Measurements of the topside ionosphere over Arecibo during the total solar eclipse 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 the 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 altitude range 146-2412 km. The Sheffield University plasmasphere ionosphere model was used to interpret the measurements. The electron temperature was found to have decreased by 600 K at 400 km altitude, but the magnitude of the decrease becomes smaller with increasing altitude. This is shown to be the result of the lesser degree of obscuration of the solar disk at latitudes north of Arecibo. Conjugate point photoelectron heating effects are also shown to play a significant role in the electron energy balance during the eclipse. The H+ ion temperature exhibited a response to the eclipse, with temperatures being around 200 K lower than expected at the time of maximum obscuration. There was relatively little variation observed in the O+ temperature. The response of the topside ionosphere is characterized by a downward motion arising from the contraction of the plasma due to reduced plasma temperatures. This is most clearly seen in the O+-H+ transition altitude which falls by 200 km. The transition altitude fully recovers within 2 hours after the eclipse. The location of the transition altitude acts to mitigate the effects of the eclipse on the topside electron densities.