Date of Award

Fall 2018

Project Type

Dissertation

Program or Major

Physics

Degree Name

Doctor of Philosophy

First Advisor

Nathan Schwadron

Second Advisor

Eberhard Möbius

Third Advisor

Martin Lee

Abstract

Two groups of pickup ions (PUIs) exist based on their origin: interstellar and inner source. Interstellar PUIs originate as neutral atoms from the local interstellar medium (LISM) that penetrate the heliosphere, become ionized, then are picked up by the solar wind. Observations of interstellar PUIs are used to derive the inflow longitude of interstellar gas. This is done by observing the peak longitude of the focusing cone – a region of increased density due to the hyperbolic trajectories of interstellar PUIs. However, observations of pickup He+ derive a longitude consistently higher than the longitude derived from interstellar neutral atom observations. In our first study, we propose that this is due to PUI transport in interplanetary space. We do so using the Energetic Particle Radiation Environment Module (EPREM) which is capable of modeling the production and transport of ions anywhere in the heliosphere. Results indicate that the difference in observations is indeed due to the transport of PUIs.

Inner source PUIs are currently not well understood, but are suggested to be produced by the interaction between solar wind ions and interplanetary dust particles (IDPs). In our second study, we compare production mechanisms based on the interaction of solar wind ions and chondritic porous (CP) IDPs. We do so by using the Stopping Range of Ions in Matter (SRIM) and EPREM to simulate the production and transport of inner source C+ and O+ produced by five mechanisms: solar wind recycling, neutralization, backscattering, sputtering, and sputtering-induced recycling (SIR). This is the first study to consider backscattering and SIR. We compare the velocity distribution function (VDF) and C+/O+ abundance ratio to observations from the Charge-Time-Of-Flight instrument (CTOF) on-board the SOlar and Heliospheric Observatory (SOHO). Obersvations of the inner source PUI C+ and O+ reveal a new constraint: a broad VDF at 1 AU with possible cutoff near twice the solar wind speed – suggesting that inner source PUIs are injected locally into the solar wind at near zero speeds. In light of this constraint and our model-data comparison, backscattering and SIR satisfy the most production constraints. However, based on the observed intensity of the inner source, it appears that sputtering and SIR are the dominant production mechanisms at or near 1 AU.

The amount of inner source PUIs produced and their velocity distribution depend on the composition, density, porosity, and size of the IDPs. In our third study, we expand upon the previous study by considering chondritic smooth (CS) IDPs to get a more complete description of inner source PUI production. We simulate the production and transport of C+ and O+ PUIs using SRIM and EPREM. We consider five production mechanisms: solar wind recycling, neutralization, backscattering, sputtering, and SIR. Comparisons are made to SOHO/CTOF. Results indicate that sputtering is the dominant mechanism. This results in an inner source PUI composition that resembles the dust grains (i.e., rich in C, Mg, Si), rather than the solar wind (i.e., rich in H, He, and O). However, observations show that the inner source composition resembles that of the solar wind. In order to resolve this contradiction, we discuss the possibility that the IDP population close to the sun is dominated by CP IDPs rather than CS IDPs.

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