摘要:

A loss in monochromaticity, or equivalently coherence, occurs when a laser beam is propagated through the atmosphere. One factor causing the loss in coherence is the scattering from the randomly moving atmosphere molecules. In this short note, the effect on a communications system's performance of the loss of coherence arising from molecular scattering is studied. The results are given for the propagation, over a horizontal path of lengthL, of an ideal‐monochromatic, collimated laser beam with a radiusRwithL≫Rand with the geometric optics condition (λL)½≪Rsatisfied (where λ is the laser wavelength). For the examples studied, it is shown that molecular scattering gives rise to only an inconsequential background noise in the receiver for typical communication systems. In particular, for λ = 1 μm andR= 5 cm and 5 m, the background noise is 74 and 54 db down, respectively. It will be necessary, however, for the communications engineer (and the radar engineer as well) to examine his particular case to ensure that the effects of molecular scattering are negligible or, if they are not negligible, that they are acc

ISSN:0048-6604    

DOI:10.1029/RS006i006p00605

年代:1971

数据来源: WILEY

摘要:

The theory for the drift of ions in theFregion is briefly reviewed, and it is shown that, for the restricted case of no external electric fields or neutral air motion, the velocity of the ions along the magnetic field lines in the upper part of theF2region can be derived from the continuity equation. Experimental results for the electron density, electron and ion temperatures, and O+vertical velocity obtained by the incoherent (Thomson) scatter technique at Millstone Hill are presented for two 24‐hour periods taken a year apart in March 1969 and 1970. The largest vertical velocities encountered occur around sunrise and sunset and give rise to vertical fluxes of the order of 5 × 108ions/cm2/sec through 600‐km altitude. It is shown that these fluxes are a result primarily of the growth or decay of the layer, though in the case of sunset the thermal contraction resulting from the decrease in the electron temperature with time assists in establishing the observed fluxes. The vertical velocity observed near 300 km is compared with the expected ambipolar diffusion velocity that should result from the observed pressure gradients in the plasma and from acceleration due to gravity. The difference between the observed and computed velocities is attributed to thermospheric neutral winds which have diurnal behavior similar to those determined by Vasseur (1969a, b) from incoherent scatter observations in Fr

ISSN:0048-6604    

DOI:10.1029/RS006i006p00609

年代:1971

数据来源: WILEY

摘要:

Differential Faraday rotation measurements of the 40‐ and 41‐MHz beacons of satellite 1964–64A/BE‐B were made at London, Ontario (43°N, 81°W) through the use of a continuous output polarimeter of the phase comparison type, and simultaneous differential absorption measurements were made by use of the ordinary and extraordinary mode amplitudes at 40 MHz. Sufficient measurement accuracy was obtained to employ the second‐order expansion of the magnetoionic refractive and absorption index equations to extract total electron contents, the ratio of the density squared integral to the total electron content, a mean ionospheric electron collision frequency, and a parameter sensitive to the height of an electron density irregularity, from the data. Two passes, orbit 17239, March 17, 1968, and orbit 17280, March 20, 1968, were selected for analysis. Peak electron densities, mean plasma scale heights, and mean electron temperatures were computed from the reduced data by the assumption of an electron density height profile; they were found to correspond to within experimental error with comparable published measurements. During both passes examined,Fregion electron density profile broadenings were detected that were, for March 20, attributed to adiabatic expansions of theFregion electron gas, which was driven by localized neutral gas motions, and for March 17 the broadenings were attributed to thermal expansions of theFregion plasma. On both occasions, the fractional errors in the data were sufficiently small over some portions of the pass to permit a determination of the dominant ionospheric absorptio

ISSN:0048-6604    

DOI:10.1029/RS006i006p00627

年代:1971

数据来源: WILEY

摘要:

Low‐frequency pulse transmissions are received over reciprocal paths after being reflected once from the ionosphericDregion. The daytime reflection height is near 70 km and the nighttime height is near 90 km. Over an 1830‐km path between Nantucket, Massachusetts, and Jupiter Inlet, Florida, with a midpoint at 34°15′N, 75°28′W geographic (or 45°40′N, 352°20′E geomagnetic coordinates), rapid simultaneous phase and amplitude fluctuations are frequently observed at night. Statistically these events occur more frequently near the equinox (seasonally) and near dawn (diurnally). They seem to occur more frequently on the path indicated above than on higher latitude paths at quite different geomagnetic longitudes; the events have not appeared at all on paths at geomagnetic latitudes below 30°N. A possible explanation for the more frequent occurrence on the Nantucket‐Jupiter path is the fact that the midpoint lies slightly west of the magnetic conjugate to the South Atlantic anomaly. Detailed correlation of both phase and amplitude at both ends of reciprocal propagation paths tends to rule out gradients, offpath reflections, or instrumentation as causes of these events. Investigations of several different propagation paths suggest that these effects may be quite localized but that paths with similar geomagnetic latitudes can be similarly affected even when they are separated by more than 100° in geomagnetic longitude. LF ionospheric propagation is very sensitive to relatively small changes in electron densities in theDregion. The events described above become less pronounced as the path length increases; this pattern suggests that they are quite localized and that the integrating effect of longer paths tends to mask the effects. Particle fluxes capable of producing electrons in the 10‐ to 100‐electron cm−3sec−1range near 80 km at night would be expected to produce variations such as those measured. These LF phase measurements over oblique paths may prove to be an excellent method for detecting leakage or dumping of charged particles from the radiation belt

ISSN:0048-6604    

DOI:10.1029/RS006i006p00639

年代:1971

数据来源: WILEY

摘要:

The approximate image theory for quasi‐static fields in the presence of a conducting half space is treated in full generality by considering an arbitrary periodic source. The general theory is developed in terms of a magnetic Hertz vector aligned perpendicular to the plane surface of the conductor. It is shown that its solution above the conductor can be expressed as the combined Hertz potentials of the source and its image located at a certain complex depth, plus terms that become negligibly small for points somewhat farther than a skin depth from the ordinary mirror image of the source. Image approximations for the individual electric and magnetic field components are derived. The magnetic field is expressed entirely in terms of the magnetic field of the source and its complex image, but the electric field, unless it is everywhere parallel to the surface of the conductor, depends in addition on the mirror image of the source. The general theory is illustrated by its application to the particular examples of magnetic dipole and infinite line current source

ISSN:0048-6604    

DOI:10.1029/RS006i006p00647

年代:1971

数据来源: WILEY

摘要:

A formal solution to the problem of the diffraction of electromagnetic waves by a wedge with given surface impedances, moving parallel or perpendicular to its bisection plane with constant velocity, has been obtained. Characteristics of the diffracted field are studied, and approximate expressions for the Doppler shifts of the excited surface‐wave fields and the scattered field in the shadow region are evaluated for both relativistic and on‐relativistic considerations. Plots of these results for a 60° wedge and a half plane are shown to illustrate their dependence on the surface impedance, the wedge angle, and the angle of incidence. Also, the variation of the amplitude of surface waves with respect to the velocity of the moving wedge is discu

ISSN:0048-6604    

DOI:10.1029/RS006i006p00655

年代:1971

数据来源: WILEY

摘要:

The current distribution on a grid type dipole antenna immersed in a warm isotropic plasma is formulated from two viewpoints. The first method is a direct source approach; it yields a new integral equation that is an extension of Hallen's equation. The second method is based on a boundary value analysis introduced by Einarsson. The equivalence between the two methods will be shown. A solution to the integral equation valid for thin antennas shows that the current is sinusoidal in form with a propagation constantKe= (ω2–ωp2)½/C. This result will be compared with the work of Mo

ISSN:0048-6604    

DOI:10.1029/RS006i006p00665

年代:1971

数据来源: WILEY

摘要:

The theory of electromagnetic wave propagation in a uniformly accelerated simple medium is generalized to cover the case of arbitrary initial wave direction. The wave splits into two natural modes relative to the plane of acceleration and the wave vector. The case of high frequency and weak acceleration is solved in detail; to observers at rest with respect to the medium, a drag effect due to apparent gravity is demonstrated, and to inertial observers a drag effect due to medium acceleration is demonstrated. A preferred asymptotic cone of propagation determined by (μϵ– 1)½is found in the accelerated frame. Various drag effects are physically interpr

ISSN:0048-6604    

DOI:10.1029/RS006i006p00673

年代:1971

数据来源: WILEY