Showing papers on "Total electron content published in 1985"

Construction of a model of equatorial scintillation intensity

Abstract: By using a 5-year series of observations at 254 MHz taken at Huancayo, Peru, an analytical equation has been developed to yield scintillation excursions in decibels as a function of solar flux, magnetic index, local time, and day of the year; the equation, while characterizing the parameters given for the region surrounding the magnetic equator and for the Atlantic sector, has serious limitations in its application at other latitudes, other frequencies, and other longitudes in the equatorial region. Current available models of electron density and total electron content are inadequate to explain the differences between high levels of scintillation activity in years of high solar flux over the anomaly region relative to that of the magnetic equator. Newer models under development go further towards explaining levels of fading. Excursions levels of the order of 28 dB and greater have been observed in the anomaly region, while only 8–9 dB excursions have been noted at 1.6 GHz on the magnetic equator. The longitudinal characteristics indicate differences in activity over various regions of the earth particularly relative to solstice scintillation occurrence. Characteristics of the patches, such as intervals between patches and differing propagation paths when transmitting from multiple satellites, can be used to move information through the ionosphere even during nights of intense scintillations.

Ionospheric electron content depletion associated with amplitude scintillation at the equatorial anomaly crest region

Abstract: The diurnal and seasonal behaviors of the ionospheric total electron content (TEC) depletion associated with VHF amplitude scintillation observed at Lunping, Taiwan (25.00 deg N, 121.17 deg E geographic), during the period March 1977 to February 1980 were presented. Diurnally, the maximum occurrence of TEC depletion appears during the late evening hours for all seasons except winter. Seasonally, the maximum occurrence of TEC depletion is not fixed at a certain season but varies with sunspot number because of the different sunspot number dependence of the occurrence of TEC depletion in different seasons. The most probable maximum TEC depletion ranges from 0.8 to 3.6 x 10 to the 16th el/sq m, and the duration ranges from 10 to 33 min. No correlation was found between the maximum TEC depletion and the duration or between the maximum TEC depletion and the associated ionospheric scintillation index. The association between TEC depletion and range spread F is much better than that between TEC depletion and frequency spread F. 32 references.

Seasonal and day-to-day variability of total electron content at mid-latitudes near solar maximum

Abstract: Faraday rotation observations were conducted at Fort Monmouth, New Jersey (40.15°N, 74.01°W), utilizing beacon transmissions from a geostationary satellite during the period just following the maximum phase of solar cycle 21. Seasonal and day-to-day variabilities are observed. Unique representation of the data has permitted the study of the day-to-day variability of derived total electron content (TEC) values. For example, winter data indicate uniformity of day-to-day TEC values during the buildup and decay phases of the diurnal variation, and summer data indicate uniformity during the predawn and sunrise phases, whereas equinox data indicate wide variability during all phases. In addition, winter and summer data indicate that during non-magnetically-disturbed periods, TEC values at the diurnal peak, which exceed monthly mean values, tend to occur on consecutive days. During the equinoxes the peak values may occur randomly. The response of TEC to magnetic activity is also discussed. Since day-to-day variability is a most difficult quantity to predict, the results have implications for prediction improvement.

Deep depletions of total electron content associated with severe mid‐latitude gigahertz scintillations during geomagnetic storms

Abstract: Using 136-MHz Faraday rotation data obtained at three closely spaced stations, we present evidence that severe nighttime gigahertz scintillations, which appear rarely at mid-latitudes around Japan only during geomagnetic storm conditions, are closely associated with deep depletions of total electron content (TEC). The TEC depletions amount to 2–8×1016 el/m² (10–30% of the background TEC), and their durations range from 10 min to 1 hour. These depletions move northeastward or eastward with velocities between 60 and 260 m/s. The depletions are probably not counterparts of the equatorial bubbles but seem to be formed in localized regions around Japan under complicated and peculiar ionospheric conditions. There is an indication that the oscillation of the F region caused by large-scale TID's propagating from north to south (∼600 m/s) may initiate the generation of the depletion.

Semi-empirical low-latitude ionospheric model. Environmental research papers

Abstract: Since current empirical models specifying low-latitude electron density profiles severely underestimate the daytime plasma density scale-height and total electron content (TEC), a Semi-empirical, Low-latitude, Ionospheric Model (SLIM) was developed that is not only more realistic but is also computationally fast. Electron-density profiles (180 to 1800 km) are theoretically calculated as a function of latitude (every 2/sup 0/ between 24 N and 24 S dip latitude) and local time (every half-hour over 24 hours LT) by solving the time-dependent plasma-continuity equation. Assuming a Chapman-like profile, sets coefficients are then generated that reproduce these individual profiles. The coefficients themselves are easily stored, quickly retrieved, and form the basis for a fast, portable, semi-empirical computer code. This report describes briefly the input parameters used to theoretically calculate profiles and the procedures used to generate the coefficients. The SLIM profiles are compared with the Chiu and Bent empirical models for Equinox, solar-maximum conditions. Finally, electron densities, the coefficients, TEC and 6300 A airglow intensities are listed in tabular form for three seasons (Equinox, June solstice, and December solstice) and two solar-cycle periods (solar maximum and solar minimum).

Nighttime enhancement at low latitudes

Abstract: L'augmentation du contenu total d'electrons et de la frequence critique f 0 F 2 a ete etudiee d'apres les donnees de cinq stations de basse latitude couvrant des angles d'inclinaison magnetique de 0,6°S a 45,00°N. On observe une bonne correlation entre les deux series de donnees. On modelise l'effet de derive EXB des vents neutres

The SRI Ionospheric Structure and Dynamics Program - 1985 Results.

Abstract: : This report describes recent observations of high latitude ionospheric structure and dynamics from the HILAT satellite and the Sondrestrom and EISCAT incoherent scatter radars. Plasma turbulence theory and ionospheric plasma diffusion processes are also described. Measured variations in latitude and local time of total electron content, levels of radio wave scintillation, and magnetospheric electrodynamics are presented for the northern polar regions. Seasonal and solar cycle variations of these phenomena are examined. Magnetic flux tube interchange produced by magnetospheric electrostatic turbulence (rather than local instability polarization fields) is suggested as an important source of high latitude ionospheric structure. The large scale magnetospheric potential field is also shown to be capable of steepening and structuring plasma gradients. These processes would certainly structure late time nuclear produced plasma. Keywords: Radio wave scintillation; Plasma turbulence; Ionospheric irregularities; Total electron content; and Convective plasma instabilities.

Determination of Ionospheric Electron Density Profiles from Satellite UV (Ultraviolet) Emission Measurements, Fiscal Year 1984.

Abstract: : The possible use of satellite ultraviolet measurements to deduce the ionospheric electron density profile (EDP) on a global basis is discussed During 1984 comparisons were continued between the hybrid daytime ionospheric model and the experimental observations These comparison studies indicate that: (1) the essential features of the EDP and certain UV emissions can be modelled, (2) the models are sufficiently sensitive to input parameters to yield poor agreement with observations when typical input values are used, (3) reasonable adjustments of the parameters can produce excellent agreement between theory and data for either EDP or airglow but not both; and (4) the qualitative understanding of the relationship between two input parameters (solar flux and neutral densities) and the model EDP and airglow features has been verified The development of a hybrid dynamic model for the nighttime midlatitude ionosphere has been initiated This model is similar to the daytime hybrid model, but uses the sunset EDP as an initial value and calculates the EDP as a function of time through the night In addition, a semi-empirical model has been developed, based on the assumption that the nighttime EDP is always well described by a modified Chapman function This model has great simplicity and allows the EDP to be inferred in a straightforward manner from optical observations Comparisons with data are difficult, however, because of the low intensity of the nightglow Keywords: Global electron density; Satellite measurements; DMSP(Detense Meteorological Satellite Program); Ionosonde data; Total electron content; Midlatitude ionosphere; Auroral E layer; Daytime airglow; Solar flux; Neutral wind

Comparison of the calibration of ionospheric delay in VLBI data by the methods of dual frequency and Faraday rotation

Abstract: When both S-band and X-band data are recorded for a signal which has passed through the ionosphere, it is possible to calculate the ionospheric contribution to signal delay In Very Long Baseline Interferometry (VLBI) this method is used to calibrate the ionosphere In the absence of dual frequency data, the ionospheric content measured by Faraday rotation, using a signal from a geostationary satellite, is mapped to the VLBI observing direction The purpose here is to compare the ionospheric delay obtained by these two methods The principal conclusions are: (1) the correlation between delays obtained by these two methods is weak; (2) in mapping Faraday rotation measurements to the VLBI observing direction, a simple mapping algorithm which accounts only for changes in hour angle and elevation angle is better than a more elaborate algorithm which includes solar and geomagnetic effects; (3) fluctuations in the difference in total electron content as seen by two antennas defining a baseline limit the application of Faraday rotation data to VLBI

The variability of the ionospheric total electron content and its effect on satellite microwave communications

Abstract: Rotation of linearly polarized waves (Faraday rotation) passing through the ionosphere causes depolarization in frequency reuse satellite communication systems As the ionospheric total electron content (TEC) is not constant, dynamic compensation for this effect may be required This paper investigates the magnitude of the TEC variations, the time scales of the variability and the predictability of the excursions Analysis of long-term TEC measurements shows that the variations are statistical with yearly and seasonal trends strongly coupled to long term solar and geophysical effects Short-term variations are difficult to predict Data indicate that reliable Faraday rotation estimates can be made for 99·9 per cent of the time, provided that long-term solar, seasonal and geographical factors are considered Occasional bursts of solar activity limit the accuracy of long-term predictions

Formation and detection of high latitude ionospheric irregularities

Abstract: Measurements of Total Electron Content (TEC) and airglow variations show that large scale plasma patches appearing in the high-latitude ionsophere have irregular structures evidenced by the satellite phase and amplitude scintillations. Whistler waves, intense quasi-DC electric field, and atmospheric gravity waves can become potential sources of various plamsa instabilities. The role of thermal effects in generating ionospheric irregularities by these sources is discussed. Meter-scale irregularities in the ionospheric E and F regions can be excited parametrically with lower hybrid waves by intense whistler waves. Ohmic dissipation of Pedersen current in the electron gas is able to create ionospheric F region irregularities in plasma blobs or plasma patches (i.e., high ambient plasma density environment) with broad scale lengths ranging from tens of meters to a few kilometers. Through the neutral-charged particle collisions, gravity waves can excite large-scale (less than tens of kilometers) ionospheric irregularities simultaneously with forced ion acoustic modes in the E region. The large-scale ionospheric density fluctuations produced in the E region can extend subsequently alogn the earth's magnetic field to the F region and the topside ionospheric regions. These mechanisms characterized by various thermal effects can contribute additively with other processes to the formation of ionospheric irregularities in the high latitude region.

Group-Path Delays in the Trans-Ionospheric Radio Wave Propagation Derived from TEC

Abstract: The total electron content measurements made at Waltair (17.7°N, 83.3°E) by measuring the Faraday Rotation of the 136.1123 MHz Radio Beacon from the ETS-II geostationary satellite have been used to derive the range errors introduced by the ionosphere due to the group path delays of radio waves in trans-ionospheric communication systems. Corrections for oblique incidence propagation were also outlined. The mathematical relations used in the present study have been derived and the experiment is described. Diurnal and seasonal variations of the group-path delays are studied.