THE DISTANCE TO THE SOLAR-WIND TERMINATION SHOCK IN 1993 AND 1994 FROM OBSERVATIONS OF ANOMALOUS COSMIC-RAYS

Using data from the Voyager and Pioneer spacecraft, we examine the ene rgy spectra of anomalous cosmic rays (ACRs) during 1992-1994, a period of rapidly increasing intensities of these particles in the outer hel iosphere. The 1992 period appears to be in nonequilibrium, marked by a rapid decrease in the tilt of the current sheet from similar to 63 de grees to similar to 35 degrees and a complex, evolving magnetic topolo gy that affects interplanetary drifts and drift acceleration along the solar wind termination shock. In addition, large propagating interpla netary disturbances in 1991 may have weakened the shock temporarily, r educing the intensity of accelerated ions. The 1993-1994 period appear s to be in quasi-equilibrium, marked by a stable tilt of the current s heet and a steady, large-scale magnetic topology that provides stable conditions for propagation and acceleration. The ACR intensities of hi gher-rigidity particles rise slowly during this period, suggesting tha t the shock intensity has stabilized. At lower rigidities the ACR inte nsities are increasing rapidly, which we attribute to decreasing modul ation. 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, signi ficantly weaker than a strong shock (s = 4) and consistent with a term ination shock modified by the pressure of galactic cosmic rays. By com paring these 1994 observations with those made in 1987, we estimate th at the latitudinal gradient of ACR He with energies 31-57 MeV nucleon( -1) is similar to 1.9%/deg along the shock from the equator to the pol e. 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.