Browsing Physics by Author "Chu, Christina Seiman"
Hot flow anomalies at earth's bow shock and their magnetospheric-ionospheric signaturesChu, Christina Seiman; Zhang, Hui; Otto, Antonius; Ng, Chung-Sang; Sibeck, David (2017-08)Hot flow anomalies (HFAs) are typically observed upstream of bow shocks. They are characterized by a significant increase in particle temperature and substantial flow deflection from the solar wind flow direction coinciding with a decrease in density. HFAs are important to study and understand because they may play an important role in solar wind-magnetosphere coupling. They may drive magnetopause motion, boundary waves, and flux transfer events. They can excite ultra low frequency waves in the magnetosphere, drive magnetic impulse events in the ionosphere, and trigger aurora brightening or dimming. Studying HFAs will aid in the understanding of fundamental processes that operate throughout the heliosphere such as particle energization and shocks. This dissertation presents statistical and case studies of hot flow anomalies identified in Time History of Events and Macroscale Interactions During Substorms (THEMIS) satellite data from 2007-2009. The characteristics and occurrence of HFAs, their dependence on solar wind/interplanetary magnetic field (IMF) conditions and location, and their magnetospheric-ionospheric signatures, have been investigated using in-situ spacecraft observations and ground based observations. THEMIS observations show that HFAs span a wide range of magnetic local times (MLTs) from approximately 7 to 16.5 MLT. HFAs were observed up to 6.3 Earth radii (RE) upstream from the bow shock. It has been found that the HFA occurrence rate depends on solar wind and interplanetary magnetic field (IMF) conditions as well as distance from the bow shock. HFA occurrence decreases with distance upstream from the bow shock. HFAs are more prevalent when there is an approximately radial interplanetary magnetic field. No HFAs were observed when the Mach number was less than 5, suggesting there is a minimum threshold Mach number for HFAs to form. HFAs occur most preferentially for solar wind speeds from 550-600 km/s. Multiple THEMIS spacecraft observations of the same HFA provide an excellent opportunity to perform a spatial and temporal analysis of an HFA. The leading edge, tangential discontinuity inside the HFA, and trailing shock boundaries for the event were identified. The boundaries' orientations and motion through space were characterized. The HFA expansion against the solar wind was 283 km/s. The spatial structure of the HFA was deduced from multiple spacecraft observations. The HFA is thicker closer to the bow shock. The magnetospheric-ionospheric signatures of an HFA have been investigated using in-situ spacecraft observations and ground based observations. Magnetic field perturbations were observed by three GOES spacecraft at geostationary orbit and high-latitude ground magnetometers in both hemispheres. Observations from magnetometers located at different MLTs showed that the perturbation propagates tailward at 0.32°/s or 9 km/s (1.27°/s or 21 km/s) for the northern (southern) hemisphere, which is consistent with an HFA propagating tailward along the dawn flank. SuperDARN radar observations showed a change in plasma velocity shortly after the HFA was observed by THEMIS.