Interpretation of Flat Energy Spectra Upstream of Fast Interplanetary Shocks

Perri, S.; Prete, Giuseppe; Zimbardo, G.; Trotta, Domenico; Wilson, L. B.; Lario, D.; Servidio, S.; Valentini, F.; Giacalone, J.

doi:10.3847/1538-4357/acc942

Cited by 4 publications

(2 citation statements)

References 70 publications

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“…However, it is not clear whether the nearly zero spectral slope measured by [1] could be reached in a particular scattering condition. A more recent modeling attempt for the SEP events reported in [1] also suggests trapping of low energy particles as origin of upstream spectral flatness [4].…”

Section: Introductionmentioning

confidence: 90%

Modeling flat proton spectra at large solar energetic particle events at 1 AU via particle escape into a self-generated/pre-existing turbulence

Fraschetti,

R. Goldberg

2023

Proceedings of 38th International Cosmic Ray Conference — PoS(ICRC2023)

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Flat proton energy (50 keV -a few MeV) spectra have been measured in-situ at 1 AU in large solar energetic particle events in the past decades. The diffusive picture of particle acceleration at shocks propagating into media that enforce a uniform spatial diffusion far upstream (several hours at 1 AU) seems unable to reproduce such measurements. Hereby we use a method recently developed based on a 1D transport equation accounting for particle acceleration and escape, both allowed at all particle energies, not only the highest energies. The key ingredient of the model is that diffusion is contributed by both self-generated turbulence close to the shock and by preexisting turbulence far upstream, so that the upstream mean free path gradually increases out to a saturation in the pristine solar wind turbulence. The steady-state spectra agree very well with in-situ measurements, confirming that escape into a medium with mean free path spatial variation is a crucial ingredient in forging the energy spectra in the proximity of shocks.

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Section: Introductionmentioning

confidence: 90%

Modeling flat proton spectra at large solar energetic particle events at 1 AU via particle escape into a self-generated/pre-existing turbulence

Fraschetti,

R. Goldberg

2023

Proceedings of 38th International Cosmic Ray Conference — PoS(ICRC2023)

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“…These produce very nearly flat energy spectra. This phenomenon is not presently well understood, but it may be related to the way in which particles escape from near the shock, where they are confined by turbulent magnetic fields (Perri et al 2023), adiabatic cooling (Prinsloo et al 2019), or perhaps a balance between the injection rate (at very low energies) at the shock and their escape upstream (Lario et al 2018).…”

Section: Introductionmentioning

confidence: 99%

Analyses of ∼0.05–2 MeV Ions Associated with the 2022 February 16 Energetic Storm Particle Event Observed by Parker Solar Probe

Giacalone,

Cohen,

McComas

et al. 2023

ApJ

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We present analyses of 0.05–2 MeV ions from the 2022 February 16 energetic storm particle event observed by Parker Solar Probe's (PSP) IS⊙IS/EPI-Lo instrument at 0.35 au from the Sun. This event was characterized by an enhancement in ion fluxes from a quiet background, increasing gradually with time with a nearly flat spectrum, rising sharply near the arrival of the coronal mass ejection (CME)–driven shock, becoming nearly a power-law spectrum, then decaying exponentially afterward, with a rate that was independent of energy. From the observed fluxes, we determine diffusion coefficients, finding that far upstream of the shock the diffusion coefficients are nearly independent of energy, with a value of 1020 cm2 s−1. Near the shock, the diffusion coefficients are more than 1 order of magnitude smaller and increase nearly linearly with energy. We also determine the source of energetic particles, by comparing ratios of the intensities at the shock to estimates of the quiet-time intensity to predictions from diffusive shock acceleration theory. We conclude that the source of energetic ions is mostly the solar wind for this event. We also present potential interpretations of the near-exponential decay of the intensity behind the shock. One possibility we suggest is that the shock was overexpanding when it crossed PSP and the energetic particle intensity decreased behind the shock to fill the expanding volume. Overexpanding CMEs could well be more common closer to the Sun, and this is an example of such a case.

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Three-dimensional modelling of the shock–turbulence interaction

Trotta

Pezzi

Burgess

et al. 2023

Monthly Notices of the Royal Astronomical Society

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The complex interaction between shocks and plasma turbulence is extremely important to address crucial features of energy conversion in a broad range of astrophysical systems. We study the interaction between a supercritical, perpendicular shock and pre-existing, fully-developed plasma turbulence, employing a novel combination of magnetohydrodynamic (MHD) and small-scale, hybrid-kinetic simulations where a shock is propagating through a turbulent medium. The variability of the shock front in the unperturbed case and for two levels of upstream fluctuations is addressed. We find that the behaviour of shock ripples, i.e., shock surface fluctuations with short (a few ion skin depths, di) wavelengths, is modified by the presence of pre-existing turbulence, which also induces strong corrugations of the shock front at larger scales. We link this complex behaviour of the shock front and the shock downstream structuring with the proton temperature anisotropies produced in the shock-turbulence system. Finally, we put our modelling effort in the context of spacecraft observations, elucidating the role of novel cross-scale, multi-spacecraft measurements in resolving shock front irregularities at different scales. These results are relevant for a broad range of astrophysical systems characterised by the presence of shock waves interacting with plasma turbulence.

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Interpretation of Flat Energy Spectra Upstream of Fast Interplanetary Shocks

Cited by 4 publications

References 70 publications

Modeling flat proton spectra at large solar energetic particle events at 1 AU via particle escape into a self-generated/pre-existing turbulence

Modeling flat proton spectra at large solar energetic particle events at 1 AU via particle escape into a self-generated/pre-existing turbulence

Analyses of ∼0.05–2 MeV Ions Associated with the 2022 February 16 Energetic Storm Particle Event Observed by Parker Solar Probe

Three-dimensional modelling of the shock–turbulence interaction

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