Abstract
Analyzing data from the Ion Composition Analyzer on board the Rosetta spacecraft, we studied a flow pattern of accelerated cometary ions (40–80 eV) inside and outside the diamagnetic cavity of comet 67P/Churyumov-Gerasimenko (67P). We found that the accelerated ions are intermittently observed and are ten times more frequently observed outside the cavity than inside, and they mainly flow tailward with an aberration (~20–40°). We suggest that they are accelerated by the tailward polarization electric field upstream of the comet. Because their occurrence frequency becomes lowest near perihelion where the water production rate is highest at 67P, ion-neutral collisions and/or charge exchange may play a role in controlling the occurrence frequency. The aberration pattern is different inside and outside the cavity in the cometocentric solar equatorial (CSEQ) frame but it is consistent in the comet-Sun electric (CSE) frame; the latter is rotated from the CSEQ frame about the comet-Sun line so that theZ-axis is aligned with the local motional electric field. Because the flow pattern of the accelerated ions inside the cavity in the CSE frame is the same as outside, we suggest that the flow pattern inside is determined by the flow outside, depending on the local plasma and magnetic field. Near the CSE polar plane the aberration is in the opposite direction of the motional electric field, while it is in the anti-cometward direction near the CSE equator plane. The aberration in the anti-electric-field direction near the CSE polar plane suggests that the accelerated ions are mass-loaded by local cold cometary ions, just like the mass-loading of the solar wind by cold cometary ions. The cause of the anti-cometward aberration near the CSE equator plane is still unknown, but this may indicate that the tailward-flowing cometary ions are deflected across the upstream boundaries or by an outward-pointing ambipolar electric field.
Highlights
Comets are small rocky and icy objects that originate from the Kuiper belt or the Oort cloud
The aberration pattern is different inside and outside the cavity in the cometocentric solar equatorial (CSEQ) frame but it is consistent in the comet-Sun electric (CSE) frame; the latter is rotated from the CSEQ frame about the comet-Sun line so that the Z-axis is aligned with the local motional electric field
We focus on cometary ions with energies of 40–80 eV in this study because lower energy ions are strongly affected by the negative spacecraft potential that was typical of Rosetta throughout the mission
Summary
Comets are small (typically a few to 10 km) rocky and icy objects that originate from the Kuiper belt or the Oort cloud. The solar wind ions (mainly protons) start to feel the Lorentz force of the interplanetary magnetic field (IMF) frozen into the bulk plasma and are deflected from the Sun-comet line in the opposite direction of the cometary pickup ions. These processes are referred to as the mass-loading of the solar wind (e.g., Szegö et al 2000; Behar et al 2016a). In this region after a significant mass-loading, the IMF starts to pile up and eventually stops to form a boundary inside which the magnetic field becomes zero (Goetz et al 2016a,b) This region is referred to as the diamagnetic cavity. Using one of the high time-resolution modes of the ICA, we statistically examine occurrence frequencies of the accelerated ions and their flow direction in and around the diamagnetic cavity
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