Case and statistical studies on the evolution of hot flow anomalies

Abstract

Hot flow anomalies (HFAs) frequently observed near Earth's bow shock are phenomena resulting from the interaction between interplanetary discontinuities and Earth's bow shock. We identify 199 HFA events using Cluster data from 2001 to 2010 and divide these events into four categories according to the dynamic pressure profile, namely, "-+," "+-," "M," and "W" types, where the symbols describe the profile of the dynamic pressure variations. We present case studies to show the main characteristics of each type of HFAs. Normalized superposed epoch analyses show that variations of the magnetic field magnitude of -+ and +- type HFAs are more dramatic than those of M and W types. The statistical study shows that the occurrence percentage of mature HFAs in W type HFAs is higher than that of -+, +-, and M types. Superposed epoch analysis result shows that variations of plasma parameters and magnetic field of mature HFAs are more dramatic than those of young HFAs, except for temperature. M and W type HFAs may be the later evolution stages of -+ and +- type HFAs; on the other hand, four categories of HFAs may be due to the fact that the spacecraft crossed an HFA structure along different paths. Key Point The formation of the four types of HFAs with different structure may be due to their evolution stages