INCOHERENT-SCATTER RADAR OBSERVATIONS OF THE F-REGION IONOSPHERE AT ARECIBO DURING JANUARY 1993

We report incoherent scatter radar observation of the F-region ion dri ft, plasma density, and electron and ion temperatures during the Janua ry 1993 World Day 10-day campaign. Although the observed ion drift dur ing the geomagnetically quiet period agrees with previous observation in the horizontal direction to a large extent for similar geophysical conditions, the vertical ion drift shows a very large downward compone nt throughout the entire day, which averages at about 20 m s(-1) near the F-region peak. Such a large average downward plasma drift has not been reported before at low latitudes. The downward motion of the F-re gion plasma comes mainly from the component along the geomagnetic fiel d line, which is chiefly controlled by the meridional wind during the day and ambipolar diffusion during the night. The E x B drift also con tributes to the downward motion of the F-region plasma during the nigh ttime although it is in anti-correlation with the variation introduced by the ion motion along the field line. During the geomagnetically di sturbed period on 25 January, the ion drifts showed large departure fr om the quiet period. The perturbed eastward ion drift was highly corre lated with that along the magnetic field, which may be indicative of t he presence of field-aligned current. Our observation shows that the d aytime electron concentration and electron temperature have contrastin g correlations at different altitude ranges in the F-region. They are positively correlated below 180 km, anti-correlated between 220-400 km , not well correlated between 440-620 km, and again positively correla ted at 660 km. While the positive correlation at 220-400 km has been o bserved previously and is well understood, the positive correlations b elow 180 km and at 660 km are less known and have not been systematica lly studied. Using the electron energy balance equation, we show that the contrasting relationships between electron concentration and elect ron temperature depend on the ion composition at lower altitudes; and on how electron heating via photoionization compares with thermal cond uction at higher altitudes. Our observation shows that plasma motion, electron concentration and the thermal characteristics of the F-region ionosphere are all interrelated. We also present the day-to-day varia bility of plasma drift, electron and ion temperatures as well as elect ron concentration. (C) 1997 Elsevier Science Ltd. All rights reserved.