The Composition of Inner-source Heavy Pickup Ions: SOHO/CELIAS/CTOF Results

Abstract:

It is commonly acknowledged that there is a source for pickup ions in the inner heliosphere. These so-called inner-source pickup ions are most probably created from solar wind ions interacting with interplanetary dust particles, but a production scenario consistent with all observations has not been found to date. As the composition of inner-source pickup ions should depend on their production, a precise knowledge of this could help identifying the production mechanism. For this purpose the raw pulse-height analysis data from the Charge-Time-Of-Flight (CTOF) sensor of the Charge, ELement, and Isotope Analysis System (CELIAS) on board the SOlar and Heliospheric Observatory (SOHO) was reanalyzed. An in-flight calibration of the sensor was performed to identify heavy pickup ions by their mass-per-charge. The composition of inner-source heavy pickup ions was derived by a sophisticated statistical assignment of the counts. Finally, this composition was compared to an estimated composition one would expect from a production scenario involving solar wind ions passing through nm-sized dust grains and a good agreement was found. Due to the large geometry factor of CTOF it was also possible to investigate the abundance ratio of the two most prominent inner-source heavy pickup ions, C+ and O+, at different solar wind speeds. A systematic increase of the O+/C+ abundance ratio with higher solar wind speeds was found. This is likely to be a further signature of the production mechanism of inner-source pickup ions.