The Distance to the Solar Wind Termination Shock in 1993 and 1994 from Observations of Anomalous Cosmic Rays

Abstract

Using data from the Voyager and Pioneer spacecraft, we examine the energy spectra of anomalous cosmic rays (ACRs) during 1992–1994, a period of rapidly increasing intensities of these particles in the outer heliosphere. The 1992 period appears to be in nonequilibrium, marked by a rapid decrease in the tilt of the current sheet from ∼63° to ∼35° and a complex, evolving magnetic topology that affects interplanetary drifts and drift acceleration along the solar wind termination shock. In addition, large propagating interplanetary disturbances in 1991 may have weakened the shock temporarily, reducing the intensity of accelerated ions. The 1993–1994 period appears to be in quasi-equilibrium, marked by a stable tilt of the current sheet and a steady, large-scale magnetic topology that provides stable conditions for propagation and acceleration. The ACR intensities of higher-rigidity particles rise slowly during this period, suggesting that the shock intensity has stabilized. At lower rigidities the ACR intensities are increasing rapidly, which we attribute to decreasing modulation. During 1994 days 157–209, we estimate that the shock was at 85 ± 5 AU. The shock strength s is estimated to be 2.63 ± 0.14, significantly weaker than a strong shock (s = 4) and consistent with a termination shock modified by the pressure of galactic cosmic rays. By comparing these 1994 observations with those made in 1987, we estimate that the latitudinal gradient of ACR He with energies 31–57 MeV nucleon^(−1) is ∼1.9%/deg along the shock from the equator to the pole. We estimate that the ratio of the efficiency of injection of He^+ to H^+ pickup ions into the acceleration process is 7.0 ± 1.6.

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