Author

Jinlei Zheng

Date of Award

2017

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Space Science

Committee Chair

Qiang Hu

Committee Member

Gang Li

Committee Member

Jakobus le Roux

Committee Member

Gary P. Zank

Committee Member

Gary Webb

Subject(s)

Space plasmas., Magnetic flux.

Abstract

In this dissertation, we developed a new automated small-scale magnetic flux rope detection algorithm based on the Grad-Shafranov (GS) reconstruction technique. The GS reconstruction technique is a tool to reconstruct the two and a half dimensional magnetic field and plasma structures based on in-situ spacecraft measurements in space plasmas. We applied this detection algorithm to the Wind spacecraft measurements during 1996 ~ 2016, covering two solar cycles, and successfully detected a total number of 74,241 small-scale magnetic flux rope events with durations from 9 to 361 minutes. This large number of small-scale magnetic flux ropes has not been discovered by any other previous studies through this unique approach. We have built an online database of small-scale magnetic flux ropes based on our detection results (see http://fluxrope.info). We performed statistical analysis of the small-scale magnetic flux rope events based on our newly developed database, and summarize the main findings as follows. (1) The occurrence of small-scale flux ropes has strong solar cycle dependency, which has the same trend as the occurrence of their large-scale counterparts, the magnetic clouds. (2) The small-scale magnetic flux ropes in the ecliptic plane tend to align along the Parker spiral, indicating that they belong to the general population of "flux tubes". (3) In low speed (< 400 km/s) solar wind, the flux ropes tend to have lower proton temperature and higher proton number density, while in high speed (>= 400 km/s) solar wind, they tend to have higher proton temperature and lower proton number density. (4) Both the duration and scale size distributions of the small-scale magnetic flux ropes obey a power law. (5) The waiting time distribution of small-scale magnetic flux ropes can be fitted by an exponential function (for shorter waiting times) and a power law function (for longer waiting times), suggesting the presence of both a random Poisson process and some clustering behavior. (6) The wall-to-wall time distribution obeys double power laws with the break point at 60 minutes (corresponding to the correlation length), which is consistent with the waiting time distributions of magnetohydrodynamic (MHD) turbulence simulations and the related observations. (7) The small-scale magnetic flux ropes tend to accumulate near the heliospheric current sheets (HCSs). We also performed case studies on the small-scale magnetic flux ropes downstream of interplanetary shocks, and found that the peaks of enhanced ions flux correspond to the merging edges of two adjacent flux ropes, indicating that the merging flux ropes are able to energize particles with appropriate energy bands. In summary, we have studied an exhaustive list of small-scale magnetic flux rope structures from twenty-one-year worth of Wind spacecraft data in the solar wind. We examined their properties and related to other structures. We have found evidence supporting competing views on the origin of this type of structures. However, we conclude that we have found strong evidence in support of the mechanism of their self-generation in MHD turbulence, especially on scales smaller than the correlation length.

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