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

Abstract

We report observations of an interplanetary (IP) shock observed by Parker Solar Probe (PSP) on 2024 September 29 at similar to 07:50:29 UTC. PSP was only similar to 17.07 Rs from the Sun, making this one of the closest observed IP shocks to date. The IP shock was a weak (Mf similar to 1.2), quasi-perpendicular (theta Bn similar to 50 degrees), and of moderate speed (Vshn similar to 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, similar to MeV energy protons with an inverse velocity arrival and synchrotron radiation (due to similar 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.