We present results of in situ ionospheric density measurements performed during the TSS-1R tether mission. The almost equatorial orbit of TSS-1R was such that the descending node was syncronous with the day-to-night terminator and the measurements refer to 4 consecutive magnetic equator crossings. The most important feature of the observations is given by the indication of very deep density minima, centered at the magnetic equator, with density values up to a factor 100 smaller than those predicted by the IRI ionospheric model. We will show that a plausible explanation of these pronounced minima can be found in terms of the prereversal enhancement of the E_¿B_ drift which is one of the dominant features of the equatorial anomaly. Our data indicate therefore that, near the magnetic equator, the IRI model needs to be updated to include more accurately the sunset strengthening of the plasma drift. ¿ 1999 American Geophysical Union