Heliosheath Properties Measured from a Voyager 2 to Voyager 1 Transient

Kavli Affiliate: John D. Richardson

| First 5 Authors: Jamie S. Rankin, David J. McComas, John D. Richardson, Nathan A. Schwadron,

| Summary:

In mid-2012, a GMIR observed by Voyager 2 crossed through the heliosheath and
collided with the heliopause, generating a pressure pulse that propagated into
the very local interstellar medium. The effects of the transmitted wave were
seen by Voyager 1 just 93 days after its own heliopause crossing. The passage
of the transient was accompanied by long-lasting decreases in galactic cosmic
ray intensities that occurred from ~2012.55 to ~2013.35 and ~2012.91 to
~2013.70 at Voyager 2 and Voyager 1, respectively. Omnidirectional (>20 MeV)
proton-dominated measurements from each spacecraft’s Cosmic Ray Subsystem
reveal a remarkable similarity between these causally-related events, with a
correlation coefficient of 91.2% and a time-lag of 130 days. Knowing the
locations of the two spacecraft, we use the observed time-delay to calculate
the GMIR’s average speed through the heliosheath (inside the heliopause) as a
function of temperature in the very local interstellar medium. This, combined
with particle, field, and plasma observations enables us to infer previously
unmeasured properties of the heliosheath, including a range of sound speeds and
total effective pressures. For a nominal temperature of ~20,000 K just outside
the heliopause, we find a sound speed of 314 (+/-) 32 km/s and total effective
pressure of 267 (+/-) 55 fPa inside the heliopause. We compare these results
with the Interstellar Boundary Explorer’s data-driven models of heliosheath
pressures derived from energetic neutral atom fluxes (the globally distributed
flux) and present them as additional evidence that the heliosheath’s dynamics
are driven by suprathermal energetic processes.

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