Large-amplitude Whistler Precursors and >MeV Particles Observed at a Weak Interplanetary Shock by Parker Solar Probe

Creators: Wilson, Lynn B.¹; Mitchell, J. Grant¹; Szabo, Adam¹; Jebaraj, Immanuel C.²; Stevens, Michael L.³; Malaspina, David M.^{4, 5}; Berland, Grant D.⁶; Kouloumvakos, Athanasios⁶; Bale, Stuart D.⁷; Livi, Roberto⁷; Halekas, Jasper S.⁸; Cohen, Christina M. S.⁹

1. Goddard Space Flight Center
2. University of Turku
3. Harvard-Smithsonian Center for Astrophysics
4. University of Colorado Boulder
5. Laboratory for Atmospheric and Space Physics
6. Johns Hopkins University Applied Physics Laboratory
7. University of California, Berkeley
8. University of Iowa
9. California Institute of Technology

Abstract

We report observations of an interplanetary (IP) shock observed by Parker Solar Probe (PSP) on 2024 September 29 at ∼07:50:29 UTC. PSP was only ∼17.07 R_s from the Sun, making this one of the closest observed IP shocks to date. The IP shock was a weak (M_f ∼ 1.2), quasi-perpendicular (θ_Bn ∼ 50°), and of moderate speed (V_shn ∼ 465 km s⁻¹). The standard shock acceleration mechanisms (e.g., Fermi acceleration) predict that such an unremarkable shock cannot generate energetic particles (i.e., over 4 orders of magnitude above thermal energies), which is supported by decades of IP shock observations near 1 au. However, ∼MeV energy protons with an inverse velocity arrival and synchrotron radiation (due to ∼MeV energy electrons) were observed upstream. This raised the question of what was different about this shock. One observation was that of a fast/magnetosonic-whistler precursor with peak-to-peak magnetic field amplitudes >700 nT, electric fields >2000 mV m⁻¹, and Poynting fluxes >230 mW m⁻². These are 2 orders of magnitude larger than any previously observed whistler precursor. To put the amplitudes in context, the lower bound Poynting flux estimates are >200 times what is necessary to drive the terrestrial aurora. Note that the normalized wave parameters (e.g., frequency) were found to be consistent with previous studies near 1 au. Thus, the precursors cannot likely generate a larger fraction of energetic particles than similar precursors near 1 au. However, the much larger amplitudes would allow for higher maximum energies. This raises important questions about inaccessible shocks in more extreme astrophysical environments and what potential energization they may have in light of these observations.

Copyright and License

Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.

Acknowledgement

Analysis software (L. B. Wilson 2022) used herein can be found on GitHub.²⁹ and referenced in V. Angelopoulos et al. (2019). All ACE, Wind, and STEREO data discussed herein are available through SPDF/CDAWeb.³⁰ PSP was designed, built, and is now operated by the Johns Hopkins Applied Physics Laboratory as part of NASA’s Living With a Star (LWS) program (contract NNN06AA01C). Support from the LWS management and technical team has played a critical role in the success of the PSP mission. The FIELDS experiment was developed and is operated under NASA contract NNN06AA01C. J.S.H., S.D.B., D.M.M., C.M.S.C., R.L., M.L.S., and A.S. are supported through PSP contracts. All PSP data were provided by the instrument teams and are soon to be released for public archival at SPDF/CDAWeb. J.G.M. acknowledges the support of the NASA PSP grant NNN06AA01C. I.C.J. is grateful for support by the Research Council of Finland (SHOCKSEE, grant No. 346902), and the European Union’s (EU’s) Horizon 2020 research and innovation program under grant agreement No. 101134999 (SOLER). The study reflects only the authors’ view and the European Commission is not responsible for any use that may be made of the information it contains. I.C.J. was also supported by the International Space Science Institute (ISSI) in Bern through ISSI International Team project No. 23-575, “Collisionless Shock as a Self-Regulatory System” and through the Visiting Scientist program. A.K. acknowledges financial support from NASA NNN06AA01C (PSP EPI-Lo) contract and NASA’s HGIO grant 80NSSC24K0555. G.D.B. acknowledges the support of the IS⊙IS instrument suite on NASA’s PSP Mission, contract NNN06AA01C. D.M.M. acknowledges the support of the NASA PSP grant NNN06AA01C.

Files

Wilson_2025_ApJ_987_31.pdf

Files (1.2 MB)

Name	Size	Download all
Wilson_2025_ApJ_987_31.pdf md5:ef295391cd1eb4e7d682a5dfddff3cb4	1.2 MB	Preview Download

Additional details

	All versions	This version
Views	0	0
Downloads	0	0
Data volume	0 Bytes	0 Bytes