Ulysses and the Reversal of the Solar Magnetic Field

In November 2000, the ESA/NASA Ulysses spacecraft flew over the south polar regions of the Sun at a distance of 2.2 AU and a latitide of 80.2S. (1 AU is the distance from the Sun to the Earth.) During this period solar activity was (and still is) close to maximum. It is during this time of maximum activity, every 11 years, that the solar magnetic field undergoes a reversal. The reversal is not instantaneous, but involves a complex restructuring of the magnetic field which appears to take at least several months....

These diagrams show how magnetic field lines in the Sun's atmosphere, the corona, vary through the solar activity cycle:

When Ulysses first flew beneath the south pole of the Sun in 1994 when solar activity was low, the Imperial College magnetometer encountered only magnetic fields pointing towards the Sun, consistent with the first of the diagrams above. When Ulysses returned to the south polar regions in November 2000 the magnetic field was found to be split into a number of sectors of opposing direction, one type pointing towards the Sun and the other pointing away from the Sun, consistent with the second diagram above. The fact that this behaviour was found at the highest latitudes reached by Ulysses suggests that the reversal of the solar field was underway at the time of the Ulysses polar passage.

The following two images from the LASCO coronagraph onboard the SOHO spacecraft, one from solar minimum activity in 1996 and the other from the present solar maximum, give a qualitative impression that the above diagrams are correct:

The SOHO/LASCO data reproduced here are produced by a consortium of the Naval Research Laboratory (USA), Max-Planck-Institut fuer Aeronomie (Germany)), Laboratoire d'Astronomie (France), and the University of Birmingham (UK). SOHO is a project of international cooperation between ESA and NASA.

SOHO continually provides images of the Sun and its atmosphere from a fixed vantage point near the Earth. Although Ulysses does not have the capability to produce such images, the images from SOHO are very useful to the Ulysses scientists in helping them to interpret their observations at high solar latitudes. The bright areas indicate where plasma (ionised gas) in the solar atmosphere is trapped in the regions where magnetic field lines form loops that return to the solar surface, and thus give us a visual record of where such magnetic loops are located.

For further information on the Ulysses magnetometer experiment please visit the Imperial College Ulysses pages. The European Space Agency Ulysses pages contain information and graphics related to the other experiments and the mission as a whole.

This page was prepared by Bob Forsyth. It was last updated on 2nd April 2001.