Solar flares, type III radio bursts, coronal mass ejections, and energetic particles

Abstract

In this correlative study between >20 MeV solar proton events, coronal mass ejections (CMEs), flares, and radio bursts it is found that essentially all of the proton events are preceded by groups of type III bursts and all are preceded by CMEs. These type III bursts (that are a flare phenomenon) usually are long‐lasting, intense bursts seen in the low‐frequency observations made from space. They are caused by streams of electrons traveling from close to the solar surface out to 1 AU. In most events the type III emissions extend into, or originate at, the time when type II and type IV bursts are reported (some 5 to 10 minutes after the start of the associated soft X‐ray flare) and have starting frequencies in the 500 to ∼100 MHz range that often get lower as a function of time. These later type III emissions are often not reported by ground‐based observers, probably because of undue attention to type II bursts. It is suggested to call them type III‐ l . Type III‐ l bursts have previously been called shock accelerated (SA) events, but an examination of radio dynamic spectra over an extended frequency range shows that the type III‐ l bursts usually start at frequencies above any type II burst that may be present. The bursts sometimes continue beyond the time when type II emission is seen and, furthermore, sometimes occur in the absence of any type II emission. Thus the causative electrons are unlikely to be shock accelerated and probably originate in the reconnection regions below fast CMEs. A search did not find any type III‐ l bursts that were not associated with CMEs. The existence of low‐frequency type III bursts proves that open field lines extend from within 0.5 radius of the Sun into the interplanetary medium (the bursts start above 100 MHz, and such emission originates within 0.5 solar radius of the solar surface). Thus it is not valid to assume that only closed field lines exist in the flaring regions associated with CMEs and some interplanetary particles originating in such flare regions might be expected in all solar particle events.