Propagation characteristics of young hot flow anomalies near the bow shock: Cluster observations

• Published in: Journal of geophysical research. Space physics 2015-06, Vol.120 (6), p.4142-4154
• Main Authors: Xiao, T., Zhang, H., Shi, Q. Q., Zong, Q.-G., Fu, S. Y., Tian, A. M., Sun, W. J., Wang, S., Parks, G. K., Yao, S. T., Rème, H., Dandouras, I.
• Format: Article
• Language: English
• Subjects: Anomalies; Boundaries; Clusters; Contraction; Dynamic pressure; Earth bow shock; Error analysis; Evolution; evolution of young HFAs; Extreme values; hot flow anomalies; Leading edges; Mathematical analysis; Propagation; Propagation velocity; Solar wind; Time measurements; timing method considering errors; Trailing edges
• ISSN: 2169-9380; 2169-9402
• DOI: 10.1002/2015JA021013

Based on Cluster observations, the propagation velocities and normal directions of hot flow anomaly (HFA) boundaries upstream the Earth's bow shock are calculated. Twenty‐one young HFAs, which have clear leading and trailing boundaries, were selected, and multispacecraft timing method considering errors was employed for the investigation. According to the difference in the propagation velocity of the leading and trailing edges, we categorized these events into three groups, namely, contracting, expanding, and stable events. The contraction speed is a few tens of kilometers per second for the contracting HFAs, and the expansion speed is tens to more than hundred kilometers per second for expanding events. For the stable events, the leading and trailing edges propagate at almost the same speed within the error range. We have further investigated what causes them to contract, expand, or stay stable by carefully calculating the thermal pressure of the young HFAs which have two distinct ion populations (solar wind beam and reflected flow). It is found that the extreme value of the sum of the magnetic and thermal pressure inside the HFAs compared with that of the nearest point outside of the leading edges is higher for expanding events and lower for contracting events, and there is no significant difference for the stable events, and the total pressure (sum of thermal, magnetic, and dynamic pressure) variation has a significant effect on the evolution for most (70%) of the HFAs, which implies that the pressure plays an important role in the evolution of young HFAs. Key Points The propagation properties of HFAs are studied using Cluster multipoint data The speed of HFA edges is calculated using timing method considering errors The pressure plays an important role in the evolution of young HFAs