Interpretation of flat energy spectra upstream of fast interplanetary shocks

Abstract

Interplanetary shocks are large-scale heliospheric structures often caused by eruptive phenomena at the Sun, and represent one of the main sources of energetic particles. Several interplanetary shock crossings by spacecraft at $1$ AU have revealed enhanced energetic-ion fluxes that extend far upstream of the shock. Surprisingly, in some shock events, ion fluxes with energies between $100$ keV and about $2$ MeV acquire similar values (which we refer to as ``overlapped'' fluxes), corresponding to flat energy spectra in that range. In contrast, closer to the shock, the fluxes are observed to depend on energy. In this work, we analyze three interplanetary shock-related energetic particle events observed by the Advanced Composition Explorer spacecraft where flat ion energy spectra were observed upstream of the shock. We interpret these observations via a velocity filter mechanism for particles in a given energy range. This reveals that low energy particles tend to be confined to the shock front and cannot easily propagate upstream, while high energy particles can. The velocity filter mechanism has been corroborated from observations of particle flux anisotropy by the Solid-State Telescope of Wind/3DP.

References in corpus (13)

• Small Scale Energy Cascade of the Solar Wind Turbulence
• Simulations and Theory of Ion Injection at Non-relativistic Collisionless Shocks
• The origin of galactic cosmic rays
• Relativistic electrons produced by foreshock disturbances
• Turbulence/wave transmission at an ICME-driven shock observed by Solar Orbiter and Wind
• Single-spacecraft techniques for shock parameters estimation: A systematic approach
• Parameter estimation of superdiffusive motion of energetic particles upstream of heliospheric shocks
• First observations of irregular surface of interplanetary shocks at ion scales by Cluster
• Observation-based modelling of the energetic storm particle event of 14 July 2012
• On the transmission of turbulent structures across the Earth's Bow Shock
• Multi-spacecraft observations of shocklets at an interplanetary shock
• Particle transport in hybrid PIC shock simulations: A comparison of diagnostics
• ULF wave transmission across collisionless shocks: 2.5D local hybrid simulations

Cited by in corpus (3)

• Three-dimensional modelling of the shock-turbulence interaction
• Flat Spectra of Energetic Particles in Interplanetary Shock Precursors
• Numerical investigation of particle acceleration at interplanetary shocks: diffusive and superdiffusive scenarios