SIMULTANEOUS, DUAL-POINT, IN-SITU MEASUREMENTS OF IONOSPHERIC STRUCTURES USING SPACE TETHERS - TSS-1R OBSERVATIONS

First ever simultaneous, dual-point, in situ measurements of natural i onospheric structures using widely spaced tethered sensors, dying in f ormation, were made during the reflight of the Tethered Satellite Syst em (TSS-1R) mission. A ''target-of-opportunity'' observation provided a direct comparison of structured ionospheric irregularity features at two altitudes near the South American geomagnetic equator at approxim ately 2000 hours local time and at an altitude of approximate to 300 k m. With the TSS-1R satellite and space shuttle separated by a vertical distance of 10 km, correlated plasma signatures detected by plasma in struments at each end indicated a strong eastward displacement in the irregularity features and possible growth of steepened features at the higher altitude. Observations made by a SUNDIAL ground station ionoso nde located north of the flight path also indicated considerable sprea d-F activity at the time. A SUNDIAL corrected ionospheric model indica ted that the shuttle was flying near the F-peak. Therefore, the strong ly correlated in situ observations were most likely associated with ir regularities in their early development or modulations near the F-peak due to equatorial spread-F (ESF). While the TSS-1R system was not opt imized for dual-point in situ ionospheric measurements, and the tether break eliminated additional equatorial zone observations planned for later in the mission, the results reported here nevertheless indicate that vertically correlated plasma features can exist and can also have strong structural variations as a function of altitude. Such variatio ns need closer examination in order to understand their effects on rad iowave scintillation. These observations also demonstrated the feasibi lity of tethered sensors in the study of ionospheric irregularities us ing controlled vertical sampling.