PICKUP PROTONS AND PRESSURE-BALANCED STRUCTURES - VOYAGER-2 OBSERVATIONS IN MERGED INTERACTION REGIONS NEAR 35-AU

Five pressure-balanced structures, each with a scale of the order of a few hundredths of an astronomical unit (AU), were identified in two m erged interaction regions (MIRs) near 35 AU in the Voyager 2 plasma an d magnetic held data. They include a tangential discontinuity, simple and complex magnetic holes, slow correlated variations among the plasm a and magnetic held parameters, and complex uncorrelated variations am ong the parameters. The changes in the magnetic pressure in these even ts are balanced by changes in the pressure of interstellar pickup prot ons. Thus the pickup protons probably play a major role in the dynamic s of the MIRs. The solar wind proton and electron pressures are relati vely unimportant in the MIRs at 35 AU and beyond. The region near 35 A U is a transition region: the Sun is the source of the magnetic field, but the interstellar medium is the source of pickup protons. Relative to the solar wind proton gyroradius, the thicknesses of the discontin uities and simple magnetic holes observed near 35 AU are at least an o rder of magnitude greater than those observed at 1 AU. However, the th icknesses of the tangential discontinuity and simple magnetic holes ob served near 35 AU (in units of the pickup proton Larmor radius) are co mparable to those observed at 1 AU (in units of the solar wind proton gyroradius). Thus the gyroradius of interstellar pickup protons contro ls the thickness of current sheets near 35 AU. We determine the inters tellar pickup proton pressure in the PBSs. Using a model for the picku p proton temperature, we estimate that the average interstellar pickup proton pressure, temperature and density in the MIRs at 35 AU are (0. 53 +/- 0.14) x 10(-12) erg/cm(3), (5.8 +/- 0.4) x 10(6) degrees K and (7 +/- 2) x 10(-4) cm(-3), respectively.