Seps: Space Weather Hazard in Interplanetary Space

In the largest and most hazardous of solar energetic particle (SEP) events, acceleration takes place at shock waves driven out from the Sun by fast CMEs. Multi-spacecraft studies show that the particles from the largest events span more than 180 degrees in solar longitude; the events can last for several days. Protons streaming away from the shock generate waves that trap particles in the acceleration region, limiting outflowing intensities but increasing the efficiency of acceleration to higher energies. Thus, early intensities are bounded, but at the time of shock passage, they can suddenly rise to a peak. These shock peaks extend to >500 MeV in the largest events, creating a serious 'delayed' radiation hazard. At high energies, spectra steepen to form a 'knee.' This spectral knee can vary from ∼10 MeV to ∼1 GeV depending on shock conditions, greatly affecting the radiation hazard. Elements with different charge-to-mass ratios differentially probe the wave spectra near shocks, producing abundance ratios that vary in space and time. These abundance ratios are a tool that can foretell conditions at an oncoming shock.