Statistical Study of Hot Flow Anomaly Induced Ground Magnetic Ultra-Low Frequency Oscillations

Pc5 ULF waves play an important role in transporting energy and particles in the coupled magnetospheric and ionospheric system. They are known to be initiated by dynamic pressure fluctuations upstream of the magnetopause, including those induced by hot flow anomalies (HFAs). However, the role of HFAs in generating magnetospheric and ground magnetic Pc5 ULF oscillations has not been investigated statistically yet. Thus, in this paper, we investigate the contribution of HFAs to ground magnetic Pc5 ULF oscillations and analyze how the characteristics of HFAs influence these oscillations, based on the coordinated observations between the THEMIS probes and the ground magnetometers at high latitudes during the years 2008, 2009 and 2019. We find that HFAs can serve as a notable source of ground magnetic Pc5 ULF oscillations, with about 18.9% of Interplanetary Magnetic Field (IMF) discontinuity-induced HFAs associated with discernible enhancements in Pc5 ULF wave power, whereas spontaneous HFAs play a comparatively minor role in generating these oscillations. Furthermore, we observe that the cores of HFAs are likely to contribute more significantly to modulating the induced ground magnetic Pc5 ULF oscillations than their compressed boundaries. More dynamic pressure reductions within HFA cores correspond to stronger ground magnetic Pc5 ULF oscillations. Additionally, HFAs can propagate with the IMF discontinuity along the bow shock, continuously generating ground magnetic Pc5 ULF oscillations during their propagation. This research sheds light on the mechanisms underlying Pc5 ULF wave generation and underscores the role of HFAs in driving magnetospheric-ionospheric interactions. Ultra Low Frequency (ULF) waves, specifically Pc5 waves, are important for transporting energy and particles between the Earth's magnetic field and its upper atmosphere. These oscillations start because of changes in pressure outside the Earth's magnetosphere, such as hot flow anomalies (HFAs). However, it has not been studied how HFAs contribute to ground Pc5 oscillations in detail yet. So, in this paper, we look at how HFAs make Pc5 oscillations in the Earth's geomagnetic field and how the features of HFAs affect these oscillations. We studied data from THEMIS probes and ground magnetometers in 2008, 2009, and 2019. We found that about 18.9% of triggered HFAs caused noticeable increases in ground magnetic Pc5 wave power. Spontaneous HFAs didn't have as big of an effect. We also found that the core of HFAs is more important for making Pc5 waves than the edges. When the pressure reduction inside HFAs increases, Pc5 waves get stronger. Also, we noticed that HFAs move together with discontinuities in the solar wind, creating Pc5 oscillations as they move. This research helps us understand how Pc5 oscillations are made and shows that HFAs are important for connecting different parts of Earth's space environment. HFAs can be an external source of ground magnetic Pc5 ULF oscillations, while SHFAs have weaker effects on generating these oscillations The cores of HFAs contribute more to modulating their induced intensity of ground magnetic Pc5 ULF oscillations than their boundaries HFA-induced ground magnetic Pc5 ULF oscillations can propagate following the propagation direction of HFAs