# Solar Energetic Particle Events with Short and Long Onset Times

## TL;DR

This study investigates the factors influencing the timing of solar energetic particle events, focusing on shock development, CME interactions, and active region properties to improve space weather prediction accuracy.

## Contribution

It identifies key factors like shock development and CME interactions that control proton release times, offering insights into SEP event variability.

## Key findings

- Proton release times vary significantly even with similar CME speeds and source locations.
- Shock development and Alfvén Mach number thresholds influence SEP onset timing.
- CME-CME interactions and active region characteristics affect proton release times.

## Abstract

Gradual solar energetic particle (SEP) events, usually attributed to shock waves driven by coronal mass ejections (CMEs), show a wide variety of temporal behaviors. For example, TO, the >10 MeV proton onset time with respect to the launch of the CME, has a distribution of at least an order of magnitude, even when the source region is not far from the so-called well-connected longitudes. It is important to understand what controls TO, especially in the context of space weather prediction. Here we study two SEP events from the western hemisphere that are different in TO on the basis of >10 MeV proton data from the Geostationary Operations Environmental Satellite, despite similar in the CME speed and longitude of the source regions. We try to find the reasons for different TO, or proton release times, in how the CME-driven shock develops and the Alfv\'en Mach number of the shock wave reaches some threshold, by combining the CME height-time profiles with radio dynamic spectra. We also discuss how CME-CME interactions and active region properties may affect proton release times.