Large gradual solar energetic particle events.

Area of Science:

• * Space Physics
• * Solar Physics
• * Astrophysics

Background:

• * Solar energetic particles (SEPs) originate near the Sun through two primary mechanisms: impulsive acceleration during solar flares and gradual acceleration at coronal mass ejection (CME)-driven shock waves.
• * Large gradual SEP events, particularly those involving high-energy protons, present significant radiation hazards to astronauts and space-based technology.
• * Current understanding of these events is limited by the smearing of observational data in Earth orbit due to complex physical processes and transport effects.

Purpose of the Study:

• * To provide a comprehensive review of the current state of knowledge regarding large gradual SEP events.
• * To identify key scientific questions that need to be addressed to improve our understanding of SEP acceleration and transport.
• * To highlight the anticipated contributions of NASA's Solar Probe Plus and ESA's Solar Orbiter missions in advancing SEP research.

Main Methods:

• * Review of existing literature and theoretical models on SEP acceleration and transport.
• * Analysis of observational data from past and current space missions.
• * Discussion of the scientific objectives and capabilities of upcoming Solar Probe Plus and Solar Orbiter missions.

Main Results:

• * SEPs are accelerated by distinct mechanisms associated with solar flares and CME-driven shocks.
• * Large gradual SEP events pose significant risks due to high-energy particle fluxes.
• * Direct in-situ measurements near the Sun are crucial for disentangling complex physical processes.

Conclusions:

• * A deeper understanding of SEP acceleration and transport is essential for space weather prediction and astronaut safety.
• * Upcoming missions like Solar Probe Plus and Solar Orbiter will provide unprecedented opportunities to study SEPs in their near-Sun source regions.
• * Direct sampling of the near-Sun environment will enable better discrimination of acceleration sites and mechanisms, leading to improved models of SEP events.