Local Interstellar Medium

Scientific Investigation 1: Improve understanding of the composition and properties of the local interstellar medium. 

IMAP will help us understand the composition and properties of the interstellar material near our solar system from direct measurements of matter from the interstellar medium that moves through our heliosphere as electrically neutral atoms. Current uncertainties in measurements have prevented accurate determination of the conditions imposed by the interstellar medium. These conditions help control the shape and structure of the boundaries that surround our solar system and separate it from the local interstellar medium.  IMAPwill improve existing estimates by using an articulation platform to view the inflow of electrically neutral interstellar atoms from widely separated vantage points with the IMAP-Lo instrument. IMAP-Lo is optimized to resolve the distribution of Interstellar Stellar Neutral (ISN) atoms with high sensitivity. 


The galactic environment of the heliosphere. IBEX suggests that the heliosphere is inside the local interstellar cloud (LIC).

The improvements are essential because prior to IMAP, direct measurement of interstellar neutral atoms by IBEX and from Ulysses presented flow speed differences up to ~10% and directional differences up to a few degrees. While some of these discrepancies have been resolved, systematic effects were uncovered that impact the observations in different ways, including the discovery of a secondary neutral population. IMAP will make precise measurements of ISN He, O, and H to constrain the interstellar flow parameters and meaningfully differentiate the flow parameters of O, He, Ne, and H. The precise determination of Local Interstellar Medium (LISM) flow properties makes it possible to provide the first determination of outer heliosheath properties that influence our understanding of all secondary neutral populations, the fundamental physical mechanisms by which the interstellar medium interacts with the SW outflow including interstellar flow around the heliopause, the charge-exchange coupling with neutral populations within the outer heliosheath, and the physical processes controlling interactions with the interstellar magnetic field. Ultimately, IMAP’s precise measurements are critical for establishing how the interstellar flow interacts with and influences global heliospheric structure. 

IMAP will derive precise O and H flow speeds. The accurate determination of H, He, and O flow properties is fundamental for IMAP to help us understand how the ISM interacts with our solar system. This advance is key for understanding the properties of the local interstellar flow, which in turn controls how the interstellar medium physically interacts with the SW at the heliopause. IMAP enables more detailed understanding of the interstellar conditions, which is equally critical in determining how the Ribbon and Belt form in the LISM. Precise determination of the ISN parameters and the secondary components provides tight constraints on the ionization state of the LISM.

The LISM is the only locally accessible sample of extrasolar material and thus provides the state of galactic matter today in juxtaposition to solar material from 4.5 billion years ago. Hence, it provides us with critical insight into Big Bang nucleosynthesis and the galactic evolution in our neighborhood, which is representative of the galactic habitable zone. The Interstellar deuterium to hydrogen ratio (D/H) is a powerful probe of nucleosynthesis and the chemical evolution of the Milky Way galaxy, yet observations in the interstellar medium are scarce and line-of-sight integrated. The D/H ratio provides input on stellar evolution, and its contribution to the local matter inventory and the Ne/O ratio has ramifications on the balance between volatile and refractory elements, which requires observation of gas phase and dust composition in the LISM. 

IMAP will provide the first accurate in situ measurements of the flux, size distribution, and composition of Interstellar Dust (ISD) particles flowing through our solar system. Itwill make detailed composition measurements  to fill a significant knowledge gap in our current limited compositional measurements of ISD particles. The Cassini Cosmic Dust Analyzer (CDA) orbiting Saturn found 36 ISD particles all with similar chemical makeup of magnesium-rich silicate and oxide composition with iron inclusions. The Stardust mission returned even fewer 7 candidate ISD particles with diverse elemental composition, including sulfides in some of the particles that contradict the CDA results. The higher IMAP count rate provides an unparalleled opportunity to directly sample and thereby discover the chemical makeup of solid matter in our galactic environment with unmatched resolution, and move beyond the limitations of these prior measurements. 

IMAP also incorporates detailed information of the UV glow from inflowing H and He for complementary ionization and radiation pressure and SW measurements needed to interpret direct-sampling of ISN H, Ne, and O. Observations of the backscatter He glow enable assessment of the SW electron temperature inside 1 AU at all heliolatitudes, and constrains electron ionization rate of ISN gas. These measurements provide complementary knowledge of ionization rates, which cull interstellar neutral atoms, thereby influencing the spatial and velocity distributions of ISN H, He, O, Ne, and D. Direct ionization rate measurements significantly improve our understanding of the ISN flow properties. Current knowledge of electron ionization rate inside 1 AU is only thought to be accurate to within an order of magnitude, but likely significantly attenuates the flux of the indirect beams of ISN atoms.