Statistical study of favorable foreshock ion properties for the formation of hot flow anomalies and foreshock bubbles

Terry Z. Liu
Assistant Researcher
Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles


When the supersonic solar wind is blocked by Earth’s magnetic fields, a bow shock is formed. The bow shock slows down and heats most of the incoming solar wind particles forming the magnetosheath. A few solar wind particles, on the other hand, are reflected by the bow shock. When the magnetic field lines are nearly parallel to the bow shock normal, these reflected particles can easily travel along the magnetic field lines far away from the bow shock, forming the foreshock (see review by Eastwood et al., 2005). Due to the interaction between the backstreaming foreshock particles and incoming solar wind particles, the foreshock is very dynamic with many transient structures, called foreshock transients. Especially when the foreshock particles encounter a sudden change of the magnetic field, a discontinuity, a foreshock transient with very significant plasma and field perturbations can form. While these foreshock transients are brought by the solar wind towards the bow shock, they can energize particles and cause pressure disturbances, resulting in many space weather effects (see review by Zhang et al., 2022). Therefore, it is very important to know under what conditions those foreshock transients occur.


To answer this question, we utilized NASAs THEMIS mission and MMS mission to conduct a statistical study. We used THEMIS spacecraft and MMS spacecraft to observe foreshock transients and measure the ambient foreshock properties. We thus obtained the portion distributions of all the foreshock quantities whenever there is a foreshock transient. We then used MMS spacecraft to measure the general portion distributions regardless of the presence of foreshock transients. By comparing between the portion distributions, we determined the foreshock properties that favor the presence of foreshock transients. We found that foreshock transients more likely occur when foreshock ions have larger ratio of their density to solar wind density, larger kinetic energy (see Figure), larger ratio of kinetic energy to thermal energy, and larger ratio of perpendicular temperature to parallel temperature (relative to the magnetic field). These results support a foreshock transient formation model proposed by our team (An et al., 2020; Liu et al., 2020): these favorable quantities indicate a strong current driven by the foreshock ions, which results in significant magnetic field variation and the corresponding plasma variation. Previous statistical studies have identified the solar wind conditions that favor the foreshock transient formation (see review by Zhang et al., 2022). By connecting between the foreshock properties and solar wind properties, our results also provide a more fundamental understanding of the favorable solar wind conditions.

Figure 1. Normalized 2-D event number distribution of foreshock ion density ratio and kinetic energy in the shock normal incidence frame (NIF). It shows that large product of the two quantities (suggesting strong foreshock ion-driven currents) favor the presence of foreshock transients.


Through a statistical study using THEMIS and MMS missions, we revealed the foreshock properties that favor the presence of foreshock transients. Because foreshock transients can cause many space weather effects, it is very necessary to predict their formation. Foreshock transients should be universal, but without in-situ observations, we cannot know whether foreshock transients could occur given a certain shock environment. Our work is one necessary step towards predictive models, which will be used to forecast the space weather effects of foreshock transients and determine their role in shock physics in general.


An, X., T. Z. Liu, J. Bortnik, A. Osmane, V. Angelopoulos (2020). Formation of foreshock transients and associated secondary shocks. ApJ, 901:73 (16pp), https://doi.org/10.3847/1538-4357/abaf03

Eastwood, J. P., E. A. Lucek, C. Mazelle, K. Meziane, Y. Narita, J. Pickett, and R. A. Treumann (2005), The Foreshock, Space. Sci. Rev., 118, 41–94, https://doi.org/10.1007/s11214-005-3824-3.

Liu, T. Z., X. An, H. Zhang, and D. Turner (2020), Magnetospheric Multiscale (MMS) observations of foreshock transients at their very early stage, ApJ, 902:5 (15pp), https://doi.org/10.3847/1538-4357/abb249

Zhang, H., Zong, Q.-G., Connor, H. et al. Dayside transient phenomena and their impact on the magnetosphere and ionosphere. Space Sci Rev (2022). DOI: 10.1007/s11214-021-00865-0

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Emmanuel Masongsong / emasongsong @ igpp.ucla.edu