Interstellar Probe to Explore the Sun’s Influences Propagating Beyond the Heliosphere Parisa Mostafavi�, G. P. Zank�, D. J. McComas�, L. Burgala4, J. Richardson5, G. Webb2, E. Provornikova1, P. Brandt1 1. Johns Hopkins University Applied Physics Lab, 2. University of Alabama in Huntsville, 3. Princeton University, 4. NASA Goddard Space Flight Center, 5. Massachusetts Institute of Technology

Introduction The heliosphere is controlled by the motion of the Sun through the interstellar space. Although humankind has explored the region of space about the Earth quite extensively, the distant heliosphere beyond the planets remains almost entirely unexplored with only the two Voyager spacecraft, New Horizons, and the early Pioneer 10 and 11 spacecraft returning the in-situ observations of this most distant region. Moreover, remote observations of energetic neutral atoms (ENAs) from the Interstellar Boundary Explorer (IBEX) at 1 au and Cassini/INCA at 10 au revealed interesting structure related to the interstellar medium. Voyager 1 and 2 crossed the heliopause in 2012 and 2018, respectively, and are both continue to make in-situ measurements of the very local interstellar medium (VLISM; the nearby region of the LISM affected by physical processes associated with the heliosphere) for the first time.

Voyager 1 and 2 have identified and partially answered many interesting questions about the outer heliosphere and the VLISM while raising numerous new questions, many of which have profound implications for the detailed structure and properties of the heliosphere and our place in the galaxy. One particularly interesting topic is the influence of the large-scale disturbances and small-scale turbulences generated by the dynamical Sun on the VLISM. This requires a spacecraft dedicated to the study of the outer heliosphere and the ISM to directly measure these shocks, pressure waves, and turbulences with suitable instrumentation. The technology for the interstellar probe mission will be available by 2030 and it can finally help us to understand the transition from the VLISM to the LISM. Pickup ions Very Local Interstellar Medium New Horizons observations: Heliospheric shocks and pressure waves PUIs dominate the internal New Horizons observation propagate into the heliosphere are partially pressure of the outer transmitted and partially reflected at the HP, and heliosphere (outside ~ 20 au) are then observed in the VLISM (Gurnett et al. (McComas et al. 2017). 2013). PUIs in the IHS and VLISM change the plasma properties, Observed shocks by Voyager 1 in the VLISM: typically increasing the sound speed with a concomitant 2012 VLISM shock was extremely broad. Frequency–time spectrogram of electron plasma oscillations detected by the V1, and the corresponding local magnetic field strength from the magnetometer. Two vertical weakening or even elimination dashed lines indicate two observed shocks in the VLISM. (Gurnett & Kurth 2019) McComas et al. (2017) of the interstellar bow shock. Mostafavi & Zank (2018) showed that the weak VLISM is collisional with To understand the basic physics of the outer heliosphere and the ISM, it respect to the thermal plasma and determined that the weak VLISM shock structure is due to thermal particle collisions and not mediated by wave- is necessary to directly measure PUIs in the heliosphere and the LISM 8.7 days and then understand their basic properties as a function of distance. particle interactions. However, heliospheric shocks è are guided by collisionless processes. Termination Shock The collisionality of the VLISM and the properties of shocks ≈1.4 need to be studied more extensively with regard to distance Although Voyager 1 and 2 are Voyager 2 observations from the HP and orientation of the LISM magnetic field. not instrumented to measure Interstellar Probe to directly measure these shocks with PUIs, the Voyager 2’s crossing of 2012 VLISM shock; Burlaga et al 2013 suitable instrumentation. the heliospheric termination Turbulence in VLISM: different than turbulence in the heliosphere. shock (HTS) indicated that the M > 1 Shocks may be responsible in part for localized small-scale turbulence and waves in the VLISM. primary dissipation mechanism Richardson et al. (2008) at the HTS is not provided by Supersonic Fraternale et al. (2020) found an increase in transverse magnetic field fluctuations a few days before Voyager 1 thermal gas since the entered the electron foreshock of 2012 VLISM shock. downstream of the HTS is Turbulence may be related to the observed anisotropy of galactic cosmic rays (GCRs), and this anisotropy may supersonic with respect to the provide a remote-sensing diagnostic of VLISM shock waves (Gurnett & Kurth 2019). GCR streaming due to thermal gas. (Richardson et al. reflection at the shock front may be responsible for the observed turbulence. 2008) and thus it is completely Interstellar Probe can study turbulence in the interstellar medium with suitable instrumets. � > 1 mediated by PUIs (Mostafavi et � al. 2018). PUIs are reflected Subsonic Summary Combined Mach number Mach Combined preferentially at the cross-shock • PUIs dominate the internal pressure in the electrostatic potential of the �� < 1 outer heliosphere and inner heliosheath Simulation of HTS crossing. Mach number HTS and acquire almost all the through the HTS with and without the è The evolution of PUIs in the dissipative heating of the bulk inclusion of PUIs. (Mostafavi et al. 2018). heliosphere and interstellar medium needs flow energy. The dissipation process at the HTS is therefore due to be studied with in-situ observations. primarily to the nearly-isotropically PUIs and not to thermal • The origin and evolution of VLISM pressure protons. waves are not well understood yet. • The very different physics of the outer heliosphere and the VLISM affects the Inner Heliosheath properties of shocks and turbulence in Voyager 2 observations these regions. show that the IHS is dominated by a large energetic particle pressure A spacecraft that explores the heliospheric boundary regions, the VLISM and the transition to the (Decker et al. 2015) and the LISM is essential if we are to ultimately understand our place in the galaxy. The future Interstellar plasma beta, when the Probe mission will directly explore the outer heliosphere and the interstellar medium by making energetic particle pressure both in-situ and remote measurements of these regions. is included, is very large. è Note that Voyager Low energetic particle pressure, magnetic field cannot measure PUIs. pressure, and thermal proton pressure. b: Plasma beta Acknowledgement with and without the inclusion of low energy particles Parisa Mostafavi acknowledges the support by John Hopkins University Applied Physics Laboratory independent R&D funds.