Changes of Pickup Ion Velocity Distribution Due to Solar Wing Compressions andInterplanetary Shocks
Abstract
We present the first systematic analysis to determine pickup ion (PUI) cut-off speed variations, during compression regions, and interplanetary shocks. This study is motivated by the attempt to remove or correct these effects on the determination of the longitude of the interstellar neutral gas flow from the flow pattern related variation of the PUI cutoff with ecliptic longitude (Moebius et al. 2015 ApJ 815, 20). Using 2007-2014 STEREO A PLASTIC, and ACE SWICS observations we identify compression regions in the solar wind and analyze the PUI velocity distribution function (VDF). Characterizing individual compressions and shocks by changes in solar wind parameters and combining them in superposed epoch analyses allows us to resolve the PUI population with adequate statistics and identify SW parameter relations. This method yields positive cut-off shifts in compression regions that are sustained through the peak solar wind speed and into the rarefaction region. This sustained heating, into regions of expanding SW, seems to either be associated with PUI transport from the preceding compression, or it may be caused by turbulence in the SW. Combining events based on similar SW parameters allows us to relate PUI heating to SW parameters.We find a correlation between PUI heating and the velocity gradients in both compression and shocks, with the latter causing the strongest effects. Also, the heating of PUIs and the SW are strongly correlated, thus perhaps suggesting a common mechanism. This heating may, in part, be understood in terms of motional electric fields in SW velocity gradients, similar to those produced at quasi-perpendicular shock fronts, causing PUI's to be accelerated along the plane of the compression. Because the PUI heating also shows a correlation with the strength of SW velocity fluctuations, the presence of turbulence may also play a role. Fundamentally, this study sheds light on the physical mechanisms that lead to energy transfer between the SW and the embedded PUI population, while providing criteria to select time periods when the PUI cut-off reflects most closely the interstellar flow velocity rather than SW related effects.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMSH13C2954B
- Keywords:
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- 2124 Heliopause and solar wind termination;
- INTERPLANETARY PHYSICSDE: 2126 Heliosphere/interstellar medium interactions;
- INTERPLANETARY PHYSICSDE: 2152 Pickup ions;
- INTERPLANETARY PHYSICSDE: 7845 Particle acceleration;
- SPACE PLASMA PHYSICS