The Science of Our Sun’s Astrosphere: In-Situ Ions from the Voyagers and Remotely Sensed ENAs from Cassini

The path-breaking observations of the two Voyager spacecraft over the past two decades, have revolutionized our understanding of interplanetary space within our solar bubble. The crossings of the Voyagers (V1, V2) of the Termination Shock (TS) led to the discovery of the previously unknown reservoir of ions and electrons that constitute the heliosheath, whereas the combination of in-situ particle and fields measurements from V1 and V2 with remote images of ~5.2 to 55 keV ENAs from Cassini/INCA at 10 AU, revealed a number of previously unanticipated heliospheric structures such as the “Belt”, a region of enhanced particle pressure inside the heliosheath. The V1 and V2 crossings of the heliopause (HP) pinpointed the extent of the upwind heliosphere’s expansion into the VLISM and its rough symmetry. We will provide a brief discussion for the contribution of the >28 keV Voyager 1 & 2/LECP observations that established “ground truth” to the ENA images from Cassini/INCA towards addressing longstanding, fundamental questions for the heliosphere’s interaction with the Very Local Interstellar Medium (VLISM), such as the shape and properties of the ion spectra inside the heliosheath, the acceleration of low energy ions and Anomalous Cosmic Rays (ACR) in the heliosheath, the pressure balance and plasma beta in the heliosheath that dictate the magnitude of the magnetic field upstream at the heliopause, the thickness of the heliosheath, the effects of the solar cycle through the outward propagating solar wind that result in an “breathing” (inflating and deflating) heliosphere, together with the implications of these measurements towards addressing the global shape of the heliosphere. The crossings of V1 and V2 from the HP revealed that the primary driver of the interaction of the heliosphere with the VLISM is the pressure of the interstellar (IS) magnetic field, whereas this interaction is more complex than previously thought: The V1 crossing of the HP was associated with the discovery of a flow stagnation region, possibly due to flux tube interchange instability. Further, V1 showed the existence of a radial inflow of low energy ions within the HS for ~9 AU before the HP, and a small radial outflow over a spatial scale of at least 33 AU past the HP, that corresponds to an ion population leaking from the HS into interstellar space. The use of these observations drive the requirements for the particle and fields measurements for a possible future Interstellar Probe mission.