COMPARISON OF INCOHERENT-SCATTER RADAR AND DIGISONDE MEASUREMENTS WITH FIELD LINE INTERHEMISPHERIC PLASMA MODELED RESULTS AT MIDDLE AND LOWLATITUDES

Comparisons of modeled and measured ionospheric characteristics are ma de for data recorded during the January 1993 10-day world day campaign . The peak of the F region ''hmF2'' measured by Digisondes at Ramey (l ocated 50 km west of Arecibo) and at Puerto Madyrn, which is located c lose to the conjugate point, are used to drive the field line interhem ispheric plasma (FLIP) model. In the summer hemisphere for times when the geomagnetic conditions are quiet, the modeled and measured NmF2 ar e in good agreement, displaying median differences of approximately 13 %. In the winter hemisphere, good agreements in NmF2 are observed duri ng midday and evening hours. However, FLIP underestimates NmF2 values at night and in the postsunrise sector. Comparison of the Arecibo ISR and FLIP parallel velocities also show good agreement during daytime, while large differences are observed at night. The discrepancies in th e Arecibo/Ramey measurements and FLIP modeled results, at night, is re lated to the effect of a 4-hour periodic variation in hmF2 and NmF2. F urther analysis identifies, for the first time, this 4-hour variation and discusses its characteristics as being consistent with the effect of a 4-hour tide. The signature of the 4-hour periodic temporal variat ion in the height of the ionosphere is only clearly observable at F re gion heights during the winter months and at low solar activity. The 4 -hour periodic temporal variation is only observed at the tropical lat itude stations Arecibo/Ramey and is not observed at the midlatitude st ation Puerto Madryn. A 4-hour tidal component is not present in the MS IS86 model which FLIP uses for its neutral concentration. Therefore, w hen tidal forcing is moving the ions and neutrals to lower altitudes, FLIP artificially calculates large recombination rates. The loss in io nization increases and to maintain the nighttime ionosphere the downwa rd flow of ionization also increases.