## Doctoral Dissertations

Summer 2019

Dissertation

Physics

#### Degree Name

Doctor of Philosophy

Eberhard Möbius

#### Abstract

Recent solar conditions include a prolonged solar minimum (2005-2009) and a weak solar maximum. The Heliospheric Magnetic Field (HMF) strength was consistently weaker in solar cycle 24 compared to the previous maxima during the space age. These anomalies may indicate that we are entering an era of persistent decline in solar activity. In my first study, I investigated past solar secular (grand) minima, especially the Maunder period (1645-1715) to gain further insight into grand minima. I found the timescale parameters associated with the magnetic flux balance in the heliosphere. I also investigated the existence of a floor in the heliospheric magnetic flux, in the absence of coronal mass ejections (CMEs), and showed that a floor $\leq 1.49$ nT is sufficient to successfully describe the HMF evolution.

As a result of the unprecedentedly low solar activity, the fluxes of galactic cosmic rays (GCRs) have increased to levels never reported previously in the space age, which might limit safe human space exploration over long-term missions (e.g., to Mars). In my second study, I used data from the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) on the Lunar Reconnaissance Orbiter (LRO) to examine the correlation between the heliospheric magnetic field, solar wind speed, and the modulation potential of the GCRs through cycle 24. I applied this correlation to past secular minima conditions, including the Dalton minimum (1790-1830) and the Gleissberg minimum (1890-1920) as extreme scenarios, to estimate the deep space radiation environment throughout cycle 25. I showed that these scenarios could lead to significant increases in dose rates (up to $\sim 60\%$). I used these results to predict the most conservative permissible mission durations (PMD) based on 3$\%$ risk of exposure-induced death (REID) in interplanetary space.

Variations in the level of solar activity affect our heliosphere's interaction with the Very Local Interstellar Medium (VLISM), as well. As the sun moves through the LISM, neutral atoms travel through the heliosphere and can be detected by IBEX. We consider Interstellar neutral (ISN) hydrogen atoms with a drifting Maxwellian distribution function in the LISM that travel on almost hyperbolic trajectories to the inner heliosphere. They are subject to solar gravity and radiation pressure as well as ionization processes. For ISN H, the radiation pressure, which exerts an effective force comparable to gravitation, decelerates individual atoms and shifts the longitude of their observed peak relative to that of ISN He. I used the peak longitude of the observed flux in the lowest energy channel of IBEX-Lo to investigate how radiation pressure shifts the ISN H signal over almost an entire solar cycle (2009 to 2018). Thus, I have created a new methodology to determine the Ly$\alpha$ effective radiation pressure over gravity ($\mu_{eff}$) from IBEX ISN H data. My analysis indicates an increase of $\mu_{eff}$ with solar activity albeit with substantial uncertainties. My study of IBEX H response functions prepares for future IMAP data, which will enable a significant reduction of the uncertainties and improvements in our understanding of the effects of radiation pressure on interstellar neutral atoms.

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