Evidence of subphotospheric activity associated with large solar flares

Observations at soft X-ray wavelengths show that magnetic regions remain 'quiet' for many hours before intense long-lived brightenings (lasting 8 hr or more) associated with major optical flaring develop there. It is argued that this sequence of events would not occur if a reservoir of energy formed in the solar atmosphere during these hours prior to its release in a flare. The alternative explanation outlined here is that the magnetic configuration existing during the 'quiet' period impairs the flow of energy upwards into the solar atmosphere: this 'missing energy' is stored within the convective zone. Flaring follows when convective instability at this reservoir leads to a violent disturbance of the magnetic configuration. The duration of the associated X-ray brightening indicates that the disturbance has a lifetime of more than half a day. Several major optical flares generally develop during each of the X-ray brightenings referred to. This study shows that conditions producing these optical enhancements usually follow the initial X-ray brightening by tens of minutes. (author)