EUVE MEASUREMENT OF THE LOCAL INTERSTELLAR WIND AND GEOCORONA VIA RESONANCE SCATTERING OF SOLAR HE-I 584-ANGSTROM LINE EMISSION

We present results from EUVE measurements obtained during the all-sky survey of interplanetary and geocoronal He I 584-Angstrom emission. Th e data consist of count rates from the long wavelength spectrometer an d the long wavelength photometric band (520-750 Angstrom) of scanner C obtained over a 1-year period from July 1992 to July 1993. During thi s period, EUVE was in survey mode so that the scanners made 5 degrees x 360 degrees sweeps of the sky in a plane perpendicular to the Sun-Ea rth line, while the spectrometers were aligned with the antisolar dire ction. The interplanetary He I signal is morphologically consistent wi th previous observations with similar observing geometry, such as Prog noz 6 [Dalaudier et al., 1984]. However, unlike the Prognoz 6 data, th e EUVE measurements were made from low Earth orbit (520 km) and so con tain geocoronal emission as well. As a result, along sight lines where the relative speed between the interplanetary wind and the Earth is a t a minimum, extinction of the interplanetary signal through resonance scattering by the He geocorona occurs. We believe this to be the firs t detection of line extinction of the local interstellar He wind emiss ions by the geocorona. We find that the geocoronal extinction signatur es provide a new means of determining the interstellar He wind vector and emission line profile and add further constraints on the values of other interplanetary and solar He parameters and the morphology of th e He geocorona. On the basis of model fits of the observed interplanet ary emission and geocoronal extinction, we determine values for the in terplanetary wind ecliptic longitude lambda = 76.0 +/- 0.4 degrees, la titude phi = -5.4 +/- 0.6 degrees (downwind direction), speed upsilon( 0) = 26.4 +/- 1.5 km s(-1), and temperature T-0 = 6900 +/- 600 K. In a ddition, assuming an interplanetary He density of 0.01 cm(-3), we dete rmine an average solar He I 584-Angstrom line center flux of 1.4 +/- 0 .3 x 10(10) photons cm(-2) s(-1) Angstrom(-1) for the data analyzed he re.