Are Solar Energetic Particle Events and Type II Bursts Associated with Fast and Narrow Coronal Mass Ejections?

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Gradual solar energetic ( $E >10~\mbox{MeV}$ ) particle (SEP) events and metric through kilometric wavelength type II radio bursts are usually associated with shocks driven by fast ( $V \geq 900~\mbox{km}\,\mbox{s}^{-1}$ ) and wide ( $W \geq 60^{\circ} $ ) coronal mass ejections (FW CMEs). This criterion was established empirically by several studies from solar cycle 23. The characteristic Alfven speed in the corona, which ranges over 500 – 1500 km s−1 at heights ${\geq}\, 2~R_{\text{o}}$ , provides the minimum $V$ requirement for a CME to drive a shock, but the general absence of SEP events or type II bursts with fast and narrow ( $W < 60^{\circ}$ ) CMEs has not been explained. We review and confirm the earlier studies with a more comprehensive comparison of SEP events and type II bursts with fast and narrow (FN) CMEs. We offer an explanation for the lack of SEP event and type II burst associations with FN CMEs in terms of recent heuristic arguments and modeling that show that the response of a magnetized plasma to the propagation of a CME depends on the CME geometry as well as on its speed. A clear distinction is made between a projectile that propagates through the medium to produce a bow shock, and a 3D piston that everywhere accumulates material to produce a broad shock and sheath. The bow shock is unfavorable for producing SEP events and type II bursts, but the $60^{\circ} $ cut-off is not explained.