Structure of the Heliospheric Termination Shock Revisited

Details Structure of the Heliospheric Termination Shock Revisited Structure of the Heliospheric Termination Shock Revisited

Dec 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003agufmsh12c..06a&link_type=abstract

American Geophysical Union, Fall Meeting 2003, abstract #SH12C-06

Physics

7839 Nonlinear Phenomena, 7851 Shock Waves, 7871 Waves And Instabilities

Scientific paper

With the possibility that the Voyager spacecraft are flirting with a termination shock crossing, the question of just what might be observed is assuming greater importance. Although the shock is expected to be quasi-perpendicular, suprathermal pickup ions and superthermal anomalous cosmic rays are likely to play a fundamental role in determining the shock structure and associated scale-lengths. This has led to a model termination shock in which upstream pickup ions decelerate the incident solar wind flow, reflected pickup ions determine the scaling of the foot ahead of a subshock transition, and anomalous cosmic rays create a smooth precursor ahead of the shock which scales with the anomalous cosmic ray diffusive length scale. The general picture is therefore one of a smoothed shock with well-defined length scales. However, the structure of a cosmic ray mediated termination shock is unlikely to be as simple as suggested. We show that the gradient in shock accelerated anomalous cosmic rays can generate strong compressive instabilities in the foreshock and strong incompressible instabilities along the face of the quasi-perpendicular termination shock. The latter are particularly effective in promoting the growth of large magnetic fields at the shock. These cosmic ray gradient driven hydrodynamic and MHD instabilities are analogous to a generalized form of Rayleigh-Taylor instability. Secondly, we show that the pickup ions themselves can drive a buoyancy-like hydrodynamic instability which is further amplified in combination with the cosmic ray driven instabilities. The picture of the termination shock that emerges is instead one that is highly turbulent on hydrodynamic and MHD scales and bears little resemblance to the conventional smoothed cosmic ray mediated shock model. In view of the highly turbulent nature of the shock, we reconsider particle transport in the presence of a highly perturbed flow field and find that the usual cosmic ray transport equation is modified substantially. The new transport equation is solved for conditions appropriate to the termination shock and the resulting anomalous cosmic ray spectra described.

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