2023 THEMIS SCIENCE NUGGETS
Predictive model of foreshock transients
Terry Z. Liu
Assistant Researcher,
Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles
Introduction
The sun continuously ejects fast plasma flow, named solar wind. The solar wind cannot directly hit the Earth surface but is blocked by the Earth’s magnetic field forming a bow shock. When the solar wind ions reach the bow shock, some of them stream back forming a region called foreshock. In the foreshock, there are many localized transient structures, i.e., foreshock transients (see review by Zhang et al. (2022)), which can cause disturbances in the near-Earth space environment that are harmful to spacecraft, electronic instruments, astronauts, etc. It is thus necessary to forecast the presence of foreshock transients so that we can prevent or minimize the damage. However, these structures are locally formed near the bow shock meaning that they cannot be monitored in advance. Therefore, a predictive model of foreshock transients is a must.
Results
Hot flow anomalies (HFAs) and foreshock bubbles (FBs) are two types of foreshock transients with the most significant disturbances. They form through interaction between the foreshock ions and sudden changes of magnetic field (discontinuities). Based on a formation model proposed by Liu et al. (2020) and An et al. (2020), Liu et al. (2023a) derived analytical equations of expansion speed based on how foreshock ions transfer energy through foreshock ion-driven current against induced electric field. The equations work well in comparison with THEMIS observations, MMS observations, and numerical simulations. Liu et al. (2023b) derived analytical equations of foreshock ion-driven current, which are consistent with ARTEMIS-MMS conjunction observations and numerical simulations. Combing these equations, Liu et al. (2023c) derived a model that predicts whether there will be significant foreshock disturbances, given solar wind parameters (including discontinuity parameters) and foreshock ion parameters as input. The model values show reasonable correlation with observed dynamic pressure decreases caused by foreshock disturbances (see figure). This result indicates that when the model values are large (small), there will be very likely significant (negligible) disturbances near Earth due to bow shock’s response to the large (small) dynamic pressure decreases.
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| Figure 1. The model values using background parameters as input vs. the observed dynamic pressure disturbances. The model values show moderate correlation with the observed dynamic pressure decreases. The figure is from Liu et al. (2023c). |
Conclusion
In summary, Liu et al. (2023a, 2023b, 2023c) established a model that can predict the presence of significant foreshock transient disturbances, given background solar wind and foreshock ion parameters. Theoretically, the foreshock ion parameters can be predicted by the solar wind parameters. Thus, our model in principle can forecast space weather disturbances driven by foreshock transients 8 min to 1 hour ahead based on real-time solar wind observations at lunar orbit and L1. In the future, more work is still needed to improve the model.
References
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/s11214-021-00865-0Liu, 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
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
Liu, T. Z., Vu, A., Zhang, H., An, X., and Angelopoulos, V. (2023a). Modeling the expansion speed of foreshock bubbles. Journal of Geophysical Research: Space Physics, 128, e2022JA030814. https://doi.org/10.1029/2022JA030814
Liu, T. Z., Angelopoulos, V., Vu, A., and Zhang, H. (2023b). Foreshock ion motion across discontinuities: Formation of foreshock transients. Journal of Geophysical Research: Space Physics, 128, e2022JA031161. https://doi.org/10.1029/2022JA031161
Liu, T. Z., Vu, A., Angelopoulos, V., and Zhang, H. (2023c). Analytical model of foreshock ion interaction with a discontinuity: A statistical study. Journal of Geophysical Research: Space Physics, 128, e2022JA031162. https://doi.org/10.1029/2022JA031162
Biographical Note
Terry Liu is an assistant researcher at University of California, Los Angeles. He received his PhD at UCLA in 2018. His research focus on transient phenomena in the foreshock and magnetosheath and their impacts.
Please send comments/suggestions to
Emmanuel Masongsong / emasongsong @ igpp.ucla.edu