Magnetotails at unmagnetized bodies: Comparison of comet Giacobini‐Zinner and Venus

Abstract

Both Comet Giacobini‐Zinner (G‐Z) and Venus have magnetotails consisting of draped interplanetary magnetic field lines. This field line draping is caused by a velocity shear between regions of greater flow speeds away from the bodies and lesser flow speeds near to the bodies. Data obtained within the Venus magnetotail by the Pioneer Venus Orbiter and within the G‐Z tail by the International Cometary Explorer traversal of G‐Z have previously been combined with stress balance considerations to infer many of the physical characteristics of these two magnetotails. In the present paper we compare and contrast these physical characteristics and thereby examine those aspects of the interactions with the solar wind and draped magnetotail forming processes which are common at the two bodies, and those which are different. We find that the near ionopause environs play a crucial role in the tail formation process at both Venus and G‐Z and that draping at the two very different sized bodies occurs on ionopause scale sizes. On the other hand, ion densities, downtail mass fluxes, tailward J × B forces, and lobe betas are factors of ∼104, 50, 100, and 20 times greater in the G‐Z tail than in Venus', while bulk flow speeds and ion temperatures are factors of ∼15 and 240 times lower. These large quantitative differences in the properties within the two magnetotails are attributable to the significantly greater upstream mass loading of the solar wind by the extended neutral atmosphere at G‐Z (comets in general) compared to the gravitationally bound atmosphere of Venus.