Title and Abstract

Title:  Observational Signatures of Small-Scale Magnetic Flux Ropes at 1 AU and Their Magnetic Connectivity to the Sun

Abstract:  In this work, the small-scale magnetic flux ropes (SMFRs) are identified by applying a cylindrical force-free fitting model to Wind satellite observations of plasmas and magnetic fields obtained near the Earth from 1995 to 2014, covering more than one solar cycle. A total of 235 SMFRs are identified during which suprathermal electrons exist often in the form of a beam. It is found that the majority (about 45 %) of the identified SMFRs were accompanied by unidirectional beams (strahl). A much smaller percentage of SMFRs (~10.7%) were associated with bidirectional beams. Only a small percentage (25 events, ~7.2%) of SMFRs indicate (sunward) conic distributions. The remaining ~37.0% of SMFRs were associated with complex electron distributions. The unidirectional beams and most of the conics (together corresponding to ~50% of the total 235 SMFRs) imply open-field SMFRs with only one end connected to the Sun. For ~37.7% of the uni-beam SMFRs, the local IMF field polarity was orthogonal or inverted (possibly due to interchange reconnection). An in-depth analysis was further performed on the 25 SMFRs accompanied by bi-directional beams to determine whether they indeed mean closed field lines. For this analysis, the expected effects were checked by the connection to the Earth’s bow shock and mirroring by a compressed magnetic field such as corotating interaction regions, high-speed streams, interplanetary coronal mass ejections, or interplanetary shocks, which all can lead to counterstreaming electrons even if the field lines are open. Based on this analysis on the solar wind conditions around the bidirectional beams, it is found that more than half of the bidirectional beams were not necessarily closed-field-line SMFRs. In conclusion, the identified SMFRs studied in this thesis are primarily of open field structure, whereas closed field structure flux ropes exist at a much lower rate. This is the first report on the statistical significance of the suprathermal electrons in SMFRs and its implication on the SMFR field connectivity to the Sun. Suggestions on future works on a few related issues are provided.