Geomagnetic response to magnetic clouds of different polarity

The polarity of a magnetic cloud refers to its changing magnetic field direction. It is classified as S‐N polarity when the magnetic field rotates from southward to northward and N‐S polarity when the field is initially northward and rotates southward. A study of 29 magnetic cloud events has found that 40–45% of magnetic clouds, independent of polarity, are followed by a fast solar wind stream which compresses the tail end of the cloud. The compression results in an increase in the solar wind plasma density and in 64% of the cases an increase in the magnetic field strength towards the latter part of the cloud. Such tail end compression can have a significant effect upon geomagnetic storm intensity if the magnetic cloud is of N‐S polarity. This is because only in the N‐S polarity case does the compression coincide with the southward IMF portion of the cloud. To test the “geoeffectiveness” of N‐S versus S‐N magnetic clouds three selected magnetic cloud events, two of S‐N polarity and one of N‐S polarity, are investigated in terms of their geomagnetic response through measured and estimated D st values. It is found that there is an increased geoeffectiveness of N‐S polarity clouds due to both an increased solar wind dynamic pressure and a compressed southward field associated with a following fast solar wind stream.