Q/A Dependence on Shock Geometry Using Multi-Spacecraft Observations
Thursday, 18 December 2014
In many Solar Energetic Particle (SEP) events, the particle energy spectra show clear roll-over or double power law features. These features can be described by a break energy, which has a charge-to-mass (Q/A) dependence that varies from event to event (Cohen et al. 2003, 2005, Mewaldt et al. 2005). Here Q is the charge state and A is the relative atomic mass of energetic particles. A (Q/A)^2 dependence of the break energy emerges naturally from a numerical model (Li et al. 2005) of particle acceleration at a parallel CME-driven shock. Shallower dependence, e.g., (Q/A)^(2/9) has also been suggested by Li et al. (2009) for a quasi-perpendicular shock. Li et al. (2009) also discussed the most general case of an oblique shock, in which the (Q/A) dependence of the break energy is between the parallel and the perpendicular cases. The combination of STEREO A&B and ACE give us the opportunity to examine the same SEP event from different vantage points in which the shock obliquities may differ. We compare the time intensity profile and spectra of different species such as H, He4, C, O and Fe in selected SEP events (Richardson et al. 2014) that were observed by SIT and LET onboard STEREO A&B and ULEIS and SIS onboard ACE. Q/A scalings which reveal the information concerning shock geometry at different spacecraft are obtained.