LAUSR

An abrupt decrease in the solar wind pressure and its effect on the magnetosphere and ionosphere during the event occurring on April 4, 1971, are studied. This event differs fundamentally from a typical sudden commencement (SC) of a geomagnetic storm or from a positive sudden impulse (SI+) and is determined as a negative sudden impulse (SI–). The geomagnetic variations at different latitudes and the cosmic radio emission in the auroral zone are analyzed. From the data of low-latitude geomagnetic observatories, several subsequent negative impulses observed with a periodicity of ~45 min were found. At the same time, a sudden decrease in the absorption of cosmic radio emission in the auroral zone was revealed. Possible physical explanations of the observed changes are discussed.