#### Date of Award

Spring 2016

#### Project Type

Dissertation

#### Program or Major

Physics

#### Degree Name

Doctor of Philosophy

#### First Advisor

Eberhard Möbius

#### Second Advisor

Harald Kucharek

#### Third Advisor

Martin Lee

#### Abstract

Our Sun is immersed in a local galactic environment which is composed of a warm, dilute, and partially ionized gas. Due to the Sun’s motion relative to this environment, the interstellar neutral (ISN) gas flows through the heliosphere providing the opportunity to perform in-situ observations of the ISN gas from Earth’s orbit. The Interstellar Boundary Explorer (IBEX) has observed the ISN gas flow over the past 7 years from a highly elliptical orbit around the Earth. The first observations of ISN H, O, and Ne were recorded by IBEX, along with the most detailed observation of ISN He. Since He is minimally influenced by ionization and charge exchange, the ISN He flow provides a sample of the pristine interstellar environment. Surprisingly, the analysis of the early IBEX observations of the ISN He flow in 2009 and 2010 with two separate analysis techniques indicated a somewhat different ISN He parameter set than the previous neutral gas observations with Ulysses GAS. One analysis technique employs the Warsaw Test Particle Model (WTPM) to simulate the ISN atom trajectories numerically from the observer position to the boundary of the heliosphere at 150 AU with spatial and temporal dependent ionization rates. A computational intensive global chi-squared minimization of the WTPM to IBEX-Lo observations is performed to characterize the ISN He flow. The second analysis technique, and the subject of this study, takes advantage of simplifications possible due to the IBEX viewing geometry of ISN atoms close to perihelion in their hyperbolic trajectories. The analytical model is based on Liouville’s Theorem with the assumption that the ISN He distribution is a drifting Maxwellian in the local interstellar medium. The analytical model is used to make a 3-step approach to determining the ISN He flow vector and temperature, rather than a global chi-squared minimization. The first step determines the location of the peak ISN He flux at Earth orbit to fix the relationship between the ISN He flow speed and the ecliptic longitude flow direction at infinity. The second step observes the ISN He latitude distribution peak as a function of observer position at Earth orbit to determine the ISN He flow ecliptic longitude and latitude at infinity. The third step uses the observed ISN He flow latitude width to determine the ISN He temperature. Each step is performed independently, rather than a global chi-squared minimization, which helps separate the influence of other particle populations. Using the first 6 years of IBEX observations along with an improved understanding of secondary particle populations and varying the IBEX observation strategy, we have refined the derived ISN He flow parameters. The new parameter set becomes more similar to the past results from the Ulysses observations but with the added consequence of a much higher temperature: velocity 27.0 + 1.4(-1.3) km s$^{-1}$, longitude 74.5{\textdegree} $\pm$ 1.7{\textdegree}, latitude -5.2{\textdegree} $\pm$ 0.3{\textdegree}, and temperature 8710 + 540(-740) K.

#### Recommended Citation

Leonard, Trevor, "Analyzing the Interstellar Neutral He Bulk Flow Parameters with IBEX-Lo" (2016). *Doctoral Dissertations*. 2246.

https://scholars.unh.edu/dissertation/2246