Incoherent scatter radar and Digisonde observations at tropical latitudes, including conjugate point studies

In this paper we compare ionospheric measurements made during the January 1993 10-day World Day Campaign with three separate instruments: the Arecibo incoherent scatter radar, the Ramey Digisonde (located 50 km west of Arecibo), and the Puerto Madryn, Argentina, Digisonde (located 400 km from the magnetic conjugate point of Arecibo). F layer peak heights from Ramey agree well with those from Arecibo throughout the entire 10-day period, with a median difference of 4 km, and along with the F peak density show effects of a 4-hour periodic temporal variation. We relate this 4-hour variation to the familiar midday biteout in the electron density that is commonly observed at Arecibo in the winter months. However, this periodic temporal variation is not observed at Puerto Madryn, which is located at a higher geographic latitude. Only occasionally do the Arecibo incoherent scatter radar and the Ramey Digisonde velocity measurements agree. Of course, it must be remembered that while the Doppler shift in an incoherent scatter radar echo is a direct measure of the line-of-sight plasma velocity component, the Doppler shift of a Digisonde echo (or any ionosonde echo for that matter) is a measure of the time rate of change of the electrical distance to the reflection point. Differences between the velocity components obtained by the two techniques are indications of the importance of chemistry and divergent transport instead of simple motions. Over the ten-day campaign and in the average of the 10 days, the Arecibo and Ramey horizontal velocities show good agreement only for a short period at night from 0000 to 0700 LT (0400 to 1100 UT). During this time the HF radio waves were reflected from relatively large zenith angles and the ionosphere was fairly high An increased westward component of the horizontal velocity measured at Arecibo and Ramey around 0330 LT (0730 UT), is correlated with sunrise in the summer conjugate hemisphere, but the drift velocity was smaller by about a factor of 2 at the southern station. Since we doubt that the field lines are not equipotentials, the difference observed could be due to Puerto Madryn not being located at the exact conjugate point for Arecibo and Ramey.