Measurements of the topside ionosphere over Arecibo during the total solareclipse of February 26, 1998

The Arecibo incoherent scatter radar facility was operated on February 26, 1998, and was used to observe the total solar eclipse that occurred over th e Caribbean. A maximum of 87% obscuration was observed over Arecibo at 1430 LT(1830 UT). The radar was operated using an experimental technique, which uses a 300 mu s single/multi- frequency pulse, to gather data from the alt itude range 146-2412 km. The Sheffield University plasmasphere ionosphere m odel was used to interpret the measurements. The electron temperature was f ound to have decreased by 600 K at 400 km altitude, but the magnitude of th e decrease becomes smaller with increasing altitude. This is shown to be th e result of the lesser degree of obscuration of the solar disk at latitudes north of Arecibo. Conjugate point photoelectron heating effects are also s hown to play a significant role in the electron energy balance during the e clipse. The H+ ion temperature exhibited a response to the eclipse, with te mperatures being around 200 K lower than expected at the time of maximum ob scuration. There was relatively little variation observed in the O+ tempera ture. The response of the topside ionosphere is characterized by a downward motion arising from the contraction of the plasma due to reduced plasma te mperatures. This is most clearly seen in the O+-H+ transition altitude whic h falls by 200 km. The transition altitude fully recovers within 2 hours af ter the eclipse. The location of the transition altitude acts to mitigate t he effects of the eclipse on the topside electron densities.