Heliosheath Plasma Flow Slowdown Observed by Voyager 1: Explanation and Implications of the Transition Layer

A striking feature in the approach of Voyager 1 to the heliopause was the nearly linear decrease during 2.5 years (2007.8-2010.3) in the inferred radial plasma flow velocity (Vr) from 60km/s to zero (Krimigis et al., Nature, 474, 359-361, 2011). However, the LECP energetic ion intensities (40keV < E < 4MeV) remained relatively constant during this time. Subsequently, after remaining near zero for 8 months (2010.3-2010.9), the radial flow went negative (-25km/s < Vr < 0) and has remained so for the past 6 months (2011.0-2011.5). The azimuthal flow has been small and positive (10 < Vphi < 25km/s) throughout these past 14 months in this heliosheath transition layer. A satisfactory explanation of this striking signature in the radial flow should quantitatively predict both the 2.5 yr linear decrease and the immediately following zero and then small negative radial flows. The signature does indeed naturally follow from the generalized transport equations for a composite thermal and non-thermal plasma (Roelof, AIP Conf. Proc., 1302, 133-141, 2010). The familiar gradP and JxB terms (along with weak pitch-angle scattering of particles along field lines) sustain the nearly constant non-thermal energetic ion intensities throughout the heliosheath in the presence of small temporal disturbances, while the much more slowly responding inertial term VgradV of the thermal plasma (that contains most of the mass) balances a frictional term produced by charge exchange of the non-thermal energetic population (that exerts most of the pressure) with the interstellar H-atoms. The relation between the latter two terms contains no free parameters or functions, involving only the observed VGR1 flow decay rate (dVr/dt=-18.8km/s/yr), the density of interstellar H-atoms, and the charge-exchange rate for 1-10keV non-thermal protons. The quantities agree within a factor of 2, i.e., within observational uncertainties. The zero velocity follows naturally from this frictional slowing-down process, and Vr eventually goes negative because the rest frame of the charge-exchange process is that of the interstellar neutral H-atoms themselves, which at the latitude of VGR1 has a radial velocity of (-26 km/s)cos(34 deg)=-22.5 km/s. Mass is conserved during the slowing down through weak gradients in the transverse plasma flow. The foregoing explanation implies that the plasma flow in the transition layer comes to rest approximately in the frame of the neutral interstellar medium, even though VGR1 has apparently not yet entered the ionized interstellar medium.