Interplanetary H Lyα Observations from a Sounding Rocket

We describe a Self-Compensating All-Reflecting Interferometer used to measure diffuse interplanetary H Lyα emission (λ ~ 1216 Å) of interstellar origin at high spectral resolution (λ/Δλ ~ 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 Lyα 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 Å, 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.