Exploring the Past and Future Boundary Conditions of the Heliosphere with Interstellar Probes

Abstract:

Multiple studies indicate the physical properties of interstellar material around and inside of the heliosphere are ordered by the interstellar magnetic field (ISMF), radiation from the hot star eps CMa, and the kinematics of the ancient expanding Loop I super-bubble [1]. We observe interstellar material from the perspective of the solar motion toward the Hercules constellation, which requires a correction to obtain cloud velocities in the Local Standard of Rest (LSR). The Sun is embedded in a flow of interstellar material that arrives from a direction within several degrees of the nearest star, alpha Cen, after correcting to the LSR. The LSR velocity of this cloud is perpendicular to the ISMF found from the IBEX ribbon and interstellar polarization data [1,2,3]. A worthy goal of Interstellar Probe missions would be to utilize our knowledge of the kinematics of nearby interstellar gas in order to explore the future and past galactic environments of the heliosphere. Selection of spacecraft trajectories aligned with the direction of the ambient interstellar wind through the LSR projected to a future or past position of the Sun will provide detailed data on the heliosphere boundary conditions for additional points in time. These twin interstellar probe missions would probe both the past and future interstellar environments of the Sun, traverse the ISMF at different angles, and sample cosmic rays and energetic ions at different angles relative to the magnetic field. The two probes would see interstellar material with different ionization conditions because of the expected cloud patchiness and ionization gradients from the eps CMa radiation field [1]. These options require understanding the angle between the solar apex motion and the velocity vector of the current interstellar cloud enveloping the heliosphere. Non-thermal characteristics of the gas velocity field and filling factor uncertainties for the nearest interstellar gas will be discussed. Selection of trajectories is key to accurately constraining the heliosphere boundary conditions during different epochs. Refs: [1] Frisch et al. ARAA 2011 [2] Schwadron et al. Astra 2015 [3] Frisch et al. ApJ 2015 and to be submitted.