Physics – Plasma Physics
Scientific paper
Apr 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008jgra..11304102m&link_type=abstract
Journal of Geophysical Research, Volume 113, Issue A4, CiteID A04102
Physics
Plasma Physics
6
Solar Physics, Astrophysics, And Astronomy: Magnetic Fields, Solar Physics, Astrophysics, And Astronomy: Radio Emissions, Space Plasma Physics: Wave/Particle Interactions (2483, 6984), Space Plasma Physics: Radiation Processes
Scientific paper
A recent theory for radio events at 2-3 kHz observed by the Voyager spacecraft suggests that the emission is generated when shocks associated with global merged interaction regions (GMIRs) enter a region just beyond the heliopause nose that is primed with an enhanced level of superthermal electrons. In this GMIR/priming theory the superthermal electrons are accelerated by lower hybrid waves generated by pick-up ions. For this acceleration to be efficient the pick-up ion ring speed v r and the Alfvên speed v A must satisfy the inequality v r /v A $\lesssim$ 5, implying that the local magnetic field B must be sufficiently large. Here this constraint is used to predict which regions generate radio emission by calculating the draping of the interstellar magnetic field B ∞ over the heliopause using the convected field equations and a gas-dynamic simulation of the solar wind-VLISM interaction. The size and shape of the regions with large |B| are predicted to depend on the orientation of B ∞ relative to the interstellar flow velocity. For sufficiently perpendicular orientations the high |B| region is a linear band parallel to B ∞ in the plane of the sky, centered near where the surface is closely parallel to B ∞, but the band shape is only a ~10% effect compared with a circular surrounding region. The magnetic amplification factor increases with decreasing distance to the heliopause nose and increasingly perpendicular orientation of B ∞, with factors $\gtrsim$5 typical within axial and transverse distances to the nose of 5 and 35 AU, respectively. Combining the magnetic amplification with plausible neutral and plasma parameters, the constraint v r /v A $\lesssim$ 5 requires B ∞ $\gtrsim$ 0.06 nT for the GMIR/priming theory to operate within the draping region. A recently proposed constraint, that B be nearly perpendicular to the normal vector $\hat{n to the GMIR surface for effective electron acceleration by the GMIR shock, is also considered. A supporting argument is provided for the previous claim that this constraint predicts strong emission in a band perpendicular to B ∞: Calculations show that the shock-accelerated electrons produce significant emission only for distances parallel to B that are small (~1 AU) compared with the macroscopic regions on the shock where B . $\hat{n ~ 0. This predicted source orientation agrees well with observations of the source and an independent estimate of the direction of B ∞ based on Lyman-α observations. It is argued that the B . $\hat{n ~ 0 constraint is a natural component of the GMIR/priming theory. The large, relatively circular nature of the draping region where v r /v A $\lesssim$ 5 will plausibly lead to the constraint B . $\hat{n ~ 0 determining the intrinsic source shape in the plane of the sky.
Cairns Iver H.
Mitchell Jeffrey J.
Pogorelov Nikolai V.
Zank Gary P.
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