense wind shears have provided ideal conditions for evolution of multiple mesospheric bores revealed as frontal systems in OH images. The downward motion associated with the fronts appeared to have brought air rich in H and O from higher altitudes into the region below 85 km wherein the temperatures 15 were also relatively high. This would have liberated sodium atoms from the reservoir species and suppressed the re-conversion of atomic sodium into reservoir species so that the lower altitude sodium layer could form and the column abundance could increase. The presented observations also reveal the importance of mesospheric frontal systems in bringing about significant variation of minor species over shorter temporal intervals. [ABSTRACT FROM AUTHOR]

The results of coupled thermo-optical modeling of Tm,Ho solid-state laser operation are reported. A rate dynamics model integrated together with a TEM00 distribution for the total number of stimulated photons is coupled with a two-dimensional time-dependent heat transfer model. The heat transfer model includes absorption, heat release, and transfer inside the crystal as well as the thermal effect of the spontaneously emitted infrared radiation. In water cooled crystal operation this radiation is shown to be absorbed within the water boundary layer, producing significant inhibition of dissipation of the heat released inside the crystal. This effect leads to crystal superheating and significant inhibition of laser energy output. Absorption loss, in particular, due to water vapor present in the cavity is found to decrease significantly laser pulse energy. Numerical results are compared to previous (Tm,Ho:yttrium lithium fluoride) and new (Tm,Ho:ceramic yttrium aluminum garnet) experimental data and discussed within the framework of the computational model and simplified analytical approximations. [ABSTRACT FROM AUTHOR]

The model verification, simulation and analysis of the mechanisms strongly limiting lasing in codoped Tm,Ho solid-state lasers are described. The simulations have shown that significant deterioration of laser operation is caused due to absorption loss in water vapor present in the resonator and also in undoped sections of the operating crystals.