Large plasma density gradients in the low-altitude low-altitude ionosphereobserved during the TSS-1R mission

We present results of in situ ionospheric density measurements performed du ring the TSS-1R tether mission. The almost equatorial orbit of TSS-1R was s uch that the descending node was syncronous with the day-to-night terminato r and the measurements refer to 4 consecutive magnetic equator crossings. T he most important feature of the observations is given by the indication of very deep density minima, centered at the magnetic equator, with density v alues up to a factor 100 smaller than those predicted by the IRI ionospheri c model. We will show that a plausible explanation of these pronounced mini ma can be found in terms of the prereversal enhancement of the (E) under ba r x (B) under bar drift which is one of the dominant features of the equato rial anomaly. Our data indicate therefore that, near the magnetic equator, the IRI model needs to be updated to include more accurately the sunset str engthening of the plasma drift.