Foreshock fluctuation and its behaviors

The foreshock region is a turbulent area that forms before a quasi-parallel shock. It is caused by the interaction between reflected particles from the bow shock and oncoming waves in the solar wind. Typically, the foreshock is located on the dawn side. However, when the interplanetary magnetic field (IMF) points in a radial or anti-radial direction, the foreshock region will move to the nose of the bow shock, covering almost the entire dayside magnetospheric system. This change triggers various phenomena in the magnetospheric system, such as magnetopause expansion and the generation of foreshock transients like sHFA. Our previous studies have shown that foreshocks behave differently depending on different upstream cone angles. The fluctuation is enhanced when approaching the bow shock with an IMF near the Parker spiral structure, but for radial IMF, it remains almost constant. In this study, we investigate the frequency, polarization, and amplitude of fluctuations for different cone angles to uncover the underlying mechanisms.