Observational Evidence for High-Mach Number Regime of Coronal Shock Waves During Powerful Solar Particle Events

Abstract:

Identifying the physical mechanisms that produce the most energetic particles is a long-standing observational and theoretical challenge in astrophysics. Strong shock waves have been proposed as efficient accelerators both in the solar physics and astrophysical contexts via various acceleration mechanisms. The proposed processes rely on shock waves being super-critical or moving several times faster than the characteristic speed of the medium they propagate through (a high M_A). Using recent imaging of the NASA STEREO, SOHO and SDO spacecraft, we provide the first observations of the time-dependent 3-dimensional distribution of the expansion speed and M_A of a coronal shock wave. These observations show that the high-energy particles measured near Earth are produced at the time of the sharp rise in the shock Mach number (>10) magnetically connected to Earth. These findings provide direct evidence to energetic particles being accelerated during the formation of a strong coronal shock. Using our new technique, we study the longitudinal spread and timing of a number of other energetic particle events during cycle 24.