The plasma environment of comet 67P/Churyumov-Gerasimenko, the Rosetta mission target comet, is explored over a range of heliocentric distances throughout the mission: 3.25 AU (Rosetta instruments on), 2.7 AU (Lander down), 2.0 AU, and 1.3 AU (perihelion). Because of the large range of gas production rates, we have used both a fluid-based magnetohydrodynamic (MHD) model as well as a semi-kinetic hybrid particle model to study the plasma distribution. We describe the variation in plasma environs over the mission as well as the differences between the two modeling approaches under different conditions. In addition, we present results from a field aligned, two-stream transport electron model of the suprathermal electron flux when the comet is near perihelion.

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Three-dimensional Helios plasma and field data are used to investigate the relative changes in direction of the velocity and magnetic field vectors across tangential discontinuities, (TDs) in the solar wind at solar distances of 0.29-0.50 AU. It is found for tangential discontinuities with both Av and AB/B large that Av and AB are closely aligned with each other, in agreement with the unexpected results of previous studies of tangential discontinuities observed at 1 AU and beyond. It is shown that this effect probably results from the destruction by the Kelvin-Helmholtz instability of TDs for which Av and AB are not aligned. The observed decrease in the number of interplanetary discontinuities with increasing solar distance may be associated with the growth of the Kelvin-Helmholtz instability with decreasing Alfv6n speed.Tsurutani, B. T., and E. J. Smith, Interplanetary discontinuities: Temporal variations and the radial gradient from 1 to 8.5 AU, J. Geophys. Res., 84, 2773, 1979.

Conditions in the protosolar nebula have left their mark in the composition of cometary volatiles, thought to be some of the most pristine material in the solar system. Cometary compositions represent the end point of processing that began in the parent molecular cloud core and continued through the collapse of that core to form the protosun and the solar nebula, and finally during the evolution of the solar nebula itself as the cometary bodies were accreting. Disentangling the effects of the various epochs on the final composition of a comet is complicated. But comets are not the only source of information about the solar nebula. Protostellar disks around young stars similar to the protosun provide a way of investigating the evolution of disks similar to the solar nebula while they are in the process of evolving to form their own solar systems. In this way we can learn about the physical and chemical conditions under which comets formed, and about the types of dynamical processing that shaped the solar system we see today.This paper summarizes some recent contributions to our understanding of both cometary volatiles and the composition, structure and evolution of protostellar disks.

The location of the Venus bow shock is found to be sensitive to both the angle of the interplanetary magnetic field (IMF) to the solar wind flow and the phase of the solar cycle. We attribute the former dependence to the effect of planetary ion pickup by the magnetosheath convection electric field analogous to cometary ion pickup by the solar wind electric field. We attribute the latter dependence to the solarcycle variation in the density of exospheric neutrals in the magnetosheath analogous to the cometary response to distance from the sun. We also find both a north-south and a pole-equator asymmetry in the location of the bow shock, controlled by the direction of the IMF in the plane perpendicular to the solar wind flow, for those data where the perpendicular component dominates. These observations suggest how the comet-solar wind interaction should also respond to the combination of the IMF orientation and the intensity of the ionizing solar radiation. 1984 Season 3.5-ß E ß • 3.0-ß ß • ß .• ß ß ß ß ß ß ß ß . . ....,.;.'3.." ß ß ß % e• ee e ß / .,. 2.0-ee ß ß ß ß --• ß ß ß ß ß • .

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Initial measurements of the Venus bow shock obtained by Pioneer Venus in 1979 near solar maximum indicated that the bow shock was on average 2.44 RV from the center of the planet in the terminator plane. This is 0.35 RV further from Venus than observed by Venera 9/10 in 1976. In the past this discrepancy has been attributed to some effect of the solar cycle. Recent measurements by Pioneer Venus support this interpretation. In 1980 the distance to the bow shock reached a maximum of 2.45 RV and since then has been almost steadily declining toward the distance measured by Venera near solar minimum. The variation in bow shock position is well correlated with the sunspot number and the F 10.7 cm flux over this period. We attribute this behavior to the variation in the neutral atmosphere of Venus with the solar cycle and its subsequent effect on the mass‐loading of the solar wind.