Morphology of Storms in the Ionosphere

抄録

A review is given on the morphology of storms in the ionosphere based on the results obtained during the world-wide cooperative observations, IGY (1957-1958) and IGC (1959). Ionospheric disturbances are classified into two categories: disturbances that take place in the lower ionosphere and F region ionospheric storms.
E region disturbances are mainly caused by an abnormal ionizing agent penetrating into the lower ionosphere. Sudden ionospheric disturbances are due to an outburst of solar X rays associated with chromospheric flares, while polar blackout phenomena are produced by incoming energetic particles. It is shown that solar cosmic ray particles (sub-relativistic) ejected from an intense flare are the source of a prolonged blackout in the polar cap region, known as polar cap absorption, and auroral particles consisting presumably of energetic electrons are responsible for auroral blackouts. Average features of these two types of polar blackout are described in some detail with other related geophysical phenomena.
F region storms accompanying large electron density variations are closely connected with geomagnetic storms. The disturbances are world-wide, their features being markedly different both with respect to geomagnetic storm-time as well as time of day. The general morphological features of storms are, therefore, described in terms of the storm-time variation, Dst, and the disturbance diurnal variation, Ds. A theoretical interpretation of ionospheric F₂ storms is still incomplete; however, two dominant theories which may explain the main features of electron density variations are discussed in the light of recent knowledge of the ionosphere. It is concluded that the Dst variation may be produced by the effect of thermal heating of the ionosphere during storms, whereas the Ds variation may be related in some way to electrodynamical drift motions of electrons due to the interaction of the geomagnetic field with the electric field induced by the enhanced dynamo-action in the ionosphere. Other disturbance effects in the F region and their connection with exospheric phenomena are also discussed briefly.