SPRG Seminars
September 24, 2013:
"Voyager 1 enters the disturbed interstellar plasma"
Edmond C. Roelof, Applied Physics Laboratory, John Hopkins University
The consensus of the Voyager-1 Science Working Team is that the spacecraft (launched in 1997) entered the disturbed interstellar plasma on 25 August 2012. It has remained in it ever since. This crossing has been deemed the "heliopause", and it is the outer boundary of the hot (non-thermal) plasma of the heliosheath (the region beyond the solar wind termination shock, crossed 16 December 2004) . Precursor activity in the heliosheath had been detected as early as the beginning of May of 2012. However, the consensus announcement of the heliopause crossing was delayed until just this month because the suite of then-available measurements were not consistent with the properties of the heliopause predicted by theory and computer simulations. The definitive measurements were later made in Oct-Nov 2012 and March 2013, when plasma waves were detected at frequencies corresponding to in situ electron densities of 0.06 and 0.08 /cc, respectively. These densities are ~60 to 80 times the plasma densities within the helioheath, and are comparable to the densities estimated for the local interstellar plasma. Nonetheless, the instruments on Voyager-1 are currently detecting the influence of solar wind disturbances on the galactic cosmic ray population. Consequently, Voyager-1 is not in the "pristine" interstellar plasma, but rather is still in the interstellar plasma that is locally "disturbed" by having to flow around the obstacle of the heliosheath.