Effects of the solar wind termination shock on the modulation of Jovian and galactic electrons in the heliosphere

Abstract

A drift-modulation model with a solar wind termination shock (TS), including a Jovian electron source, was used to study the effects of this shock on electron spectra and radial intensity profiles. With electron measurements available up to ∼70 AU at ∼16 MeV from Pioneer 10, we compared computations with these observations using new diffusion coefficients and a new local interstellar electron spectrum (LIS). Satisfying the constraints imposed on the diffusion tensor by the observations, the effects of varying the location of the heliospheric TS and the location of the heliospheric boundary were investigated and illustrated. We find that Jovian and especially low-energy galactic electrons can be reaccelerated effectively at the TS. Reasonable compatibility between the model and the measurements is produced with the shock radius present at ∼90 AU and the outer boundary radius at 120-130 AU. The intensity of the Jovian electrons is too low at the TS to contribute meaningfully to the TS-accelerated and combined galactic and Jovian electron intensity at all energies. The shock acceleration results in moderate but still significant changes on the combined galactic and Jovian electron radial intensity profiled at 16 MeV with respect to nonshock solutions. A change in the radial gradient from positive upstream to negative downstream close to the shock (within ∼5 AU) is caused. These effects can be enlarged by decreasing the shock radius while keeping the outer boundary radius fixed or, more effectively, by moving the outer boundary outward while keeping the shock radius fixed. Between ∼100 MeV and ∼2 GeV the effects of the TS on modulated spectra are large and quite significant, even at Earth. The higher the LIS is made, the more noticeable the shock's effect on the 16 MeV radial profile (gradients) becomes. If the distance between the shock and outer heliospheric boundary was kept constant, the impact of the TS on the 16 MeV radial profile remains qualitatively unchanged, regardless of the position of the TS. In conclusion we consider the location of the outer boundary with respect to the shock location as more significant to electron modulation than the actual location of the termination shock. This model and approach confirm that a considerable amount of modulation can take place in the outer heliosphere, especially in the region between the TS and the modulation boundary, depending on the energies considered and the value of the LIS. Copyright 2002 by the American Geophysical Union.