Interplanetary H Lyα Observations from a Sounding Rocket

We describe a Self-Compensating All-Reflecting Interferometer used to measure diffuse interplanetary H Ly alpha emission (lambda similar to 1216 Angstrom) of interstellar origin at high spectral resolution (lambda/Delta lambda - 100,000). The instrument is capable of resolving the separation between the Doppler-shifted, interplanetary neutral H emission from the Earth's geocoronal emission line. We present the optical layout and mechanical design that enable the instrument to maintain alignment through a sounding rocket flight. We find an H Lya spectrum, including both geocoronal emission and interplanetary scattered features, which we have fitted using Gaussian line profiles. The interplanetary H line is fitted with a 7900 +/- 2400 K line width. The geocoronal line width is fitted with a 1900 +/- 500 K Gaussian. The Doppler shift between the geocoronal and interplanetary line is 0.145 Angstrom, corresponding to an apparent interplanetary wind velocity of 16 +/-7 km. s(-1). We show how full line-profile measurements of interplanetary H emission can advance current studies of the heliospheric interface region and very local interstellar medium.