We investigated the relationship between post‐midnightF‐region field aligned irregularities (FAIs) andF‐layer altitude by analyzing data of a 30.8‐MHz radar installed 5at Kototabang, Indonesia (0.2°S, 100.3°E; geomagnetic latitude 10.4°S) and an ionosonde installed at Chumphon, Thailand (10.7°N, 99.4°E; geomagnetic latitude 3.3°N). Chumphon is located near the geomagnetic equator on approximately the same meridian as Kototabang. Case studies show that the altitude of theF‐layer rose at Chumphon a half hour before the post‐midnight FAIs appeared at Kototabang. The Doppler velocity of theE‐region FAIs observed simultaneously by the 30.8‐MHz radar was downward, indicating that theF‐layer uplift was not caused by the electric field. We also investigated seasonal variations of the post‐midnight FAI occurrence and theF‐layer altitude. Both the post‐midnight FAIs and the uplift of theF‐layer were frequently seen around midnight between May and August. The seasonal variation of the midnightF‐layer uplift around the geomagnetic equator coincided with that of the post‐midnight FAI occurrence at Kototabang. These results suggest that the uplift of theF‐layer would play an important role in the generation of post‐midnight FAIs. We evaluated the linear growth rate of the Rayleigh‐Taylor instability based on the altitude of theF‐layer observed at Chumphon. The result shows that the uplift of theF‐layer can enhance the growth rate because gravity‐driven eastward electric current increases. Therefore, we interpret that the observed FAIs were accompanied by plasma bubble, the growth rate of which was reinforced by the upliftedF‐layer.
This paper reports the experimental and theoretical investigation of the Talbot effect beyond the paraxial limit at optical frequencies. Au hole array films with periodicity 0 a comparable to the wavelength of coherent illumination λ were used to study the non-paraxial Talbot effect. Significant differences from the paraxial (classical) Talbot effect were observed. Depending on the ratio of 0 / a λ , the interference pattern in the direction perpendicular to the hole array was not necessarily periodic, and the self-image distances deviated from the paraxial Talbot distances. Defects within the hole array film or above the film were healed in the self-images as the light propagated from the surface.
A set of holographic filters was developed to convert the Gaussian intensity distribution of a collimated laser beam into a uniform one. The design and the fabricating method of the holographic filters are presented and experimental results are shown.
A digital phase-measuring interferometer with a laser-diode source has been developed that is based on a fringe-scanning technique with a stepwise wavelength change by variation of the laser injection current. The phase is changed to produce a relative phase difference between the beams in the two arms of the interferometer. Calibrated phase shifts used for a phase-extraction algorithm are derived from one-dimensional least-squares fits to cosine fringe functions to achieve accurate results. Experimental results are presented.
A digital phase measuring interferometer with a frequency-modulated laser diode using the integratedbucket technique is described. The injection current is continuously changed to introduce a time- varying phase difference between the two beams of an unbalanced Twyman-Green interferometer. The intensity of the interference patterns is integrated with a CCD array sensor for intervals of one-quarter period of the fringe. Using the intensity data a microcomputer calculates the phase to be detected. Some experimental results with the interferometer are presented; the rms repeatability obtained was lambda/80.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.