Proton spectral radiation environment of the outer heliosphere and local interstellar medium
Abstract
Proton flux measurements at plasma to cosmic ray energies from the Voyager 1-2, Interstellar Background Explorer (IBEX), and Cassini spacecraft each provide partial basis for development of time-averaged flux models with applications to dynamics of particle acceleration, pressure balance at the heliopause and space weathering of extreme Kuiper Belt, Oort Cloud, and interstellar (e.g., 1l'Oumuamua) objects. Each spacecraft presently covers a unique range of space and energy with Voyager 1 reported to be in the local interstellar medium, Voyager 2 still in the heliosheath, and with IBEX and Cassini (through Sept. 2017 end of mission) remotely measuring energetic neutral atoms (ENAs) from some combination of heliosheath and local interstellar sources. Voyager 2 provides the only in-situ measurements of plasma, suprathermal, and higher energy proton distributions in the heliosheath, while IBEX and Cassini indirectly provide measurements of the suprathermal proton flux spectra from models for the ENA sources. Voyager 1 provides the only direct measurements of proton and heavier ion fluxes beyond the heliopause at few MeV and higher energies, while lower energy fluxes are respectively precluded at plasma and suprathermal energies by an inoperative sensor and penetrating cosmic ray background. The so-called secondary-ENA IBEX source beyond the heliopause in the very local interstellar medium could include a mixture of ENAs from charge-exchange interactions of heliospheric and interstellar suprathermal ions, although solar cycle dependence suggests a dominance of the heliospheric source. It has been suggested that remote observations of interstellar molecular cloud ionization rates limit the suprathermal flux spectrum, but sub-MeV ions would not penetrate into such clouds and would more likely arise internally as pickup ions in stellar wind flows within the clouds. Current limits on pressure balance at the heliopause do not preclude a substantial suprathermal component from the local interstellar environment. Future Voyager-2 measurements beyond the heliopause will directly sample the plasma environment but suprathermal fluxes would still be hidden beneath penetrating background and could only be measured by a future interstellar probe. We use moment and flux distribution data from the spacecraft sources, as available from the NASA Space Physics Data Facility (SPDF), the NASA Virtual Energetic Particle Observatory (VEPO), and instrument team publications, to compile time-averaged data models for the proton flux spectra in the heliosheath and in the local interstellar medium. These models are then applied to sample calculations of total proton pressures, re balance at the heliopause, and of space weathering parameters for radiolysis and sputtering on more distant icy bodies of the Kuiper Belt and Oort Cloud.
- Publication:
-
42nd COSPAR Scientific Assembly
- Pub Date:
- July 2018
- Bibcode:
- 2018cosp...42E.694C