科研之友
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摘要
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 Alfven speed v(A) must satisfy the inequality v(r)/v(A) less than or similar to 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(infinity) 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 vertical bar B vertical bar are predicted to depend on the orientation of B infinity relative to the interstellar flow velocity. For sufficiently perpendicular orientations the high vertical bar B vertical bar region is a linear band parallel to B(infinity) in the plane of the sky, centered near where the surface is closely parallel to B(infinity), but the band shape is only a similar to 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(infinity), with factors greater than or similar to 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 vr/vA less than or similar to 5 requires B(infinity) greater than or similar to 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 (n) over cap 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(infinity): Calculations show that the shock-accelerated electrons produce significant emission only for distances parallel to B that are small (approximate to 1 AU) compared with the macroscopic regions on the shock where B.(n) over cap approximate to 0. This predicted source orientation agrees well with observations of the source and an independent estimate of the direction of B(infinity) based on Lyman-alpha observations. It is argued that the B . (n) over cap approximate to 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) less than or similar to 5 will plausibly lead to the constraint B . (n) over cap approximate to 0 determining the intrinsic source shape in the plane of the sky.
- 出版日期2008-4-9
