  Magnetic reconnection plays a crucial role in large dissipative events in
space plasmas, such as solar flares and geomagnetic substorms, but there has
been much confusion regarding reconnection onset responsible for the explosive
events. The main theme of the present paper is to propose a new physical
concept on reconnection onset and to clarify physical mechanism responsible for
flares and substorms. Reconnection rate can be defined as inductive electric
field along X neutral line, so it is proposed that large-scale reconnection can
occur when ambient reconnection (inductive) electric fields effectively
penetrate into X line. In the solar corona or geomagnetic tail without
effective Coulomb collision, reconnection electric fields are shielded (cut
off) around X line, so reconnection cannot occur and magnetic energy can be
stored in the form of current sheets. It is demonstrated that when plasmas
become effectively collisional and dissipative around X line, shielding of
reconnection electric fields around X line is violated, and reconnection
drastically grows to eventually cause the (Petschek-like) fast reconnection
mechanism responsible for flares and substorms.
