Skip to main content

Thank you for visiting nature.com. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

  • Letter
  • Published:

Autogenous and efficient acceleration of energetic ions upstream of Earth’s bow shock

Nature volume 561, pages 206–210 (2018)Cite this article

Subjects

Abstract

Earth and its magnetosphere are immersed in the supersonic flow of the solar-wind plasma that fills interplanetary space. As the solar wind slows and deflects to flow around Earth, or any other obstacle, a ‘bow shock’ forms within the flow. Under almost all solar-wind conditions, planetary bow shocks such as Earth’s are collisionless, supercritical shocks, meaning that they reflect and accelerate a fraction of the incident solar-wind ions as an energy dissipation mechanism1,2, which results in the formation of a region called the ion foreshock3. In the foreshock, large-scale, transient phenomena can develop, such as ‘hot flow anomalies’4,5,6,7,8,9, which are concentrations of shock-reflected, suprathermal ions that are channelled and accumulated along certain structures in the upstream magnetic field. Hot flow anomalies evolve explosively, often resulting in the formation of new shocks along their upstream edges5,10, and potentially contribute to particle acceleration11,12,13, but there have hitherto been no observations to constrain this acceleration or to confirm the underlying mechanism. Here we report observations of a hot flow anomaly accelerating solar-wind ions from roughly 1–10 kiloelectronvolts up to almost 1,000 kiloelectronvolts. The acceleration mechanism depends on the mass and charge state of the ions and is consistent with first-order Fermi acceleration14,15. The acceleration that we observe results from only the interaction of Earth’s bow shock with the solar wind, but produces a much, much larger number of energetic particles compared to what would typically be produced in the foreshock from acceleration at the bow shock. Such autogenous and efficient acceleration at quasi-parallel bow shocks (the normal direction of which are within about 45 degrees of the interplanetary magnetic field direction) provides a potential solution to Fermi’s ‘injection problem’, which requires an as-yet-unexplained seed population of energetic particles, and implies that foreshock transients may be important in the generation of cosmic rays at astrophysical shocks throughout the cosmos.

This is a preview of subscription content, access via your institution

Access options

Buy this article

  • Purchase on Springer Link
  • Instant access to full article PDF
Buy now

Prices may be subject to local taxes which are calculated during checkout

Fig. 1: Overview of the MMS burst data during the HFA.
Fig. 2: MMS observations of energetic ions and context data during and after the encounter with the HFA.
Fig. 3: Conceptual scenario for the HFA observed by MMS on 28 December 2015.
Fig. 4: Comparing observations and theory.

Similar content being viewed by others

Data availability

All MMS data used for this study are publicly available via the MMS Science Data Center at https://spdf.gsfc.nasa.gov/pub/data/mms.

References

  1. Wilson, L. B. III et al. Quantified energy dissipation rates in the terrestrial bow shock: 1. Analysis techniques and methodology. J. Geophys. Res. 119, 6455–6474 (2014).

    Google Scholar 

  2. Wilson, L. B. III et al. Quantified energy dissipation rates in the terrestrial bow shock: 2. Waves and dissipation. J. Geophys. Res. 119, 6475–6495 (2014).

    Google Scholar 

  3. Eastwood, J. P. et al. The foreshock. Space Sci. Rev. 118, 41–94 (2005).

    Google Scholar 

  4. Schwartz, S. J. et al. An active current sheet in the solar wind. Nature 318, 269–271 (1985).

    Google Scholar 

  5. Thomsen, M. F. et al. On the origin of hot diamagnetic cavities near the Earth’s bow shock. J. Geophys. Res. 93, 11311–11325 (1988).

    Google Scholar 

  6. Paschmann, G. et al. Three-dimensional plasma structures with anomalous flow directions near the Earth’s bow shock. J. Geophys. Res. 93, 11279–11294 (1988).

    Google Scholar 

  7. Schwartz, S. J. et al. Conditions for the formation of hot flow anomalies at Earth’s bow shock. J. Geophys. Res. 105, 12639–12650 (2000).

    Google Scholar 

  8. Omidi, N. & Sibeck, D. G. Formation of hot flow anomalies and solitary shocks. J. Geophys. Res. 112, A01203 (2007).

    Google Scholar 

  9. Wang, S., Zong, Q. & Zhang, H. Hot flow anomaly formation and evolution: cluster observations. J. Geophys. Res. 118, 4360–4380 (2013).

    Google Scholar 

  10. Fuselier, S. A. et al. Fast shocks at the edges of hot diamagnetic cavities upstream from the Earth’s bow shock. J. Geophys. Res. 92, 3187–3194 (1987).

    Google Scholar 

  11. Wilson, L. B. III et al. Relativistic electrons produced by foreshock disturbances observed upstream of Earth’s bow shock. Phys. Rev. Lett. 117, 215101 (2016).

    CAS  Google Scholar 

  12. Liu, T. Z. et al. Statistical study of particle acceleration in the core of foreshock transients. J. Geophys. Res. 122, 7197–7208 (2017).

    Google Scholar 

  13. Giacalone, J. & Burgess, D. Interaction between inclined current sheets and the heliospheric termination shock. Geophys. Res. Lett. 37, L19104 (2010).

    Google Scholar 

  14. Fermi, E. On the origin of cosmic radiation. Phys. Rev. 75, 1169–1174 (1949).

    Google Scholar 

  15. Liu, T. Z. et al. Fermi acceleration of electrons inside foreshock transient cores. J. Geophys. Res. 122, 9248–9263 (2017).

    Google Scholar 

  16. Burch, J. L. et al. Magnetospheric Multiscale overview and science objectives. Space Sci. Rev. 199, 5–21 (2016).

    Google Scholar 

  17. Liu, T. Z., Turner, D. L., Angelopoulos, V. & Omidi, N. Multipoint observations of the structure and evolution of foreshock bubbles and their relation to hot flow anomalies. J. Geophys. Res. 121, 5489–5509 (2016).

    Google Scholar 

  18. Turner, D. L. et al. First observations of foreshock bubbles upstream of Earth’s bow shock: characteristics and comparisons to HFAs. J. Geophys. Res. 118, 1552–1570 (2013).

    Google Scholar 

  19. Chu, C. et al. THEMIS satellite observations of hot flow anomalies at Earth’s bow shock. Ann. Geophys. 35, 443–451 (2017).

    Google Scholar 

  20. Jones, F. C. & Ellison, D. C. The plasma physics of shock acceleration. Space Sci. Rev. 58, 259–346 (1991).

    Google Scholar 

  21. Caprioli, D., Pop, A.-R. & Spitkovsky, A. Simulations and theory of ion injection at non-relativistic collisionless shocks. Astrophys. J. 798, L28 (2015).

    CAS  Google Scholar 

  22. Giacalone, J. Particle acceleration at shocks moving through an irregular magnetic field. Astrophys. J. 624, 765–772 (2005).

    Google Scholar 

  23. Park, J., Caprioli, D. & Spitkovsky, A. Simultaneous acceleration of protons and electrons at nonrelativistic quasiparallel collisionless shocks. Phys. Rev. Lett. 114, 085003 (2015).

    Google Scholar 

  24. Caprioli, D., Yi, D. T. & Spitkovsky, A. Chemical enhancements in shock-accelerated particles: ab initio simulations. Phys. Rev. Lett. 119, 171101 (2017).

    Google Scholar 

  25. Zhang, H. et al. Spontaneous hot flow anomalies at quasi-parallel shocks: 1. Observations. J. Geophys. Res. 118, 3357–3363 (2013).

    Google Scholar 

  26. Omidi, N., Zhang, H., Sibeck, D. & Turner, D. Spontaneous hot flow anomalies at quasi-parallel shocks: 2. Hybrid simulations. J. Geophys. Res. 118, 173–180 (2013).

    Google Scholar 

  27. Hietala, H., Sandroos, A. & Vainio, R. Particle acceleration in shock-shock interaction: model to data comparison. Astrophys. J. 751, L14 (2012).

    Google Scholar 

  28. Balogh, A. & Treumann, R. A. Physics of Collisionless Shocks (Springer, New York, 2013).

    Google Scholar 

  29. Torbert, R. B. et al. The FIELDS instrument suite on MMS: scientific objectives, measurements, and data products. Space Sci. Rev. 199, 105–135 (2016).

    Google Scholar 

  30. Mauk, B. H. et al. The Energetic Particle Detector (EPD) investigation and the Energetic Ion Spectrometer (EIS) for the Magnetospheric Multiscale (MMS) mission. Space Sci. Rev. 199, 471–514 (2016).

    CAS  Google Scholar 

  31. Blake, J. B. et al. The Fly’s Eye Energetic Particle Spectrometer (FEEPS) sensors for the Magnetospheric Multiscale (MMS) mission. Space Sci. Rev. 199, 309–329 (2016).

    Google Scholar 

  32. Pollock, C. et al. Fast plasma investigation for Magnetospheric Multiscale. Space Sci. Rev. 199, 331–406 (2016).

    Google Scholar 

  33. Young, D. T. et al. Hot plasma composition analyzer for the Magnetospheric Multiscale mission. Space Sci. Rev. 199, 407–470 (2016).

    Google Scholar 

  34. Sonnerup, B. U. Ö. & Scheible, M. in Analysis Methods for Multi-Spacecraft Data (eds Paschmann, G. & Daly, P. W.) Ch. 8, 185–220 (ISSI/ESA, 1998).

  35. Schwartz, S. J. in Analysis Methods for Multi-Spacecraft Data (eds Paschmann, G. & Daly, P. W.) Ch. 10, 249–270 (ISSI/ESA, 1998).

  36. Liu, T. Z., Lu, S., Angelopoulos, V., Lin, Y. & Wang, X. Y. Ion acceleration inside foreshock transients. J. Geophys. Res. 123, 163–178 (2018).

    Google Scholar 

  37. Drury, L. O. An introduction to the theory of diffusive shock acceleration of energetic particles in tenuous plasmas. Rep. Prog. Phys. 46, 973–1027 (1983).

    CAS  Google Scholar 

Download references

Acknowledgements

This work was supported by NASA contract NNG04EB99C at Southwest Research Institute, a NASA grant (NNX16AQ50G) and research supported by the International Space Science Institute’s (ISSI) International Teams programme. We thank all of the MMS team and the SPEDAS software developers for their publicly available data and software products. D.L.T. thanks T. Phan, S.-H. Lee and D. G. Sibeck for discussions and NASA’s From Earth to the Solar System collection.

Reviewer information

Nature thanks H. Zhang and the other anonymous reviewer(s) for their contribution to the peer review of this work.

Author information

Authors and Affiliations

  1. Space Sciences Department, The Aerospace Corporation, El Segundo, CA, USA

    D. L. Turner, J. F. Fennell, J. H. Clemmons, J. B. Blake, R. G. Gomez & R. G. Gomez

  2. NASA Goddard Space Flight Center, Greenbelt, MD, USA

    L. B. Wilson III, D. J. Gershman, L. Avanov & B. L. Giles

  3. Department of Earth, Planetary, and Space Science, University of California, Los Angeles, CA, USA

    T. Z. Liu, R. J. Strangeway & C. T. Russell

  4. Applied Physics Laboratory, Laurel, MD, USA

    I. J. Cohen, J. Westlake & B. H. Mauk

  5. Imperial College London, London, UK

    S. J. Schwartz

  6. School of Electrical Engineering, Aalto University, Espoo, Finland

    A. Osmane

  7. Rudolf Peierls Centre of Theoretical Physics, University of Oxford, Oxford, UK

    A. Osmane

  8. Department of Physics and Astronomy, University of Iowa, Iowa City, IA, USA

    A. N. Jaynes

  9. Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO, USA

    T. Leonard & D. N. Baker

  10. Institute For the Study of Earth, Oceans, and Space, University of New Hampshire, Durham, NH, USA

    R. B. Torbert

  11. Southwest Research Institute, San Antonio, TX, USA

    R. B. Torbert, J. Broll, S. A. Fuselier & J. L. Burch

  12. Departoment of Physics and Astronomy, University of Texas at San Antonio, San Antonio, TX, USA

    J. Broll & S. A. Fuselier

Authors
  1. D. L. Turner
    View author publications

    You can also search for this author in PubMed Google Scholar

  2. L. B. Wilson III
    View author publications

    You can also search for this author in PubMed Google Scholar

  3. T. Z. Liu
    View author publications

    You can also search for this author in PubMed Google Scholar

  4. I. J. Cohen
    View author publications

    You can also search for this author in PubMed Google Scholar

  5. S. J. Schwartz
    View author publications

    You can also search for this author in PubMed Google Scholar

  6. A. Osmane
    View author publications

    You can also search for this author in PubMed Google Scholar

  7. J. F. Fennell
    View author publications

    You can also search for this author in PubMed Google Scholar

  8. J. H. Clemmons
    View author publications

    You can also search for this author in PubMed Google Scholar

  9. J. B. Blake
    View author publications

    You can also search for this author in PubMed Google Scholar

  10. J. Westlake
    View author publications

    You can also search for this author in PubMed Google Scholar

  11. B. H. Mauk
    View author publications

    You can also search for this author in PubMed Google Scholar

  12. A. N. Jaynes
    View author publications

    You can also search for this author in PubMed Google Scholar

  13. T. Leonard
    View author publications

    You can also search for this author in PubMed Google Scholar

  14. D. N. Baker
    View author publications

    You can also search for this author in PubMed Google Scholar

  15. R. J. Strangeway
    View author publications

    You can also search for this author in PubMed Google Scholar

  16. C. T. Russell
    View author publications

    You can also search for this author in PubMed Google Scholar

  17. D. J. Gershman
    View author publications

    You can also search for this author in PubMed Google Scholar

  18. L. Avanov
    View author publications

    You can also search for this author in PubMed Google Scholar

  19. B. L. Giles
    View author publications

    You can also search for this author in PubMed Google Scholar

  20. R. B. Torbert
    View author publications

    You can also search for this author in PubMed Google Scholar

  21. J. Broll
    View author publications

    You can also search for this author in PubMed Google Scholar

  22. R. G. Gomez
    View author publications

    You can also search for this author in PubMed Google Scholar

  23. S. A. Fuselier
    View author publications

    You can also search for this author in PubMed Google Scholar

  24. J. L. Burch
    View author publications

    You can also search for this author in PubMed Google Scholar

Contributions

D.L.T. performed the data analysis, interpretation and manuscript preparation. L.B.W., T.Z.L., S.J.S. and A.O. contributed to data interpretation, multipoint analysis and development of the theory for comparison to the observations. J.F.F., J.H.C., J.B.B., A.N.J., T.L. and D.N.B. contributed to the development, operation and data processing of the FEEPS energetic particle telescopes, data quality assurance and interpretation of those data. I.J.C., J.W. and B.H.M. contributed to the development, operation and data processing of the EIS instruments, data quality assurance and interpretation of those data. R.J.S. and C.T.R. contributed to the development, operation and data processing of the fluxgate magnetometers and related data quality assurance. D.J.G., L.A. and B.L.G. contributed to the development, operation and data processing of the fast plasma instruments and related data quality assurance. R.B.T. contributed to the development, operation and data processing of the FIELDS instrument suite and related data quality assurance. J.B., R.G.G. and S.A.F. contributed to the development operation and data processing of the HPCA instruments, data quality assurance and interpretation of those data. J.L.B. is the PI of MMS science and contributed with quality assurance and interpretation of the data.

Corresponding author

Correspondence to D. L. Turner.

Ethics declarations

Competing interests

The authors declare no competing interests.

Additional information

Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Extended data figures and tables

Extended Data Fig. 1 Solar-wind ion species observed by MMS HPCA.

a, Magnetic-field vector and strength from MMS-2. b–f, FEEPS total energetic ion (b) and HPCA ion (c–f) composition flux energy spectra (omni-directional) combined from all four spacecraft (MMS-X). HPCA observed many fewer He+ ion than He2+ ions (α particles) and no O+ ions (all zero counts despite energy channels up to about 38 keV) at E > 10 keV during the observation of the energetic ions between the HFA and the magnetosheath. The absence of low-charge-state oxygen in HPCA data (O+, O2+) indicates that the energetic CNO ions observed by EIS were at a high charge state. This confirms that the accelerated ions were high-charge-state ions (q/m ≥ 2) and thus of solar-wind origin. Like FEEPS, HPCA also shows that the most energetic beam of α particles and protons is streaming away from the HFA.

Extended Data Fig. 2 Conceptual model of the Fermi acceleration trap created by an HFA evolving along Earth’s bow shock.

At the top of this schematic is the solar wind, which is the ‘upstream’ direction to which some ions within the trap might escape. The region at the bottom of the schematic is the magnetosheath plasma, which is the ‘downstream’ direction to which some ions might also escape. The HFA sheath and shock region, with some finite thickness d, lies between the two horizontal black lines. The HFA core region lies between the HFA sheath/shock and magnetosheath regions. The HFA core has a characteristic length scale L and is perpetually converging on the magnetosheath such that dL/dt < 0 at any point along the core. Critical to the model (and as observed and simulated in HFAs), the magnetic-field strength within the HFA core (less than 1 nT) is much, much weaker than that in the HFA sheath or shock and in Earth’s bow shock or magnetosheath (more than 10 nT); this geometry ensures that some subset of ions within the HFA core can remain trapped between the two regions of higher field strength, owing to differences in the ion gyroradii (rc) in the different regions. We note that the orientations of the magnetic fields are sketched arbitrarily here. Three example ion trajectories are sketched: (i) escaping to the solar wind; (ii) escaping to the magnetosheath; and (iii) remaining trapped within the HFA core.

Extended Data Fig. 3 Simplified model of the HFA.

This model includes a finite thickness of the HFA sheath (that is, shocked plasma) and shock d, which is used to determine the energy spectra of ions accelerated within the trap considering losses through the boundary. In this picture, the HFA sheath and shock fall between the two horizontal black lines, with the HFA core being a larger region below this and the solar wind being the region above this. Only ions at particular energies, pitch angles and gyrophase can remain trapped in the Fermi acceleration trap between Earth’s bow shock or magnetosheath and the HFA shock. For example, the two ion trajectories shown are at the same energy but different gyrophase when they enter the HFA sheath with enhanced magnetic field strength: the rightmost ion is reflected and can continue to be accelerated, whereas the leftmost ion escapes the trap and is lost to the solar wind.

Rights and permissions

Reprints and permissions

About this article

Check for updates. Verify currency and authenticity via CrossMark

Cite this article

Turner, D.L., Wilson, L.B., Liu, T.Z. et al. Autogenous and efficient acceleration of energetic ions upstream of Earth’s bow shock. Nature 561, 206–210 (2018). https://doi.org/10.1038/s41586-018-0472-9

Download citation

  • Received:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI: https://doi.org/10.1038/s41586-018-0472-9

Keywords

This article is cited by

Comments

By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.

Search

Advanced search

Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing