The injection problem in diffusive shock acceleration theory is discussed with particular focus on electrons. The following issues are addressed: Why it has been considered to be so difficult, what is the required condition, and how it can be resolved. It is argued that there exists a critical Mach number above which the electron injection is achieved. Above the threshold, back-streaming electrons reflected back upstream by the shock front can self-generate high-frequency whistler waves, which can scatter themselves as required for subsequent acceleration. The theoretical estimate is found to be well consistent with in-situ measurements, indicating this could provide a possible solution to the long standing problem in the diffusive shock acceleration theory.
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