ISSN:0048-6604    

DOI:10.1029/RS019i003p00685

年代:1984

数据来源: WILEY

摘要:

Efforts continue to update and improve empirical models of the mean morphological behavior of ionospherically produced scintillation. While often intended for engineering application, these models also can be scientifically useful to the extent that they describe not only the signal fluctuations but also the plasma density irregularities that cause them. Such a model is contained in the computer code called WBMOD, which employs the phase screen propagation theory to calculate the power law spectral index and strength (at a fluctuation frequency of 1 Hz) of phase scintillation and the standard deviation of phase, for a user‐specified system. Explicit provision is made for an ionospheric outer scale, but the relevant phase spectrum is cut off by a system parameter in most applications. The intensity scintillation indexS4is calculated from weak‐scintillation theory and corrected for the saturating effect of multiple scatter on the basis of Rice statistics. The irregularity model in WBMOD describes three‐dimensionally anisotropic (sheetlike) irregularities on the nightside of the high‐latitude scintillation zone and axially symmetric (rodlike) irregularities elsewhere. The current status of WBMOD, which is undergoing continued refinement, is described in this paper for all latitude regimes. Plans for future development of its irregularity model are stated, and it is suggested that such models should now move in the direction of incorporating irregularity d

ISSN:0048-6604    

DOI:10.1029/RS019i003p00687

年代:1984

数据来源: WILEY

摘要:

Multifrequency amplitude scintillation data at L and C bands from the equatorial region at Ascension Island are used to study the spectral index n for the scintillation index S4. The average frequency dependence of the scintillation index (S4) is found to be f−1.68. It is shown that the spectral index tends to be larger for events which occur after 2030 local time than for events which occur before 2030. This may indicate temporal evolution of the irregularity spectrum. Model studies based on a two component power law irregularity spectrum are applied to relate changes in n to spectral evolution and to show that the observations are consistent with recent in situ measurements. Possible geophysical implications are also discusse

ISSN:0048-6604    

DOI:10.1029/RS019i003p00695

年代:1984

数据来源: WILEY

摘要:

VHF nighttime scintillations, recorded during a high solar activity period at a meridian chain of stations covering a magnetic latitude belt of 3°–21°N (420 km subionospheric points) are analyzed to investigate the influence of equatorial spreadFirregularities on the occurrence of scintillation at latitudes away from the equator. Observations show that saturated amplitude scintillations start abruptly about one and a half hours after ground sunset and their onset is almost simultaneous at stations whose subionospheric points are within 12°N latitude of the magnetic equator, but is delayed at a station whose subionospheric point is at 21°N magnetic latitude by 15 min to 4 hours. In addition, the occurrence of postsunset scintillations at all the stations is found to be conditional on their prior occurrence at the equatorial station. If no postsunset scintillation activity is seen at the equatorial station, no scintillations are seen at other stations also. The occurrence of scintillations is explained as caused by rising plasma bubbles and associated irregularities over the magnetic equator and the subsequent mapping of these irregularities down the magnetic field lines to theFregion of higher latitudes through some instantaneous mechanism; and hence an equatorial control is established on the generation of postsunset scintillation‐producing irregularities in the entire low‐lati

ISSN:0048-6604    

DOI:10.1029/RS019i003p00707

年代:1984

数据来源: WILEY

摘要:

An ionospheric bubble is first modeled based on in situ data measured on board a satellite. The effect such a bubble may have on radio signals propagating through it is next simulated by solving numerically a parabolic equation. Considerable pulse distortion is found. When the computational technique is applied to a frozen but moving bubble, these results imply that the pulse excess arrival time, the pulse width, and its skewness will fluctuate with time, leading to the pulse jitter problem often observed experimentally.

ISSN:0048-6604    

DOI:10.1029/RS019i003p00719

年代:1984

数据来源: WILEY

摘要:

Simultaneous measurements of fast Faraday polarization fluctuations (FPF's) and amplitude scintillations at 136 MHz during April 1977 to February 1980 and of amplitude scintillations at 136 MHz and 244 MHz during August 1981 to December 1982 made at Calcutta (27° dip subionospheric) have been utilized to investigate the nature of ionospheric irregularities during different seasons near the equatorial anomaly crest in the Indian sector. The irregularities occurring in the equinoxes under high solar activity conditions are found to be responsible for intense VHF/UHF scintillations and FPF's. The June solstice is characterized by irregularities producing slow and moderate to weak VHF scintillations but no significant FPF's and UHF scintillations. The occurrence of FPF's is mostly confined to premidnight hours and reduces significantly with respect to the amplitude scintillation occurrence in postmidnight hours. The results are assessed in terms of the strength of irregularities and the longitudinal behavior of global equatorial scintillation activities

ISSN:0048-6604    

DOI:10.1029/RS019i003p00725

年代:1984

数据来源: WILEY

摘要:

The geomagnetic storm of April 11–14, 1981, was the most severe storm reported since the August 1972 events. Observations of equatorial scintillations in the VHF through microwave range of frequencies (250 MHz to 1.5 GHz) by two stations, Huancayo, Peru, situated near the magnetic equator, and Ascension island, close to the equatorial anomaly crest, are presented. Unusually high levels of activity were recorded at both the stations even around sunrise. At Huancayo, triggered by a reversal of electric field and therefore a rise in the height of theFlayer, the 1.5‐GHz transmissions from the satellite MARISAT (15°W) exhibited a scintillation index of 9 dB, which is one of the highest levels recorded at Huancayo over a period of several years. At Ascension island, severe depletions in ionospheric electron content associated with scintillations were detected for the first time in our observations near sunrise. The dual‐station multisatellite observations are discussed in terms of plasma processes at the equator. The reversal of direction after a deep magnetic bay caused the reversal of direction of the electric field, produced an upward bubble velocity, and generatedFlayer irregularities. This bay, however, was unable to produce an upwelling after sunrise because ofElayer conductivity. The postmidnight and sunrise behavior of theFlayer irregularity region has been verified by observations of additional severe magnetic storms during years of high sunspot activity. One example, observations on March 5, 1981, at Manila, is shown. The localized generation of patches during periods of magnetic activity indicates the possibility of bubble formation during magnetically quiet periods without a triggering wave. A hypothesis is proposed for instabilities forming from longitudinal (east‐west) electron density gradients existing in this case during the magnetic bay; in the case of magnetically quiet postsunset activity, steep and irregular gradients in electron density may exist across the longitudinal plane at the magnetic equator, producing conditions for the generation of insta

ISSN:0048-6604    

DOI:10.1029/RS019i003p00731

年代:1984

数据来源: WILEY

摘要:

During September 1982 a group of U.S., West German, and Brazilian organizations evaluated the trigger mechanism of equatorial ionospheric bubbles in an ionospheric modification experiment in Brazil. The experiment involved the release of two barium chemical packages at an unstable point in the nighttimeFlayer. At this point, they would trigger a bubble of low ion density to rise into the middle and upperFregions. Following the release of the barium, an aircraft equipped with a UHF satellite receiver was flown in the shadow of the bubble. It mapped the growth of the resulting irregularities from their initial appearance until they grew to a width of 100 km or more. The experiment demonstrated that a bubble could be triggered by proper placement of the barium ions and that the resultant irregularities would have the appearance of naturally triggered equatorial ionospheric irregularities.

ISSN:0048-6604    

DOI:10.1029/RS019i003p00741

年代:1984

数据来源: WILEY

摘要:

The day‐to‐day variability in ionospheric electron content (IEC) is studied using the diurnal IEC maximum data obtained from multistations located in the latitude range between 15.0°N and 30.0°N in the Indian zone during solar minimum. Depending on the location of the observing station, the changes in IECmaxvalues, of about ±20 to 40%, are found to occur in the form of single day abnormality, alternate day abnormality, and long‐term periodic fluctuations. The magnitude of fluctuations is found to be maximum at a station which is near the crest of the equatorial anomaly belt. The long‐term periodic variations, other than annual and semiannual variations, are different for different locations and seasons and show a periodicity of about 45 days in winter in the equatorial anomaly region only and about 27 days in summer at all the stations. The changes are not always correlated with solar or magnetic activity changes. It is observed that short‐as well as long‐term variations in IECmaxat stations within and near the crest of the equatorial anomaly belt are controlled mainly by the electrojet stre

ISSN:0048-6604    

DOI:10.1029/RS019i003p00749

年代:1984

数据来源: WILEY

摘要:

Faraday rotation observations were conducted at Haifa, Israel (32.87°N, 35.09°E), and Athens, Greece (37.97°N, 23.72°E), during the maximum phase of the current solar cycle using the VHF beacon of the SIRIO satellite. The subionospheric points (at 420 km) are (29.9°N, 27.9°E) and (34.5°N, 18.4°E), and the subionosphericLshell values are 1.24 and 1.37, respectively. Expected latitudinal and local time differences in total electron content (TEC) for the two locales are observed. However, the Haifa data are characterized by generally occurring, seasonally independent, large postsunset electron content maxima which are absent for the Athens data. Furthermore, the postsunset increases would appear to be a solar maximum phenomenon, as they are not observed during the minimum phase of the solar cycle. The postsunset increases are attributed to electron fluxes arriving from the equatorial regions along the magnetic lines of force. The correlation coefficients of hourly TEC at the Haifa/Athens locales exhibit a seasonally independent diurnal variation with minimum values at night and maximum values generally at the end of the buildup phase of TEC variation. The daytime ratios of the standard deviation of TEC to the average TEC are generally seasonally independent and behave quite similarly at the two locales, with daytime values bel

ISSN:0048-6604    

DOI:10.1029/RS019i003p00757

年代:1984

数据来源: WILEY