Scintillation

About: Scintillation is a research topic. Over the lifetime, 14022 publications have been published within this topic receiving 187694 citations.

Scintillation boundary during quiet and disturbed magnetic conditions

Abstract: Earlier studies of ionospheric scintillations outlined the lower boundary of the high-latitude region where intense scintillations at 40 MHz were observed. The quiet-day scintillation boundary reached a lower position of 57° invariant latitude at 2200 LT. Dyson's (1969) recent observations with a Langmuir probe have verified the existence of a lower latitude boundary of small-scale irregularities. The boundary concept has been extended to include the effects of magnetic storms. Observations of satellite beacon signals at 40 and 54 MHz during 1961 to 1966 indicate that the mean change in the lower boundary latitude of the irregularity region is a decrease of approximately 1.6° per unit change in local K index. This is quite similar to the change of 1.8° per unit change in Kp noted for the trough position by Rycroft and Thomas (1970). In examining the data available from high-inclination and synchronous satellites, it was noted that the change in latitude with K index is a function of time. The maximum change of latitude as a function of K index, approximately 2° to 3° per unit K, occurred between 0300 and 0600 LT; the minimum change, about 1° per unit K, occurred over a broad interval from 1600 to 0200 LT. If the irregularity structure is produced by an interaction of the plasmapause with the ionosphere, the morphologic behavior of this region of the magnetosphere can be studied by reviewing the large inventory of scintillation and spread F data that has been amassed in the course of ionospheric research.

Implementation of fast-Fourier-transform-based simulations of extra-large atmospheric phase and scintillation screens

Abstract: Fast-Fourier-transform-based simulations of single-layer atmospheric von Karman phase screens and Kolmogorov scintillation screens up to hundreds of meters in size were implemented and tested for applications with percent range accuracy. The tests included the expected and the observed structure and pupil variance functions; for the phase, the tests also included the Fried turbulence parameter r0 measured by the seeing and by a simulated differential image motion monitor. The standard compensations used to correct the undersampling at low spatial frequencies were improved, and those needed for the high spatial frequencies were determined analytically. The limiting ratios of the screen sampling step to r0 and of the screen size to the pupil aperture were estimated by means of the simulated data. Sample results are shown that demonstrate the performances of the simulations for single-layer Kolmogorov and von Karman phase screens up to 200 m in size and for Kolmogorov scintillation screens for pupils up to 50 m of aperture.

Observations and modeling of multi-frequency VHF and GHz scintillations in the equatorial region

Abstract: Amplitude scintillation spectra and statistics obtained simultaneously at VHF (257.55 MHz), L band (1541.5 MHz), and C band (3945.5 MHz) from the MARISAT communications satellite (15°W) are presented. The measurements were made at Ascension Island (7°58′S, 14°25′W, 30°S DIP) over a high elevation propagation path within a few degrees of the magnetic meridian plane. The multi-frequency data and the unique propagation geometry enable us to study the structures of the equatorial ionospheric irregularities closely. Numerical simulation is used to model a specific event. Comparisons between the modeling results and the observations indicate that the data are consistent with the recently measured (in situ) two-component power law irregularity spectrum with a shallow large-scale regime and a steeper small-scale regime, and a spectral break at about 1-km scale size.

Plastic scintillation dosimetry for radiation therapy: minimizing capture of Cerenkov radiation noise

Abstract: Over the last decade, there has been an increased interest in scintillation dosimetry using small water-equivalent plastic scintillators, because of their favourable characteristics when compared with other more commonly used detector systems. Although plastic scintillators have been shown to have many desirable dosimetric properties, as yet there is no successful commercial detector system of this type available for routine clinical use in radiation oncology. The main factor preventing this new technology from realizing its full potential in commercial applications is the maximization of signal coupling efficiency and the minimization of noise capture. A principal constituent of noise is Cerenkov radiation. This study reports the calculated capture of Cerenkov radiation by an optical fibre in the special case where the radiation is generated by a relativistic particle on the fibre axis and the fibre axis is parallel to the Cerenkov cone. The fraction of radiation captured is calculated as a function of the fibre core refractive index and the refractive index difference between the core and the cladding of the fibre for relativistic particles. This is then used to deduce the relative intensity captured for a range of fibre core refractive indices and fibre core–cladding refractive index differences. It is shown that the core refractive index has little effect on the amount of radiation captured compared to the refractive index difference. The implications of this result for the design of radiation therapy plastic scintillation dosimeters are considered.