[1] HF radar observations, 630.0 nm cusp auroral emission, and an ionospheric tomography scan are compared to investigate the mechanism by which radar cusp echoes are generated. A collocation is reported between the equatorward boundaries of broad spectral width HF radar backscatter and 630.0 nm cusp auroral precipitation. In addition, the radio tomographic image reveals spatial structures in ionospheric electron density that map closely to the precipitation. However, the large-scale horizontal gradient appears to be insufficiently steep to account alone for the rapid growth of irregularities by the gradient drift mechanism. It is proposed that the initial source of the decameter-scale features responsible for the backscatter may result from fine structure within the precipitation itself, or alternatively, from the cascade down of unstable intermediate-scale gradients created in the plasma convecting poleward from the source region. The latter process may explain the ongoing backscatter found poleward of the auroral band.

The response of the ionospheric F region to the large solar flare that occurred near 1500 UT on August 7, 1972, has been monitored by means of Faraday rotation measurements made at 17 stations in North America, Europe, and Africa. With observations spanning more than 10 hours in local time and more than 70 deg in latitude, the first truly global morphology of a flare‐induced F region event was obtained. The sizes of the individual sudden increases in the total electron content (Sitec) ranged from 1.8 to 8.6 × 1016 el/m²; on a percentage basis, all the Sitec fell within the 15 to 30% range. No obvious relationship was found between the sizes of the increases and the solar zenith angles at the various subionospheric points, nor between the observed Sitec and the sudden flare effects (SFE) seen on nearby magnetometer recordings. The latitudinal behavior provided the only simple ordering parameter found in the data, the lower latitudes having larger observed increases than the higher latitudes. Millstone Hill incoherent scatter data showed that nearly 40% of the total content enhancement observed at that site came from heights above 300 km. All the Sitec had a rise time of about 10 min, during which the Tec rate of change showed an excellent correlation with the time development of the solar radio burst monitored at 35,000 MHz.

Abstract. The accuracy of the Global Positioning System (GPS) satellite navigation system can be degraded by propagation effects on ray paths through the ionized atmosphere. The bulk of the plasma resides in the F layer, and models of total electron content have been developed to compensate for the effects of this ionization. However, use of the GPS system involves long ray paths through the tenuous hydrogen-based plasma of the protonosphere, and little is known about the electron content in this region. In the present study, the Sheffield University plasmasphere ionosphere model has been used to determine the electron content on the protonospheric section of GPS ray paths. Results are presented for stations at midlatitudes in the European and American sectors at both extremes of the solar cycle. The results are discussed in terms of the geometry of the flux tubes and the known behavior of the plasma.

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