2017 THEMIS SCIENCE NUGGETS

Statistical study of particle acceleration in the core of foreshock transients

by Terry Z. Liu
UCLA EPSS

Introduction

The Earth's foreshock, which is upstream of the bow shock, is filled with back-streaming particles reflected at the bow shock or escaping from the downstream region. These back-streaming foreshock particles can interact with solar wind particles and solar wind discontinuities forming foreshock transients. Several types of foreshock transients possessing a tenuous, hot core have been observed and simulated. Because of the low dynamic pressure in their cores, these phenomena can significantly disturb the bow shock and the magnetosphere-ionosphere system. Recent observations have also demonstrated that foreshock transients can accelerate particles which, when transported earthward, can affect space weather. Understanding the potential of foreshock transients to accelerate particles can help us understand shock acceleration at Earth and at other planetary and astrophysical systems.

To further investigate foreshock transients' potential for acceleration we conduct a statistical study of ion and electron energization in the core of foreshock transients using THEMIS observations in 2008 and 2009 (247 events). We compare ion and electron energy inside foreshock transients and the background. We correlate particle energy with solar wind parameters to find out what solar wind conditions favor the particle acceleration and the formation of foreshock transients.


Figure 1. Non-solar wind ion energization estimates in the foreshock transient core plotted against the solar wind kinetic energy: mean kinetic energy (a); mean thermal energy (b); Kappa index of the phase space density distribution (c). Normalized estimates (d, e, f, respectively) are the ratios of these quantities to the same quantities in the background foreshock.

Results

By comparing mean total energy of non-solar wind ions in the foreshock transients and the background, we find that only ~30% of events show ion energization inside foreshock transients. One possibility is that ions accelerated inside foreshock transients can leak out which mask our measurements of ion energy. This hypothesis is confirmed by our follow-up paper, Liu et al. [2017b]. Another possibility is that ions need to lose some energy to support the expansion of foreshock transients. The remaining diffuse solar wind ions may also cause underestimate of ion energy. Thus, ion energization by foreshock transients may be more common than 30%. By correlating with solar wind parameters (Figure 1), we find that faster solar wind speed can result in higher ion energy inside foreshock transients, but solar wind speed can hardly affect the ratio of ion energy inside foreshock transients to the background (probably due to leakage).

Figure 2. Electron energization estimates plotted against solar wind kinetic energy: average temperature of suprathermal tail (a); E5%, the energy above which the bins contribute 5% to the partial pressure (b); Kappa index of the phase space density distribution (c); and normalized estimates for these quantities (d, e, f, respectively) are their ratios to the background.

Conclusion

  1. Ion energization is occasionally observed in the core of foreshock transients whereas electron energization is almost always observed.
  2. Solar wind speed is the most important parameter that positively correlates with ion and electron energization.
  3. Fast solar wind speed and weak magnetic field strength favor formation of foreshock transients.

References

Liu, T. Z., V. Angelopoulos, H. Hietala, and L. B. Wilson III (2017a), Statistical study of particle acceleration in the core of foreshock transients, J. Geophys. Res. Space Physics, 122, doi:10.1002/2017JA024043.

Liu, T. Z., V. Angelopoulos, and H. Hietala (2017b), Energetic ion leakage from foreshock transient cores, J. Geophys. Res. Space Physics, 122, doi:10.1002/2017JA024257.

Biographical Note

Terry Liu is a PhD student in space physics at the University of California, Los Angeles. He is studying the Earth's bow shock and related upstream particle phenomena.

Please send comments/suggestions to
Emmanuel Masongsong / emasongsong @ igpp.ucla.edu