The coronal plasma, which is the cause of geomagnetic storms, can impinge on the Earth only when some coronal formation is pointing towards the Earth. If no such formation is directed towards the Earth, a period of geomagnetic calm follows.

It was found that certain coronal formations, governed clearly by the appropriate local magnetic fields, correspond to the individual stages of development of active centres. Above the sunspot groups, there occur either conical rarifications or cylindrical, condensed fluxes dependent on a relatively non-variable, or unstable magnetic field. The filaments which outlive the sunspot period, provided they are located in floccular fields, are appropriate to streamers of helmet shape. The final stage of the active region, characterized by the presence of filaments only (called ‘free filaments’ by the authors), does not change the arrangement of the corona appreciably, i.e. the normal shape, or so-called minimum type. On the basis of the mentioned relations, it is possible to determine the shape and direction of so far currently unobservable coronal structures from chromospheric situations.

The mentioned facts enable a unified interpretation of the geomagnetic activity during the whole of the solar cycle to be made. The differences given for some types of storms (sporadic and recurrent, sudden and gradual commencement), may be explained by the various arrangements, occurrences or absences of local magnetic fields. The paper presents examples of chromospheric situations which were used, in some cases, at the Geophysical Institute in Prague for forecasting geomagnetic activity.