Physics
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufmsh21b1603m&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #SH21B-1603
Physics
7837 Neutral Particles (2151), 7847 Radiation Processes
Scientific paper
The 2-3 kHz radio emissions observed by the Voyager spacecraft are thought to result from the acceleration of nonthermal electrons by shocks associated with global merged interaction regions (GMIRs) traveling through the outer heliosheath. It has been proposed that the nonthermal electrons are accelerated by lower-hybrid waves, generated by pick-up ions which form ring beam distribution functions in velocity space perpendicular to B. Here, we calculate the distribution function of pick-up ions formed in the outer heliosheath by charge exchange with neutrals born in the outer heliosheath (region 1), inner heliosheath (region 2), and solar wind (region 3). It is found that pick-up ions formed by charge exchange with the slow region 2 neutrals do not have ring-beam distributions as previously thought. These pick-up ions only form a shoulder without large gradients ∂ f/∂ v⊥ able to drive waves, since the neutrals are rather warm. Instead, ring-beams are formed by charge exchange with region 3 neutrals. This causes the ring speed to be a factor of ~~4 higher and the number density to decreased by a factor of ~~1/10 compared with the earlier estimates. The maximum energy available for waves and so particle acceleration increases by a factor of ~ 1.6. With this change in the source population of neutrals/pick-up ions the earlier theory for the radio emissions [Cairns and Zank, 2002, Mitchell et al., 2004] proceeds without qualitative changes. Combining this higher ring beam speed vr with constraints on the ratio vr/vA for lower-hybrid waves to accelerate electrons effectively increases the maximum magnetic field in the VLISM to ~~0.2~nT (here vA is the Alfvén speed). Calculations are ongoing for the spatial variation in the pick-up ion number density as a function of interstellar magnetic field orientation. It is found that pick-up ion kinematics is vital and that v∥ ≫ vE × B, contrary to earlier expectations. Cairns, I.~H., and G.~P. Zank (2002), Geophys. Res. Lett., 29(7), 47. Mitchell, J.~J., I.~H. Cairns, and P.~A. Robinson (2004), J. Geophys. Res., 109, 6108.
Cairns Iver H.
Mitchell Jeffrey J.
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