Browsing by Author "Ruohotie, Julia"

Small-scale flux ropes (SFRs) are structures with helical magnetic field and they are frequently observed in the solar wind. In addition to the solar wind, SFRs can also be found within larger structures, like interplanetary coronal mass ejections (ICMEs) and their sheath regions that form between the shock and the leading edge of the ICME if the ICME propagates fast enough. ICME-driven sheaths are composed of shocked and compressed solar wind plasma. SFRs can be swept from the upstream solar wind into ICME sheaths when the upstream wind is deflected and compressed into ICME sheaths. Alternatively, SFRs can be formed within ICME sheaths through number of processes. This thesis includes the first comprehensive study of the occurrence of SFRs specifically in ICME-driven sheath regions. SFRs are identified from spacecraft data in both ICME sheaths and the upstream solar wind using the wavelet analysis method. This method calculates normalized reduced magnetic helicity, normalized cross-helicity, and normalized residual energy and uses them to identify SFRs and Alfvén waves. The method is applied to 55 ICME-driven sheath regions observed by Wind spacecraft. The occurrence of SFRs is studied in three different frequency ranges between 10−2 − 10−4 Hz. SFRs are found to be common structures in ICME-driven sheaths and they are more common in ICME sheaths than in the upstream solar wind. This suggest that SFRs are at least to some extent generated within ICME sheaths. The occurrence of SFRs behaves differently in different frequency ranges. The occurrence of SFRs is relatively constant at high frequencies (smallest scale) while the occurrence of low-frequency (largest scale) SFRs increases towards the leading edge of the ICME. This suggests that high- and low-frequency SFRs are generated by different processes. The occurrence of Alfvén waves was found to be somewhat similar in the upstream solar wind and ICME sheaths. However, there was an increase in the occurrence of Alfvén waves near the shock. This indicates that SFRs and Alfvén waves are generated by different processes and shock related processes might be important in the generation of Alfvén waves while the processes near the leading edge of the ICME are important in the generation of larger scale SFRs.