Bremsstrahlung in the gravitational field of a cosmic string

In the framework of QED we investigate the bremsstrahlung process for an electron passing by a straight static cosmic string. This process is precluded in empty Minkowski space-time by energy and momentum conservation laws. It happens in the presence of the cosmic string as a consequence of the conical structure of space, in spite of the flatness of the metric. The cross section and emitted electromagnetic energy are computed and analytic expressions are found for different energies of the incoming electron. The energy interval is divided in three parts depending on whether the energy is just above electron rest mass $M$, much larger than $M$, or exceeds $M/\delta$, with $\delta$ the string mass per unit length in Planck units. We compare our results with those of scalar QED and classical electrodynamics and also with conic pair production process computed earlier.Comment: 21 pages, to appear in Phys. Rev. D., KONS-RGKU-94-0

On the detectability of internal waves by an imaging lidar

The first results of a multisensor airborne survey conducted off the western Iberian Coast are presented (including visible, lidar, and infrared imagery) and reveal the presence of internal solitary waves (ISWs) propagating into the nearshore region. For the first time, two-dimensional lidar imagery is shown to detect the presence of ISWs, and the results are interpreted in a more comprehensive framework provided by the remaining instrumentation. Sea surface roughness patterns, resulting from the ISWs, are found to be imaged in the lidar data, where specular reflection causes slicks to appear as areas of significantly reduced backscatter. Moreover, the lidar data reveal an unprecedented view into the ISW surface and subsurface structures. Possible interpretations are discussed based on the accumulation of surfactants and air bubble entrainment at the leading edge of the ISWs (where maximum convergence occurs)