INCOHERENT-SCATTER RADAR AND DIGISONDE OBSERVATIONS AT TROPICAL LATITUDES, INCLUDING CONJUGATE POINT STUDIES

In this paper we compare ionospheric measurements made during the Janu ary 1993 10-day World Day Campaign with three separate instruments: th e Arecibo incoherent scatter radar, the Ramey Digisonde (located 50 km west of Arecibo), and the Puerto Madryn, Argentina, Digisonde (locate d 400 km from the magnetic conjugate point of Arecibo). F layer peak h eights from Ramey agree well with those from Arecibo throughout the en tire 10-day period, with a median difference of 4 km, and along with t he F peak density show effects of a 4-hour periodic temporal variation . We relate this 4-hour variation to the familiar midday biteout in th e electron density that is commonly observed at Arecibo in the winter months. However, this periodic temporal variation is not observed at P uerto Madryn, which is located at a higher geographic latitude. Only o ccasionally do the Arecibo incoherent scatter radar and the Ramey Digi sonde velocity measurements agree. Of course, it must be remembered th at while the Doppler shift in an incoherent scatter radar echo is a di rect measure of the line-of-sight plasma velocity component, the Doppl er shift of a Digisonde echo (or any ionosonde echo for that matter) i s a measure of the time rate of change of the electrical distance to t he reflection point. Differences between the velocity components obtai ned by the two techniques are indications of the importance of chemist ry and divergent transport instead of simple motions. Over the ten-day campaign and in the average of the 10 days, the Arecibo and Ramey hor izontal velocities show good agreement only for a short period at nigh t from 0000 to 0700 LT (0400 to 1100 UT). During this time the HF radi o waves were reflected from relatively large zenith angles and the ion osphere was fairly high An increased westward component of the horizon tal velocity measured at Arecibo and Ramey around 0330 LT (0730 UT), i s correlated with sunrise in the summer conjugate hemisphere, but the drift velocity was smaller by about a factor of 2 at the southern stat ion. Since we doubt that the field lines are not equipotentials, the d ifference observed could be due to Puerto Madryn not being located at the exact conjugate point for Arecibo and Ramey.