L. Kersley

A preliminary experimental test of ionospheric tomography

Abstract A preliminary experiment to test the application of computerized tomography (CT) to ionospheric imaging is described. Co-ordinated observations are presented in which radio transmissions from NNSS satellites were used to measure the total electron content required for the tomographic image of electron density. Simultaneous measurements of electron density by the EISCAT ionospheric radar facility were made for comparison. The results show that a large-scale ionospheric electron density gradient reconstructed by the tomographic technique is also seen in the EISCAT observations.

The meteorological jet stream as a source of medium scale gravity waves in the thermosphere: an experimental study

Abstract Results are reported of a co-ordinated experimental study of medium scale gravity waves involving use of a multistatic incoherent scatter system together with two networks of polarimeters monitoring transmissions from a geostationary satellite during a six-day period in July 1974. The latter observations give access to the horizontal propagation parameters of the waves while the former technique yields information on the vertical structure of both waves and the ambient atmosphere. A reverse ray tracing analysis, through the atmosphere, for each gravity wave is described and its validity and accuracy discussed in detail. For the majority of the waves the reverse group path can be followed down to tropopause level and comparison with meteorological data has shown that many of the possible source regions of the observed waves appear to lie in proximity to the jet stream, with some evidence to suggest a preference for the polar side of the jet. Other wave sources are found to be close to regions of convectively unstable cold polar air. Wave energetics are discussed together with possible generation mechanisms, the latter in the form of non-linear interaction of shear flow instabilities in the jet stream and penetrative convection.

A global response of the total electron content of the ionosphere to the magnetic storm of 17 and 18 June 1972

Abstract A global study is made of the response of the total electron content of the ionosphere to the sudden commencement (S.C.) geomagnetic storm of 17 June 1972. Using data from 18 sites, a comparison is made of the seasonal differences between this storm and the 17 December 1971 ionospheric storm studied on a similar global basis by Schodel et al . (1974). When storm induced total electron content enhancements occur, they generally occur within 24 h of the S.C. and they tend to be larger in the winter hemisphere; subsequent depletions are generally more severe in the summer hemisphere. The best-defined storm effect is the positive phase evening enhancements seen in the American longitude sector. The local times of the peak enhancements show a coupled seasonal/longitudinal/latitudinal pattern with latitude being the clearest ordering parameter. The magnitudes of the subsequent negative phase depletions show a longitudinal gradient from east to west.

Atmospheric waves induced by the solar eclipse of 30 June 1973

Abstract Results are reported of ionospheric sounding and polarization rotation measurements made at a series of stations across West Africa during the total solar eclipse of 30 June 1973. A weak ionospheric disturbance identified by a minimum in both peak density and total electron content appears to be associated with the eclipse, propagating over a limited distance in the form of a bow wave. A small wave of period 18 min may also be eclipse produced, the wavelength being comparable to the size of the umbral region. Possible mechanisms for the generation of atmospheric waves and ionospheric disturbances by an eclipse are discussed but no firm conclusion has been reached concerning the source of the observed fluctuations.

A global description of the F-region during the ionospheric storm of 17 December 1971

Abstract Total electron content data—observed continuously by means of geostationary radio beacons—provide a good means to study the ionosphere during severe ionospheric storms. Data of 20 sites were collected in order to study the ionospheric storm of 17 December 1971. A remarkable increase in the total electron content occurred at midlatitude stations throughout the northern hemisphere. Stations at auroral and equatorial latitudes also showed sharp increases which were both LT and latitude dependent whereas both sites in the southern hemisphere showed substantial depletions in the total electron content.

Summer daytime scintillation and sporadic-E

Abstract Scintillation of transionospheric transmissions from geostationary satellites observed at a mid-latitude site during daytime in summer has been shown to be associated with sporadic- E . It has been found that a high critical frequency and an extended vertical structure of the sporadic- E layer, as indicated by range spread on ionograms, are linked with scintillation occurrence. There are indications that at times of scintillation the vertical distribution of ionization in the layer may be influenced by short period gravity waves.

Small scale irregularities associated with a high latitude electron density gradient: scintillation and EISCAT observations

Abstract A coordinated experiment involving scintillation observations using NNSS satellites and special program measurements with the EISCAT ionospheric radar facility is described. The results reveal the presence of sub-kilometre scale irregularities in the vicinity of a long lived steep equatorwards gradient in electron density. Evidence is presented of a southwards plasma flow which would cause the gradient to be unstable to the E Λ B gradient-drift mechanism. An instability growth time of about 4 min has been estimated from the observations. Cooler electron temperatures associated with enhanced densities rules out soft particle precipitation as an irregularity source in this case.

Power-spectra of VHF intensity scintillations from F2- and E-region ionospheric irregularities

Abstract An experiment is described for the routine study of scintillations and ionospheric irregularities at high-latitudes using NNSS satellites with additional coordinated observations by means of the EISCAT ionospheric radar facility. Early results, obtained during the development phase of the experiment, are presented of the power spectra of intensity fluctuations at 150 MHz observed at the equatorwards edge of the high-latitude irregularity zone. The spectra of 165 samples of night-time scintillation recorded during October 1982 to May 1983 show a spectral index with a mean value of −3.58 and a steepening of the spectral slope with increasing S 4 . Some examples of scintillation arising from irregularities at E -layer height show spectral indices of magnitude generally smaller than for F -region cases. A few spectra have been found with a clear break in spectral slope at around 10 Hz, suggesting two regimes for irregularities of different scale sizes.

The dependence of ionospheric slab thickness on geomagnetic activity

Abstract Monthly mean values of daytime slab thickness show a positive dependence on geomagnetic activity. In the analysis steps must be taken to minimise the effects of the larger annual and solar cycle variations.

Scintillation and EISCAT investigations of gradient-drift irregularities in the high latitude ionosphere

Abstract Scintillation observations of vhf satellite transmissions provide information on the occurrence of ionospheric irregularities with horizontal scale sizes in the sub-kilometre regime. Although incapable of sufficient resolution to observe such irregularities, incoherent scatter radar yields measurements of ionospheric plasma parameters on a larger scale. Thus, coordinated observations using both techniques allow a study of the mechanisms responsible for the production and growth of the small scale irregularities. Results are presented of scintillation measurements made at Kiruna in northern Sweden using transmissions from satellites in the Navy Navigational Satellite System (NNSS) at times when the European incoherent scatter radar facility (EISCAT) was operating in a meridional scan common programme mode. Two satellite passes are discussed in which scintillation-producing irregularities were found in the vicinity of larger scale long-lived gradients in F-region electron density. Radar measurements of plasma drift velocity indicate that the small scale structures could have originated from destabilization of the electron density gradients by the E sx B gradient-drift instability mechanism.