Showing papers on "TEC published in 1987"

A semi‐empirical low‐latitude ionospheric model

Abstract: Since current empirical models specifying low-latitude electron density profiles severely underestimate the daytime plasma density scale-height and total electron content (TEC) values, a semi-empirical low-latitude ionospheric model (SLIM) has been developed which is not only computationally fast, but also more realistic. Electron density profiles (180–1800 km) are theoretically calculated as a function of latitude (every 2° 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. Using simple exponential functions, sets of coefficients are then generated which 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 paper describes briefly the input parameters used to theoretically calculate the 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, while calculated 6300 A airglow intensities and TEC values are compared with available observations. The SLIM profiles, their coefficients, TEC and 6300 A airglow intensities are available in tabular and computer formats.

Latitudinal variations of nighttime enhancements in total electron content

Abstract: Latitudinal variations of the various characteristics of nighttime anomalous enhancements in total electron content (TEC) are presented by considering TEC data from the ATS series of satellites for a 2-month period from a number of stations in the northern hemisphere extending from 2° to 63° dip latitude. The latitudinal variations of the various TEC enhancement characteristics are found to be strikingly similar, and they reveal a pattern of cyclic variation with two distinct minima, one at 30°N and the other at 60°N (dip latitude). The TEC enhancements show a strong positive correlation with geomagnetic activity at middle and high latitudes and no significant correlation at low latitudes. The observed pattern of the latitudinal vaiations of the TEC enhancements is discussed in the light of the potential source mechanisms.

Daily observations of the development of the ionospheric equatorial anomaly by means of differential Doppler shift method

Abstract: The differential Doppler frequency shifts observed by receiving coherent radio signals at frequencies of 150 and 400 MHz transmitted from the polar orbiting satellites of U.S. Navy Navigation Satellite System have been used to deduce the latitudinal variations of the ionospheric total electron content (TEC) near the ionospheric equatorial anomaly crest region. All latitudinal variations of TEC thus obtained for each passage of an NNSS satellite are used to construct daily contour plots of TEC in a latitude versus local time coordinates. It has been shown that these contour plot of TEC can be used to investigate the behavior of TEC around equatorial anomaly crest region. Some results are presented and discussed.

Total electron content and L-band amplitude and phase scintillation measurements in the polar cap ionosphere

Abstract: The first measurements of absolute Total Electron Content (TEC) and L-band amplitude and phase scintillation was made from Thule, Greenland, a polar cap station, in early 1984. These measurements were made using signals transmitted from the Global Positioning System (GPS) satellites. The variability of the TEC, especially during the afternoon to pre-midnight hours, is large, with increases in TEC above the background values of greater than 100% not uncommon. During one disturbed time, quasi-periodic TEC enhancements having periods as short as ten minutes and amplitudes equal to the background TEC were observed for over two hours. The TEC during some of the disturbed periods in the dark Thule ionosphere exceeded mid-latitude daytime values. Amplitude scintillations were small, not exceeding 3 dB peak to peak during the entire observing period, but they were associated with the times of TEC enhancements, with some evidence for stronger scintillation occurring during the negative gradients of the TEC enhancements. Phase scintillations were highest during some of the times of enhanced TEC, and depend critically upon the phase detrend internal used.

Generalized Approach To Cooling Charge-Coupled Devices Using Thermoelectric Coolers

Abstract: This paper is concerned with the use of thermoelectric coolers (TECs) to cool charge-coupled devices (CCDs). Heat inputs to the CCD from the warmer environment are identified, and generalized graphs are used to approximate the major heat inputs. A method of choosing and estimating the power consumption of the TEC is discussed. This method includes the use of TEC performance information supplied by the manufacturer and equations derived from this information. Parameters of the equations are tabulated to enable the reader to use the TEC performance equations for choosing and estimating the power needed for specific TEC applications.

Miniature multi-temperature radiometric reference

Abstract: A miniature multi-temperature radiometric reference is placed in an intermediate image plane (20) of a thermal imaging optical system in order to correct, in a real-time manner, non-uniformities among detectors (30). The radiometric reference comprises at least two reference elements (10) mounted on a thermolelectric cooler (TEC) (1) which may have several stages (6). The reference elements (10) are mounted on the same platform (3) of the TEC (1). Each element (10) has a surface (11), having a substantially uniform temperature, within the depth of focus at the intermediate image plane (20). All of the radiometric reference surfaces (11) are maintained at different temperatures, which can be simultaneously above and below the ambient temperature. This enables determination of a normalized detector response, and subsequent correction of the response of each detector (30) as a function of temperature throughout the range characterized by the temperatures of the reference surfaces (11).

Ionospheric Scintillations/TEC and In-Situ Density Measurements at an Auroral Location in the European Sector

Abstract: : The orbiting HiLat satellite launched in 1983 offered an opportunity for studying ionospheric scintillation parameters in relation to in-situ measurements of ionization density, drift velocity, field-aligned current, and particle precipitation during the sunspot minimum period. This report discusses results of a morphological study based on observations of scintillations and total electron content (TEC) at the auroral oval station at Tromso, Norway, during the period Dec 1983 - Oct 1985. The geometrical enhancement of scintillations observed during alignment of the propagation with the local magnetic L-shell is shown to be the most consistent and conspicuous feature of scintillations in the nighttime auroral oval. The dynamics of the spatial and temporal extent of this region are illustrated in the invariant latitude/ magnetic local time grid. Steepening of phase spectral slope in the geometrical enhancement region is indicative of the presence of L-shell aligned sheet-like irregularities at long scale lengths. The seasonal variation of TEC determined from the differential Doppler measurements of HiLat transmissions is discussed in relation to the in-situ density measurements at 830 km. The results are also used to illustrate the dependence of ionospheric structure parameters on short- term variability of solar activity during the sunspot minimum period. This study provides an insight into the nature of magnetospheric coupling with the ionosphere at high latitudes.

Measurements of trans-ionospheric propagation parameters in the polar-cap ionosphere. Final report, October 1982-November 1986

Abstract: Measurements of absolute TEC (Total Electron Content), were made for the first time from the polar-cap station located at Thule, Greenland in early 1984 using dual-frequency group delay measurements from high orbit satellites. Measurements of L-band amplitude scintillation and dual-frequency differential carrier-phase scintillation were also made. The variability of the TEC was extremely large, with increases above the background values of more than 100% observed frequently. Quasi-periodic TEC enhancements with periods as short as 10 min and with amplitudes exceeding 10**17 el/m**2 column were seen over several time intervals of more than two hours. The absolute values of TEC observed in the dark polar-cap ionosphere at times exceeded the diurnal maximum values from the mid-latitude ionosphere during this initial period of observations.

Ionospheric scintillations/TEC and in-situ density measurements at an auroral location in the European sector. Environmental research papers, 2 October 1986-31 July 1987

Abstract: The orbiting HiLat satellite launched in 1983 offered an opportunity for studying ionospheric scintillation parameters in relation to in-situ measurements of ionization density, drift velocity, field-aligned current, and particle precipitation during the sunspot-minimum period. This report discusses results of a morphological study based on observations of scintillations and total electron content (TEC) at the auroral-oval station at Tromso, Norway, during the period Dec 1983 - Oct 1985. The geometrical enhancement of scintillations observed during alignment of the propagation with the local magnetic L-shell is shown to be the most consistent and conspicuous feature of scintillations in the nighttime auroral oval. The dynamics of the spatial and temporal extent of this region are illustrated in the invariant latitude/magnetic local time grid. Steepening of phase spectral slope in the geometrical enhancement region is indicative of the presence of L-shell aligned sheet-like irregularities at long scale lengths. The seasonal variation of TEC determined from the differential Doppler measurements of HiLat transmissions is discussed in relation to the in-situ density measurements at 830 km. The results are also used to illustrate the dependence of ionospheric structure parameters on short-term variability of solar activity during the sunspot-minimum period. This study provides an insight into themore » nature of magnetospheric coupling with the ionosphere at high latitudes.« less