Date of Award

Fall 2021

Project Type

Dissertation

Program or Major

Physics

Degree Name

Doctor of Philosophy

First Advisor

Noé Lugaz

Second Advisor

Antoinette Galvin

Third Advisor

Charles Farrugia

Abstract

The complexities of our nearest star, the Sun, are characterized by its magnetic field. In the absence of a magnetic field, diverse phenomena as the solar cycle, solar eruptions, solar wind, to name but a few, would be unknown to us. Coronal mass ejections, a large form of solar eruption, are an essential mechanism for the evolution of the Sun. CMEs provide a means by which the built-up magnetic flux and solar material over solar cycles are removed from the solar atmosphere into the solar wind. This spectacular phenomenon has repercussions throughout the heliosphere, driving a range of heliospheric, magnetospheric, ionospheric, atmospheric, and ground effects, collectively called "space weather." Each CME structure (shock, sheath, and magnetic ejecta) has distinctive characteristics, but all cause perturbations on different scales within regular solar wind conditions. CME-driven shock is the discontinuous transition from a supersonic (or more accurately faster than the fast magnetosonic speed) to a subsonic (or more accurately slower than the fast magnetosonic speed) solar wind, and the sheath is the region of compressed and heated solar wind plasma with higherpower of magnetic field fluctuations. In contrast, the magnetic ejecta is a magnetically-dominated region of lower proton density and kinetic temperature with minimal magnetic field fluctuations.

In this thesis, the characteristics and radial evolution of CME sheaths are investigated with multi-spacecraft observations in the inner heliosphere and single-spacecraft measurements near 1 astronomical unit (AU, the mean distance from the center of the Earth to the center of the Sun). In general, the radial evolution of CMEs is inferred from analyzing different CMEs at different heliospheric distances from the Sun. Such statistical approaches are hindered by the inhomogeneity of CMEs, leading to uncertain estimates. The results presented in this thesis provide observational evidence of the inhomogeneity of CME structures. Especially as the heliocentric distance increases, the exponential decrease of the magnetic field strength within the sheath has less CME-to-CME variability than the CME. The results also indicatethat CME expansion near 1 AU does not reflect its expansion in the innermost heliosphere.

However, multi-spacecraft observations can also lead to an erroneous treatment of the radial evolution. Our findings suggest that the primary sources of uncertainties in multi-spacecraft observations are longitudinal separations between the measuring spacecraft.

This thesis sheds new light on the physical processes responsible for the observed variabilities of CME sheaths near-Earth. The results point towards the hypothesis that such observed variabilities of CME sheaths near 1 AU are likely to be governed by the sheath formation mechanisms and intrinsic CME characteristics. One fascinating aspect of our findings is that sheath variabilities tend to be not influenced by the shocks that precede them. On the other hand, preliminary statistics from our threshold-based probabilistic forecasting model demonstrate the importance of shocks, hinting at the solar wind variations in the vicinity of shocks to be a strong indicator of an upcoming intense and prolonged southward magnetic field period.

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