No abstract

The recent discovery of a ferroelectric response to a switching electric field in nematic phases exhibited by bent-core polar molecules, particularly 1,2,4-oxdiazole derivatives, could lead to new avenues for the development of electro-optic devices. For the first time, we report ferroelectric-like switching under the influence of a triangular wave electric field in the nematic phase exhibited by unsymmetrical achiral four-ring bent-core compounds. These bent-core molecules, exhibiting a large nematic phase range (>70 C), consist of two unequal lengths in two wings and possess a polar moiety at one end and an alkyloxy chain at the other end. An anomalous variation in spontaneous polarization as a function of temperature in the nematic phase is distinctly observed, which is similar to the results reported only in the low temperature region of the nematic phase. Electro-optical, current-response and dielectric studies of aligned samples corroborate earlier reports of the proposed polar structure of the cybotactic clusters and the ferroelectric-like polar switching of these nematic phases.

We present systematic observations of daytime 150‐km echoes from Gadanki (13.5° N, 79.2° E, 6.3° dip latitude). The echoing region is confined to 142–162 km altitude and show forenoon descent and afternoon ascent displaying a ‘necklace shape’ quite similar to that of the equatorial 150‐km echoes. Signal‐to‐noise ratio, velocity and spectral width are as high as 13 dB above noise, −5 to 35 m s−1 and 2 to15 m s−1, respectively. The observed ‘necklace shape’ is first of its kind from Gadanki and the present observations have almost all characteristics of equatorial daytime 150‐km echoes. The Gadanki radar observations hence provide first experimental evidence on the occurrence of daytime 150‐km echoes outside the equatorial electrojet belt.

Understanding the structure and dynamics of the Martian thermosphere (100-220 km) is extremely important as this region, in particular the upper thermosphere, acts as a buffer zone between the reservoir of atmospheric species down below and the exosphere above from where the gaseous escape happens (e.g., Bougher, Cravens, et al., 2015). From this view point, the exobase acts as a lid on top of the thermosphere and the gaseous escape is regulated by the amount of energy, particles, and fields (both electric and magnetic) that reach the upper thermosphere. Thermospheric neutral densities, in general, decrease exponentially with an increase in altitude. Superimposed on this, there are perturbations of various scales that are due to forcings from above and below. Forcings from below include the planetary waves, thermal tides, and gravity

[1] Long-term variations of monthly mean zonal and meridional winds in the Mesosphere and Lower Thermosphere (MLT) at low-latitudes are analyzed using four medium frequency (MF) radars and three meteor radars located in the Asia-Oceania region. Radar data taken at close-by latitudes are appended to construct long-term data sets. With this, we have long-term data from five distinct latitudes within AE22 (viz., 22 N, $9 N, 0-2 N, 6-7 S and 21 S). The data length varies at different latitudes and spans a maximum of two decades during 1990-2010. The zonal winds show semiannual oscillation (SAO) at all locations with westward (eastward) winds during equinoxes (solstices). The month height pattern of SAO is similar within AE9and is different at AE22 . The westward winds in the March equinox were enhanced every two or three years during 1990-2002. We define this phenomenon as Mesospheric Quasi-Biennial Enhancement (MQBE). Such signature is not clear after 2002. The meridional winds show annual oscillation (AO), with northward and southward winds during the December and June solstices, respectively. However, the timing at which the wind direction changes does not coincide at all latitudes. The amplitude of the AO is enhanced after 2004 and 2008 at $9 N and $7 S, respectively. Orthogonal components of SAO and AO are detected with persistent phase relation, which suggests that the zonal and meridional winds are coupled. The meridional winds show long-term trends at latitudes of $9 N and $6-7 S, but not at other latitudes . The zonal winds do not show significant long-term trends.Citation: Venkateswara Rao, N., T. Tsuda, D. M. Riggin, S. Gurubaran, I. M. Reid, and R. A. Vincent (2012), Long-term variability of mean winds in the mesosphere and lower thermosphere at low latitudes,