Low Altitude Solar Magnetic Reconnection, Type III Solar Radio Bursts, and X-ray Emissions.

Type III solar radio bursts are the Sun's most intense and frequent nonthermal radio emissions. They involve two critical problems in astrophysics, plasma physics, and space physics: how collective processes produce nonthermal radiation and how magnetic reconnection occurs and changes magnetic energy into kinetic energy. Here magnetic reconnection events are identified definitively in Solar Dynamics Observatory UV-EUV data, with strong upward and downward pairs of jets, current sheets, and cusp-like geometries on top of time-varying magnetic loops, and strong outflows along pairs of open magnetic field lines. Type III bursts imaged by the Murchison Widefield Array and detected by the Learmonth radiospectrograph and STEREO B spacecraft are demonstrated to be in very good temporal and spatial coincidence with specific reconnection events and with bursts of X-rays detected by the RHESSI spacecraft. The reconnection sites are low, near heights of 5-10 Mm. These images and event timings provide the long-desired direct evidence that semi-relativistic electrons energized in magnetic reconnection regions produce type III radio bursts. Not all the observed reconnection events produce X-ray events or coronal or interplanetary type III bursts; thus different special conditions exist for electrons leaving reconnection regions to produce observable radio, EUV, UV, and X-ray bursts.

Structure and dynamics of a nanocolloidal silica gel dispersion.

We studied the structure and the dynamics of a nanocolloidal silica gel dispersed in an organic solvent [octylcyanobiphenyl (8CB)] as a function of the silica density by x-ray intensity fluctuation spectroscopy (XIFS). The silica density of the dispersed aerosil gel samples ranged from 0.03 to 0.20 g cm-3 and the autocorrelation of the silica scattering was probed over the q range from 0.03 to 0.15 nm-1 (corresponding to length scales from 42 to 209 nm) at a constant room temperature at which 8CB is in the smectic-A phase. The gel structure has a fractal dimension in this density range of df approximately 2.15. The time autocorrelation functions of the gels show clear density-dependent and complex dynamics. The gel relaxation times are very long and become bimodal with nonergodic character for densities from 0.10 to 0.16 g cm-3. In this same density range, the fluctuation contrast (strength) is a minimum while the relaxation time becomes independent of wave vector. Together, these results indicate that there is a narrow silica density range for these gels in which the dynamics changes dramatically. This suggests a complex phase diagram for the dynamics of aerosil gels as a function of densification.

Calorimetric study of the nematic to smectic-A and smectic-A to smectic-C phase transitions in liquid-crystal-aerosil dispersions.

A high-resolution calorimetric study has been carried out on nanocolloidal dispersions of aerosils in the liquid crystal 4-n-pentylphenylthiol-4'-n-octyloxybenzoate (8S5) as a function of aerosil concentration and temperature spanning the smectic-C to nematic phases. Over this temperature range, this liquid crystal possesses two continuous XY phase transitions: a fluctuation-dominated nematic to smectic-A transition with alpha approximately alphaXY=-0.013 and a mean-field smectic-A to smectic-C transition. The effective critical character of the N-SmA transition remains unchanged over the entire range of the introduced quenched random disorder while the peak height and enthalpy can be well described by considering a cutoff length scale to the quasicritical fluctuations. The robust nature of the N-SmA transition in this system contrasts with cyanobiphenyl-aerosil systems and may be due to the mesogens being nonpolar and having a long nematic range. The character of the SmA-SmC transition changes gradually with increasing disorder but remains mean field like. The heat capacity maximum at the SmA-SmC transition scales as rho with an apparent evolution from tricritical to a simple mean-field step behavior. These results may be generally understood as a stiffening of the liquid crystal (both the nematic elasticity as well as the smectic layer compression modulus B) with silica density.

Isotropic to nematic transition of aerosil-disordered liquid crystals.

We present a high-resolution study of the isotropic to nematic phase transition of a low birefringence liquid-crystal compound incorporating an aerosil gel. Calorimetry, light scattering, and microscopy data coherently combine to allow for an accurate determination of the temperature dependence of the onset of the nematic state. The nematic order develops on cooling through two distinct processes while the nematic correlation length mildly decreases. We understand the doubling of the phase transition as due to a crossover from a random-dilution regime, where the silica gel couples to the scalar part of the nematic order parameter, to a low-T random-field regime, where the coupling induces distortions in the director field.

Evolution of the isotropic-to-nematic phase transition in octyloxycyanobiphenyl+aerosil dispersions.

High-resolution ac calorimetry has been carried out on dispersions of aerosils in the liquid crystal octyloxycyanobiphenyl (8OCB) as a function of aerosil concentration and temperature spanning the crystal to isotropic phases. The liquid crystal 8OCB is elastically stiffer than the previously well studied octylcyanobiphenyl (8CB)+aerosil system and so general quenched random-disorder effects and liquid crystal specific effects can be distinguished. A double heat capacity feature is observed at the isotropic to nematic phase transition with an aerosil independent overlap of the heat capacity wings far from the transition and having a nonmonotonic variation of the transition temperature. A crossover between low and high aerosil density behavior is observed for 8OCB+aerosil. These features are generally consistent with those on the 8CB+aerosil system. Differences between these two systems in the magnitude of the transition temperature shifts, heat capacity suppression, and crossover aerosil density between the two regimes of behavior indicate a liquid crystal specific effect. The low aerosil density regime is apparently more orientationally disordered than the high aerosil density regime, which is more translationally disordered. An interpretation of these results based on a temperature dependent disorder strength is discussed. Finally, a detailed thermal hysteresis study has found that crystallization of a well homogenized sample perturbs and increases the disorder for low aerosil density samples but does not influence high-density samples.

Effect of a quenched random field on a continuous symmetry breaking transition: nematic to smectic-A transition in octyloxycyanobiphenyl-aerosil dispersions.

High-resolution x-ray diffraction and ac-calorimetric experiments have been carried out on the liquid-crystal octyloxycyanobiphenyl in which aerosil particles are dispersed. The measurements were made over a temperature range around the bulk nematic to smectic-A transition temperature. At this transition the liquid crystal breaks translational symmetry in a single direction. The silica particles, which hydrogen bond together to form a very low density gel, provide the quenched disorder. The random gel leads to observable broadening of the x-ray reflection from the smectic layers. The structure factor is well described by modeling the effect of the aerosils as a quenched random field. Dispersed aerosils are thought to pin both the direction of the translational ordering and the position of the layers. The latter appears to have the greatest effect on the x-ray line shape. We show that the aerosil surface area, as verified by small-angle scattering, equates to the variance of the random field. Calorimetric results reveal substantial change in the specific heat peak associated with the nematic to smectic-A transition. As the concentration of aerosil increases, the specific heat peak remains sharp yet decreases in magnitude and shifts in temperature in a nonmonotonic fashion. In this regime, the critical exponent alpha becomes progressively smaller. For the samples with the largest concentrations of aerosil particles the C(p)(N-A) peak becomes highly smeared and shifts smoothly to lower temperatures.