Pickup Ions in the Heliosphere
Final Report Abstract
This project investigated a population of extremely rare ions in the heliosphere, so-called pickup ions (PUIs). They can be distinguished from the much more abundant solar wind ions by either their charge state (in the case of He and heavy ions they are seen as singly charged ions) or by their non-thermal velocity distribution functions (VDFs). When we proposed this project, we knew that there were two PUI populations, one originated in the interstellar medium, the other from the so-called inner source. Because the solar system and its surrounding heliosphere move through the very local interstellar medium (VLISM), electrically neutral atoms from the interstellar can penetrate into the heliosphere where they can be ionized and subsequently picked up by the solar wind which flows radially away from the Sun. Such PUIs are clearly interstellar PUIs. Inner-source PUIs were believed to originate close to the Sun (0.05 ≤ r ≤ 0.15 AU) and to be a product of the interaction of the solar wind with interplanetary dust. The exact production mechanism was unknown, various scenarios had been proposed in the literature. This work provided answers to several questions about both the innersource and the interstellar PUIs. 1. We found that the pitch-angle-scattering process after ionization must act much slower than had previously been assumed. The key discovery here was that of the torus or ring distribution in velocity space. This discovery led to a series of papers and has changed the way researchers in this field think about the so-called injection process. It is also important for the interpretation of new observations by IBEX. 2. We gave two new constraints on the nature of the inner-source PUIs, both coming from a new data set which had just become available to us in 2012. We used the excellent mass-percharge resolution and counting statistics of the Charge Time Of flight (CTOF) instrument onboard SOHO. This allowed us to determine the composition of the inner source PUIs much better than previously possible. We found that only one scenario for the origin of inner source PUIs agreed with the compositional data, the solar-wind neutralization scenario. On the other hand, we also used the high counting statistics to investigate the variability of the flux of innersource PUIs and study its correlation with the solar wind flux. Again we found that only the solar wind neutralization scenario was able to explain the observed strong correlations. Thus, we had two independent observations nailing down this long-standing question. 3. We were able to give much more precise values for the interstellar inflow direction, essentially more than doubling the amount of information. This served as valuable input for studies investigating the interstellar medium (e.g., IBEX), and also possible time variations in the interstellar inflow direction.
Publications
- Inflow direction of interstellar neutrals deduced from pickup ion measurements at 1 AU; J. Geophys. Res.; 117, A09106, (2012)
Drews, C.; Berger, L.; Wimmer-Schweingruber, R. F.; Bochsler, P.; Galvin, A. B.; Klecker, B.; and Möbius, E.
(See online at https://doi.org/10.1029/2012JA017746) - Interstellar He+ ring-beam distributions: Observations and implications; Geophys. Res. Lett.; bf 40, 1468-1473, (2013)
Drews, C.; Berger, L.; Wimmer-Schweingruber, R. F.; and Galvin, A. B.
(See online at https://doi.org/10.1002/grl.50368) - 2D He+ pickup ion velocity distribution functions: STEREO PLASTIC observations, Astron. & Astrophys,; 575, A97, (2015)
Drews, C.; Berger, L.; Taut, A.; Peleikis, T.; and Wimmer-Schweingruber, R. F.
(See online at https://doi.org/10.1051/0004-6361/201425271) - Anisotropy of the He+ , C+ , N+ , O+ , and Ne+ pickup ion velocity distribution functions; Astron. & Astrophys.; 588, A12, (2016)
Drews, C.; Berger, L.; Taut, A.; and Wimmer-Schweingruber, R. F.
(See online at https://doi.org/10.1051/0004-6361/201527603)
