The three dimensional structure of corotating interaction regions and modeling of the heavy ion sensor
Corotating Interaction Regions (CIRs) are compression regions that form in interplanetary space at the interfaces between slow and fast solar wind streams. This dissertation studies the three-dimensional orientation of planar magnetic structures within CIRs near Earth, how their orientation evolves, and the implications for the structure and properties of parent coronal holes. This dissertation also shows our work of modeling the response of the Heavy Ion Sensor (HIS) for the Solar Orbiter mission. We will discuss the methods and results of each chapter below.
In Chapter 2, we have surveyed the properties of 153 co-rotating interaction regions (CIRs) observed at 1 AU from January, 1995 through December, 2008. We identified that 74 of the 153 CIRs contain planar magnetic structures (PMSs). For planar and non-planar CIRs, we compared distributions of the bulk plasma and magnetic field parameters. Our identification of CIRs and their features yields the following results: (1) The thermal, magnetic, and dynamic pressures within CIRs are strongly correlated. (2) There is no statistical difference between planar and non-planar CIRs in the distributions and correlations between bulk plasma and magnetic field parameters. (3) The mean observed CIR azimuthal tilt is within 1 sigma of the predicted Parker spiral at 1 AU, while the mean meridional tilt is about 20°. (4) The meridional tilt of CIRs changes from one solar rotation to the next, with no relationship between successive reoccurrences. (5) The meridional tilt of CIRs in the ecliptic is not ordered by the magnetic field polarity of the parent coronal hole. (6) Although solar wind deflection is a function of CIR shape and speed, the relationship is not in agreement with that predicted by Lee . We conclude the following: (1) PMSs in CIRs are not caused by a unique characteristic in the local plasma or magnetic field. (2) The lack of relationship between CIR tilt and its parent coronal hole suggests that coronal hole boundaries may be more complex than currently observed. (3) In general, further theoretical work is necessary to explain the observations of CIR tilt.
In Chapter 3 we study the radial evolution of planar magnetic structures in 3 corotating interaction regions (CIRs). We compare our in-situ observations with results from an analytical and a numerical model of CIRs [ Lee, 2000; Odstrcil, 2003]. We find that: (1) All 3 CIRs' meridional tilt retained its North/South orientation at ACE and Ulysses, but the evolution was not systematic. Further, the model results of CIR meridional tilt do not agree with observations. (2) All 3 CIRs rotated azimuthally with the Parker spiral as expected, however model results only describe this behavior quantitatively for 1 CIR. (3) For all 3 CIRs, the solar wind deflection angles were predicted by the coupled solar corona-solar wind models, Wang-Sheely-Arge (WSA)-Enlil and the MHD Around a Sphere (MAS)-Enlil, but neither model was able to reproduce the observed planar magnetic structures. (4) The WSA-Enlil results of azimuthal magnetic field orientation are in better agreement with observations than those based on the MAS-Enlil. We suggest that the evolution of meridional tilt from ACE to Ulysses did not agree with projections because the parent coronal holes were highly structured compared with the idealized shapes assumed in the models. We also suggest that observations of azimuthal tilt do not agree with the model results because the models may be underestimating transverse flows, whereas in reality, these flows could affect the observed azimuthal tilt of the CIR.
In Chapter 4 we characterize the expected behavior of HIS for Solar Orbiter. Solar Orbiter is scheduled to launch in January 2017, and will carry as part of its payload the Heavy Ion Sensor (HIS), one of the instruments of the Solar Wind Analyzer (SWA) instrument suite. Heavy ions of particular interest are Ne, Mg, Si, and S which have been difficult to measure in the past due to their similar mass per charge ratios in the solar wind. We have characterized the response of HIS to these species using a Monte Carlo simulation of the instrument. These simulations use a realistic count rate and account for lost energy and angular scattering of ions passing through a carbon foil, time-of-flight of the secondary electrons, and the pulse height defect within the solid-state detectors. Our results show that HIS is capable of resolving the masses and charge-states of solar wind ions, such as He, C, O, Ne, Mg, and Fe. Our results also indicate that there is some overlap between S and Si.
In Chapter 5 we summarize our results and propose future work.