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 Dst 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.