Lys/STELLA: H Lyman Alpha Spectrograph for the Interstellar Probe

The Interstellar Probe project gives an unprecedented opportunity to study the hydrogen atom distribution from the interstellar medium to the inner heliosphere. The solar H Lyman alpha emission (121.6nm) is the brightest line in the UV range. Solar Lyman alpha photons are backscattered by hydrogen atoms in the interplanetary medium producing the interplanetary glow that extends far beyond the heliopause into the interstellar medium. A Lyman alpha spectrograph will measure the LISM H number density giving the first direct measurement of this quantity just outside of the heliospheric interface. This value is one of the critical parameters defining the size and behavior of the heliospheric interace. With a high resolution spectrograph, it will be possible to differentiate between the Lyman alpha galactic emission derived from the UVS-Voyager data and the LISM H Lyman alpha emission from the line of sight velocity of the atoms. Because of resonant charge exchange between the hydrogen atoms and the protons, the H atom distribution is strongly affected when the neutrals cross the heliospheric interface region. H atoms created after charge exchange keep the velocity distribution of the protons that they were created from. Therefore, the backscattered Lyman alpha line profile will change as the interstellar probe crosses through the inner heliosheath to the outer heliosheath and then moves into the LISM, providing a test on the proton distribution in the heliosphere regions crossed by the interstellar probe. Here, we will present an instrumental design that will allow for this study bringing new information on the heliospheric interface and the very local interstellar medium.