On the Dst variation forms of sporadic and M disturbances of the geomagnetic field

Two samples of sporadic geomagnetic disturbances, with a phenomenon on the Sun-either a flare (sample 1) or fiber disappearance (sample 2)-producing each of these disturbances being observed are considered. It is detected that in 36% of sample 1 disturbances and in 50% of sample 2 ones Dst-variation had a classical form: a clearly expressed initial phase, a sharp and deep main phase and a less sharp recovery phase which is often close to exponent. In other cases reduced Dst forms, characterised by a weak development or absence of the main phase were observed. There exist four of these forms. Parallel survey of dozens of M-disturbances has shown that they possess the same reduced Dst forms, but the classical form does not occur at all. A conclusion is drawn that the presence of classical Dst form in geomagnetic disturbance is a sufficient base for its reference to sporadic ones