Evidence for the occurrence of a cylindrical current sheet over the South Pole of the Sun

Abstract:

A cylindrical current sheet was observed inside a fast speed stream from the coronal hole during the passage of Ulysses over the South solar pole in December 2006 – January 2007, when it reached maximal heliolatitude of 79.7 deg at 2.4 AU. It was characterized by sharp changes in the solar wind and IMF parameters, characteristic for cylindrical or conic-like current sheets. Interestingly, both the solar wind velocity and the density sharply decreased, but the temperature was twice as large in the point closest to the pole. We discuss solar origin of the observed structure and present modeling that can explain the observations. We develop the model of a cylindrical current sheet that surrounds a coronal hole. Modeling shows that such a sheet might experience pinching with multiplication of the structure. One of the smaller-size tubes might be observed by Ulysses. We also consider a case of streamer- and jet-like structures, as well as the possibility of formation of a relatively small cylindrical current sheet near the pole due to non-axiality of the solar rotation axis and the magnetic axis. According to polar oval observations, similar structures are sometimes formed in the terrestrial cusps during the development of geomagnetic storms.