This thesis is concerned with studying the interactions of interplanetary large-scale structures with the solar wind and with each other based on multi spacecraft in-situ observations. These structures include: interplanetary coronal mass ejections (ICMEs), stream interaction regions SIRs and heliospheric plasma sheets HPSs. During their propagation through interplanetary space, ICMEs can interact with each other or with CIRs, making space weather forecasting more difficult. Interaction can change their configurations, dynamics, magnetic field and plasma morphologies. This includes flux rope deformation, reverse shock formation, radio emission enhancement, proton temperature increases, and ICME deflections. Caution should be exercised regarding geomagnetic storm forecasting that depends upon ICME magnetic field observations close to the sun, especially when there is an interaction with co-rotating structures, because this interaction could change the profile of the magnetic field measurements. Chapter 3 is a case study that aims to understand more about this interaction based on multi spacecraft in-situ measurements. During September 09- 10, 2011 the ACE, Wind and The Solar and Heliospheric Observatory(SOHO) spacecraft measured a complex interaction between an ICME and a co-rotating interaction region CIR associated with the HPS. A forward shock, sheath, and magnetic cloud was followed by the CIR and a high-speed stream (HSS) originating from a large equatorial coronal hole. Except for a few short periods, the suprathermal electrons are unidirectional with a 180˚ pitch angle, suggesting a scenario that the magnetic field of the ICME is open through interchange reconnection. Signatures of interaction between the ICME, CIR and subsequent HSS is distributed throughout the event and not concentrated in a specific position suggesting that the structures have become entangled, or embedded. The forward shock was strong and the ICME speed is not enough to drive it. We attributed the most likely source of the shock as the ICME-CIR interaction. Distinct and Interesting features due to the ICME-CIR/HSS interaction are the heating flux discontinuity upstream of the ICME front shock, the very high proton density in the shock, the significant speed elevation within the sheath, the distortion of Bz in the magnetic cloud, the indistinct location of the stream interface, the unidirectional domination of the suprathermal electrons and the reverse shock at the rear boundary of the CIR. A few days earlier, The Solar Terrestrial Relation Observatory (STEREO) B recorded the passage of the CIR in absence of an ICME. Despite the disruption of the ICME, some general features of the CIR are preserved, showing compression of the CIR by the ICME by a factor of ∼ 4, and enabling some qualitative comparison and further insight into the interaction. Chapter 4 presents an In-Situ study of a compound stream event measured by the STEREO B on March20-25, 2011. During the period, in situ results were obtained by STEREO B measuring a compound stream containing several interacting structures. An analysis of these results suggests that the stream consists of two interplanetary coronal mass ejections (ICMEs) followed by an embedded ICME/CIR. The third ICME has merged with, and is embedded within, a CIR. A significant stream interface has appeared within the ICME3 revealing the ability of this structure to penetrate the magnetic flux rope and change its physical properties, particularly its temperature. The sudden appearance of ICME3 within the fast wind side of the CIR causes the temperature to drop suddenly to its lowest level in about 1.2 hours, from 3.89 x 105 K to 1.07 x 104 K. Conversely, the fast wind which follows the CIR influences not only the ICME3 temperature but also its plasma β. In addition, the third ICME impacts the CIR through expansion and deceleration. Penetration of a forward pressure wave driven by this combined ICME-CIR causes an increase in the temperature and plasma beta of the second ICME and part of the first. Despite the presence of signatures from four large-scale interacting structures within the compound stream, it is difficult to reconcile the in-situ sequence with remote sensing observations of CMEs and ejecta close to the Sun. Compound streams therefore remain difficult to interpret, and further understanding of the subject will depend on the future study of similar events. Chapter 5 concentrates on tracking the evolution of a CIR during its propagation through the interplanetary spiral pattern (180 degrees of longitude) and its interaction with other interplanetary structures from period 20 April 2011 to 8 May 2011 based on multi spacecraft in situ observations (STEREO B, WIND, ACE and STEREO A). The results reveal an interaction with two-magnetic cloud flux ropes embedded within the CIR at Lagrange L1 and STEREO A (STA) positions. The role of these interplanetary magnetic flux ropes was noticeable on the CIR morphology by changing its reverse shock and stream interaction (SI) behaviours, with an obvious appearance of the SI within the magnetic cloud flux rope which is embedded with the CIR. The appearance of the reverse shock at STEREO B (STB), disappearance at L1 region, and then an appearance at STEREO A, imply that the CIR reverse shock behaviour was subjected to the local interplanetary circumstance, such as transient ICME interactions. It also reveals the CIR’s ability to recover its magnetic field and plasma morphology after disruption by ICMEs. The L1 ICME changes the CIR speed and dynamic pressure profiles. consequently, the acting forces at both reverse shock sides have changed, leading to the disappearance of the reverse shock at L1. At STA, the ICME deceleration has moved the position of the CIR’s maximum speed (Vmax) back to the rear CIR boundary, similar to position at STB. The SIs location has approached the Vmax location. These two changes made the profile of the dynamic pressure near the rear of the CIR to return to a state similar to that seen at STB, and thus, the reverse shock reappears. The ICME-CIR interaction at L1 and STA effects the HCS by the growth of the heliospheric plasma sheet (HPS) at the front of the events which coincided with the existence of the magnetic clouds within the CIR.
|Huw Morgan (Goruchwylydd) & Eleri Pryse (Goruchwylydd)